Editing How to rebuild a Rochester Quadrajet 4MV carburetor
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=Introduction= | =Introduction= | ||
− | This article is written with the intention of informing, guiding and assisting the first-time builder who has never dealt with a carburetor before. It merges detailed, step-by-step directions, complete with images, links to online and print resources, and a multitude of helpful tips and pointers collected from the members of [http://www.Hotrodders.com | + | This article is written with the intention of informing, guiding and assisting the first-time builder who has never dealt with a carburetor before. It merges detailed, step-by-step directions, complete with images, links to online and print resources, and a multitude of helpful tips and pointers collected from the members of [http://www.Hotrodders.com Hotrodders] forum, all of them in their own words. |
− | The article is also meant to complement the rebuild guide books available, shedding extra light on any topic or procedure that may require further explanation. In the future, as time allows for updates, the article will | + | The article is also meant to complement the rebuild guide books available (Ruggles and the Roe books are the two mentioned most often), shedding extra light on any topic or procedure that may require further explanation. In the future, as time allows for updates, the article will site sections and passages from related books to help the reader follow along with ease. |
− | Although the article specifically deals with the Rochester Quadrajet | + | Although the article specifically deals with the Rochester Quadrajet 4MV model, the information found here is useful for working on other Rochester 4 barrel models as well. |
At times, the reader may encounter differing viewpoints, especially when dealing with certain repair options. It is up to the reader to decide which method works best. If confusion persists or further advice is needed, it is highly recommended that the reader visit the [http://www.hotrodders.com Hotrodders] forum and ask for assistance from fellow members. | At times, the reader may encounter differing viewpoints, especially when dealing with certain repair options. It is up to the reader to decide which method works best. If confusion persists or further advice is needed, it is highly recommended that the reader visit the [http://www.hotrodders.com Hotrodders] forum and ask for assistance from fellow members. | ||
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=Planning and organization= | =Planning and organization= | ||
===A. Becoming familiar with the carburetor=== | ===A. Becoming familiar with the carburetor=== | ||
− | If you have never dealt with a carburetor in your life, it is important to familiarize yourself with the various components. Please refer to the '''''Books/ Guides/ Websites /Multimedia''''' section | + | ---- |
+ | If you have never dealt with a carburetor in your life, it is important to familiarize yourself with the various components. Please refer to the '''''Books/ Guides/ Websites /Multimedia''''' section below for helpful materials. | ||
=====Books/guides/websites/multimedia===== | =====Books/guides/websites/multimedia===== | ||
− | : | + | :'''''"How to Rebuild and Modify Rochester Quadrajet Carburetors"''''' by Cliff Ruggles is a frequent recommendation by the members of [http://www.hotrodders.com Hotrodders] forum. It is a highly-reviewed book and the author is considered a leading authority on Quadrajet carburetors. |
− | :In addition to his book, Cliff Ruggles also maintains a website ([http://www.cliffshighperformance.com/ | + | :In addition to his book, Cliff Ruggles also maintains a website ([http://www.cliffshighperformance.com/ Cliff's High Performance]), with a forum dedicated to discussing carburetors. |
− | :Another highly reviewed title is | + | :Another highly reviewed title is '''''"Rochester Carburetors"''''' by Doug Roe / Bill Fisher. |
:Other titles devoted to Rochester Quadrajets are also available from retailers such as [http://www.barnesandnoble.com/ Barnes and Noble], [http://www.amazon.com Amazon], [http://www.ebay.com eBay] and its sister site [http://www.half.com Half.com]. | :Other titles devoted to Rochester Quadrajets are also available from retailers such as [http://www.barnesandnoble.com/ Barnes and Noble], [http://www.amazon.com Amazon], [http://www.ebay.com eBay] and its sister site [http://www.half.com Half.com]. | ||
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=====Carburetor identification: Locations and decoding===== | =====Carburetor identification: Locations and decoding===== | ||
− | :Once it has been determined that your Rochester Quadrajet carburetor requires a rebuild, the first thing to do is to locate the identification number. The identification number will help with ordering the correct parts or a rebuild kit. The number is usually found as a stamping in the body | + | :Once it has been determined that your Rochester Quadrajet carburetor requires a rebuild, the first thing to do is to locate the identification number. The identification number will help with ordering the correct set of parts or a rebuild kit. The number is usually found either as a tag or as a stamping in the body on the driver-side of the carburetor, running vertically up and down.'' ^1'' Should a need arise for decoding, please see the following article for further information and assistance: [[Rochester Quadrajet Carburetors: Identification and Decoding]] |
− | + | [[File:Carburetor_Identification_Number.jpg|thumb|center|750x250px|ID Number 17057525 APP 3356]] | |
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− | [[File:Carburetor_Identification_Number.jpg|thumb|center|750x250px| | + | |
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===B. Parts=== | ===B. Parts=== | ||
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− | Available through various manufacturers. For user experiences, please check out the ''Hotrodders'' forum thread, [http://www.hotrodders.com/forum/q-jet-rebuild-kit-210485.html#post1502078 | + | *Carburetor Rebuild Kits: Available through various manufacturers. For user experiences, please check out the ''Hotrodders'' forum thread, ''[http://www.hotrodders.com/forum/q-jet-rebuild-kit-210485.html#post1502078 Q-jet rebuild kit]''. |
− | + | *Individual Parts: Please refer to the '''''Procedures''''' section. | |
− | Please refer to the ''''' | + | |
− | + | *Sellers / Sources: <br />[http://www.cliffshighperformance.com/index.html Cliff's High Performance] <br />[http://quadrajetparts.com/index.php QuadrajetParts.com] <br />[http://www.carburetion.com/index.htm Carbs Unlimited] <br />[http://www.carbkitsource.com/ The Carburetor Doctor] | |
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===C. Tools=== | ===C. Tools=== | ||
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:*Carburetor stand - used to keep the carburetor in place while working on it. It is useful for testing out all linkages before re-installation. You can buy one from any performance auto parts supplier or build your own. For ideas, check out this [http://www.forabodiesonly.com/mopar/showpost.php?p=498702&postcount=2 post] on [http://www.forabodiesonly.com/ For A-Bodies Only] | :*Carburetor stand - used to keep the carburetor in place while working on it. It is useful for testing out all linkages before re-installation. You can buy one from any performance auto parts supplier or build your own. For ideas, check out this [http://www.forabodiesonly.com/mopar/showpost.php?p=498702&postcount=2 post] on [http://www.forabodiesonly.com/ For A-Bodies Only] | ||
− | <blockquote>''"I built a stand to hold the carb body so that I could functionally check all the linkage and clearances on the butterflies. It also makes assembly of the metering rods much easier. You can buy a stand, but hard to justify unless you plan on rebuilding lots of carbs. My Q- | + | <blockquote>''"I built a stand to hold the carb body so that I could functionally check all the linkage and clearances on the butterflies. It also makes assembly of the metering rods much easier. You can buy a stand, but hard to justify unless you plan on rebuilding lots of carbs. My Q-Jet book told me how to build the stand and it has been invaluable over the years."'' - '''''trees''''', [http://www.hotrodders.com Hotrodders]forum member.</blockquote> |
=====Chemicals===== | =====Chemicals===== | ||
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:*Throttle return spring | :*Throttle return spring | ||
:*Divorced-choke coil- Unscrew from the intake manifold if applicable. | :*Divorced-choke coil- Unscrew from the intake manifold if applicable. | ||
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:The carburetor is held in place by two long bolts visible on top of the air horn and either two nuts or two bolts on the back of the carburetor on top of the throttle cable bracket. Loosen these up, remove them, and set them aside in the parts organizer. | :The carburetor is held in place by two long bolts visible on top of the air horn and either two nuts or two bolts on the back of the carburetor on top of the throttle cable bracket. Loosen these up, remove them, and set them aside in the parts organizer. | ||
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===D. Dis-assembly=== | ===D. Dis-assembly=== | ||
− | + | '''''Note:''''' Be sure to have your parts organizer near by at all times. Also, take the time to match up the new parts with the old ones as soon as you remove them. If something doesn't match up, return it for the right part before beginning re-assembly. | |
====Accelerator pump actuator==== | ====Accelerator pump actuator==== | ||
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====Choke and linkage==== | ====Choke and linkage==== | ||
− | + | '''''Cautionary Note:''''' It is very important to take good, well-lit, close-up pictures of the linkages before dis-assembly. The choke linkage has several parts that must line up in a specific way during re-assembly or they will not work. | |
====Air Horn and Metering Rods==== | ====Air Horn and Metering Rods==== | ||
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*Jets | *Jets | ||
*Fuel Filter Housing | *Fuel Filter Housing | ||
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====Base/throttle body==== | ====Base/throttle body==== | ||
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====Trouble spots==== | ====Trouble spots==== | ||
Please keep an eye on the following areas during dis-assembly and rebuild. | Please keep an eye on the following areas during dis-assembly and rebuild. | ||
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=====Stripped or loose/damaged fuel inlet threads===== | =====Stripped or loose/damaged fuel inlet threads===== | ||
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=====Leaking float bowl well plugs===== | =====Leaking float bowl well plugs===== | ||
− | A common problem | + | A common problem on the early Q-jets especially, is leaky main wells. The 'cure' for this has long been to use JB Weld, etc. to seal them over. This is just a stop-gap measure, I have seen personally how the epoxy will degrade over time and quit sealing as it did at first. |
− | + | If a permanent fix is wanted, the wells can be resealed with new plugs that are peened over to hold them in place, or the wells can be drilled and tapped for short screws that will cure them for good. The primary wells seldom leak, the secondary wells on the early carbs do tend to leak more than the later carbs. | |
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Also please see post #48 on [http://www.hotrodders.com/forum/rochester-quadrajet-4mv-carburetor-removal-disassembly-rebuild-rookie-level-190558-4.html page 4] on the thread [http://www.hotrodders.com/forum/rochester-quadrajet-4mv-carburetor-removal-disassembly-rebuild-rookie-level-190558.html ''"Rochester Quadrajet 4MV Carburetor: Removal, Disassembly, Rebuild (Rookie Level")''] | Also please see post #48 on [http://www.hotrodders.com/forum/rochester-quadrajet-4mv-carburetor-removal-disassembly-rebuild-rookie-level-190558-4.html page 4] on the thread [http://www.hotrodders.com/forum/rochester-quadrajet-4mv-carburetor-removal-disassembly-rebuild-rookie-level-190558.html ''"Rochester Quadrajet 4MV Carburetor: Removal, Disassembly, Rebuild (Rookie Level")''] | ||
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Throttle return springs used as shown below should be avoided: | Throttle return springs used as shown below should be avoided: | ||
[[File:Throttle return bad.jpg|thumb|800px|left|Bad throttle return spring orientation]] <br style="clear:both"/> | [[File:Throttle return bad.jpg|thumb|800px|left|Bad throttle return spring orientation]] <br style="clear:both"/> | ||
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=====Throttle blade screw removal===== | =====Throttle blade screw removal===== | ||
If primary throttle shaft bushings are to be installed, the throttle shaft has to first be removed. After removing all the linkages and choke parts from the end of the shaft, the throttle blades will need to be removed. | If primary throttle shaft bushings are to be installed, the throttle shaft has to first be removed. After removing all the linkages and choke parts from the end of the shaft, the throttle blades will need to be removed. | ||
− | First, the ends of the original screws have to be ground away because they're "staked" at the factory | + | First, the ends of the original screws have to be ground away because they're "staked" at the factory. You may notice the ends are split a slight amount to make them impossible to loosen, back out and fall into the intake. |
[[File:STAKED QJET SCREW.jpg|border|250px|left]] | [[File:STAKED QJET SCREW.jpg|border|250px|left]] | ||
+ | Should a screw get broken off in the shaft, it can be removed by whatever method available; often drilling is done but be careful to get centered on the broken screw. A drill press is the way to do this best, with the throttle plate clamped down. There's no need to worry about matching the screw sizes side-to-side, in other words, if only one screw broke off, use the other original threaded hole along with the larger hole made for the broken screw. Just use the next larger size and use loctite as well as "peening" the ends of the screws. | ||
− | + | To do the peening, you can clamp a piece of square stock into a vice to use as an anvil to support the screw head on, then peen the ends of the screws with a small jeweler's hammer or ball peen. You don't need to simulate the factory way of staking; the main idea is to disrupt the threads on the end of the screw so they cannot fall into the engine. The loctite will do the rest. | |
− | + | If the ends of the screws look too long, they can be ground down with a dremel to the approximate length of the stock screws before peening them. | |
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− | + | Be very careful if you end up trying to find a replacement for the secondary shaft. They are not all the same. I'm curious. Why did you remove the shaft if you aren't bushing it? | |
− | + | The secondary throttle shaft do not need bushings. If the secondary shaft/throttle plate bore is that bad, the rest of the carb is likely worn badly; another core or at a least another throttle plate should be considered. | |
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=====Cleaning orifices===== | =====Cleaning orifices===== | ||
− | + | Please see posts numbers 23 through 28 on page 2 in the thread ''[http://www.hotrodders.com/forum/rochester-quadrajet-4mv-carburetor-removal-disassembly-rebuild-rookie-level-190558-2.html Rochester Quadrajet 4MV Carburetor: Removal, Disassembly, Rebuild (Rookie Level)]''. | |
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=====Throttle blades===== | =====Throttle blades===== | ||
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=====Cleaning idle tubes===== | =====Cleaning idle tubes===== | ||
− | The procedure for removal, cleaning and re-installation is covered in Ruggles' book (chapter 5, "Rebuilding the Carb", pages 77-78). It is also discussed on the thread ''[http://www.hotrodders.com/forum/1977-chevy-k20-engine-suffers-hesitation-power-loss-when-drive-213757.html "1977 Chevy K20: Engine Suffers from Hesitation/Power Loss When In Drive"]'' ([http://www.hotrodders.com/forum/1977-chevy-k20-engine-suffers-hesitation-power-loss-when-drive-213757- | + | The procedure for removal, cleaning and re-installation is covered in Ruggles' book (chapter 5, "Rebuilding the Carb", pages 77-78). It is also discussed on the thread ''[http://www.hotrodders.com/forum/1977-chevy-k20-engine-suffers-hesitation-power-loss-when-drive-213757.html "1977 Chevy K20: Engine Suffers from Hesitation/Power Loss When In Drive"]'' ([http://www.hotrodders.com/forum/1977-chevy-k20-engine-suffers-hesitation-power-loss-when-drive-213757-5.html page 5], post # 53, 58, 59). |
===E. Cleaning=== | ===E. Cleaning=== | ||
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=====Soaking the parts===== | =====Soaking the parts===== | ||
− | + | '''Cautionary Note:''' According to [http://www.hotrodders.com Hotrodders] forum member '''''trees''''': | |
− | <blockquote>''"Soaking all metallic parts is the way to go but using small brushes lightly is a good | + | <blockquote>''"Soaking all metallic parts is the way to go but using small brushes lightly is a good ideal. Do not soak non-metallic items." ^''</blockquote> |
=====Cleaning with brushes and wires===== | =====Cleaning with brushes and wires===== | ||
− | + | '''Cautionary Note:''' According to [http://www.hotrodders.com Hotrodders] forum member '''''trees''''': | |
<blockquote>''"You also need to use the correct size wire to run through all the jets and orifices. Be careful not to force the wire and score the surface." ^''</blockquote> | <blockquote>''"You also need to use the correct size wire to run through all the jets and orifices. Be careful not to force the wire and score the surface." ^''</blockquote> | ||
− | ====="White | + | ====="White Stuff" Build Up===== |
− | For information on the ''"white stuff"'' that can build up on carburetor castings, please see the | + | For information on the ''"white stuff"'' that can build up on carburetor castings, please see the discussion on the ''Hotrodders'' thread, [http://www.hotrodders.com/forum/quadrajet-211629-3.html '''''Quadrajet''''']. |
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=====Drying the parts===== | =====Drying the parts===== | ||
− | + | '''Cautionary Note:''' According to [http://www.hotrodders.com Hotrodders] forum member '''''trees''''': | |
<blockquote>''"One last rinse with the cleaner and blow dry everything. Make sure you use eye protection for this because the solvent can do serious damage!" ^''</blockquote> | <blockquote>''"One last rinse with the cleaner and blow dry everything. Make sure you use eye protection for this because the solvent can do serious damage!" ^''</blockquote> | ||
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=====Base/throttle plate===== | =====Base/throttle plate===== | ||
− | + | '''Note:''' Sometimes the Q-jet base plate may be missing a screw or two (there are only two on most carbs; three at the most). While the base plate is "helped" being held on by the two long front intake manifold mounting bolts and the two long screws at the rear of the carb, it's still a good idea to replace the missing screws. Just be sure to not over tighten these long bolts. If they're over tightened, it can warp the carb permanently. | |
=====Float/fuel bowl===== | =====Float/fuel bowl===== | ||
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:# Accelerator Pump | :# Accelerator Pump | ||
− | + | '''Note:''' When removing the accelerator pump arm roll pin, place a ~0.050" feeler gauge (or similar thickness material) behind the accelerator pump lever pin, so when you drive the pin towards the air horn lip it doesn't butt tightly against it; instead there's room to get a thin straight blade screwdriver in behind it to lever the pin back into position. | |
=====Air horn===== | =====Air horn===== | ||
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===G. Re-installation=== | ===G. Re-installation=== | ||
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− | + | '''Cautionary Note:''' Tighten the carb to the thick insulator-type gasket evenly but not too tightly. Because the gasket of that type has a lot of give to it, it can cause the base plate to warp- sometimes enough to cause the throttle shafts/blades to bind. | |
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− | [ | + | '''Cautionary Note:''' According to [http://www.hotrodders.com Hotrodders] forum member '''''cyclopsblown34''''': |
− | + | <blockquote>''"That nut on the backside of the carburetor base plate, do not overtighten it. Use a thread sealant tape or paste on it and don't wrench down hard on it, the casting is thin and will crack if over torqued. you can pretty much get it tight by feel, the sealant allows you to tighten it with less torque and still have a good seal." ^''</blockquote> | |
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==Tuning== | ==Tuning== | ||
Some basic [http://www.73-87.com/7387garage/drivetrain/myqjet.htm tips and links], from from 73-87.com. | Some basic [http://www.73-87.com/7387garage/drivetrain/myqjet.htm tips and links], from from 73-87.com. | ||
− | + | ===Basic adjustments=== | |
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− | ===Basic | + | |
[http://vetteworks.tripod.com/qjet.htm Rochester 4MC, 4MV, M4MC, and E4ME Carburetors] from vetteworks.tripod.com | [http://vetteworks.tripod.com/qjet.htm Rochester 4MC, 4MV, M4MC, and E4ME Carburetors] from vetteworks.tripod.com | ||
− | ===Idle | + | ===Idle problems=== |
*[http://www.73-87.com/7387garage/drivetrain/qjetidle.htm Q-jet Idle Problems] from 73-87.com | *[http://www.73-87.com/7387garage/drivetrain/qjetidle.htm Q-jet Idle Problems] from 73-87.com | ||
====Idle discharge ports==== | ====Idle discharge ports==== | ||
− | The idle discharge ports that are controlled by the idle mixture screws can be anywhere from around 0.050" to 0.080" diameter. If you find a lean idle condition that doesn't seem to respond to turning the screws CCW, you can gauge the port size using your small numbered drill bit index, then enlarge it one size at a time until you regain control with the screws | + | The idle discharge ports that are controlled by the idle mixture screws can be anywhere from around 0.050" to 0.080" diameter. If you find a lean idle condition that doesn't seem to respond to turning the screws CCW, you can gauge the port size using your small numbered drill bit index, then enlarge it one size at a time until you regain control with the screws. |
This is providing that the throttle plates are not open too far due to a large cam- which will over-expose the transfer slot, just like on a Holley. The cure for that is the same as you'd do with a Holley- start by using more ignition advance. This allows the primary throttle blades to be closed down some to lower the idle speed, which increases due to the advanced timing. Often, this is all that's needed, so be sure to do this FIRST, then enlarge the ports only if still needed. Normally, going larger than 0.0100" isn't needed and if the idle screws are still not responsive at this size, the problem lies elsewhere- like needing idle bypass air (or more bypass air). | This is providing that the throttle plates are not open too far due to a large cam- which will over-expose the transfer slot, just like on a Holley. The cure for that is the same as you'd do with a Holley- start by using more ignition advance. This allows the primary throttle blades to be closed down some to lower the idle speed, which increases due to the advanced timing. Often, this is all that's needed, so be sure to do this FIRST, then enlarge the ports only if still needed. Normally, going larger than 0.0100" isn't needed and if the idle screws are still not responsive at this size, the problem lies elsewhere- like needing idle bypass air (or more bypass air). | ||
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===Power piston springs=== | ===Power piston springs=== | ||
− | A power piston spring allows the power piston (aka "PP") to behave like a Holley power valve (or "PV"), that is, the enrichening system is controlled by the amount of vacuum the engine is producing- which is also an indication of how much load the engine is under. More load = less vacuum. Less vacuum = more enrichment, all else being equal. PP springs | + | A power piston spring allows the power piston (aka "PP") to behave like a Holley power valve (or "PV"), that is, the enrichening system is controlled by the amount of vacuum the engine is producing- which is also an indication of how much load the engine is under. More load = less vacuum. Less vacuum = more enrichment, all else being equal. PP springs are seldom mentioned when Q-Jet tuning is being discussed, but need to be addressed just like if it were a Holley PV. The same principals apply, as far as tuning for low vacuum cams, etc. |
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− | + | At high vacuum, the vacuum exceeds the PP spring rate and the needles are pulled down into the main jets, leaning the fuel/air mixture. Conversely, when vacuum drops, like when under a load or the accelerator pedal is whacked WFO, the PP spring rate exceeds the pull from the engine vacuum, which allows the spring to lift the needles up to their smaller diameter, or "rich" position. | |
− | + | If a “medium” PP spring = a Holley 6.5 in/Hg, a "soft" PP spring (allows enrichening to come in at a lower vacuum) would be like a 3.5 Holley PV; a “stiff” PP spring = a Holley PV of, say, 8.5 in/Hg. Longer duration cams will “like” a softer PP spring. | |
− | + | Below is an image showing a variety of different springs, another shot shows three different types of power piston. The springs vary in length, coil diameter and wire diameter (0.012"- 0.020" in the collection below, as an example). | |
{| | {| | ||
|[[File:Power piston springs 002.jpg|400px]] | |[[File:Power piston springs 002.jpg|400px]] | ||
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− | Edelbrock has four different PP springs: gold (4" | + | Edelbrock has some Q-Jet tuning parts. They show four different PP springs: gold (4" Hg), orange (5" Hg), black (6" Hg), and yellow (8" Hg) in their p/n 1980 spring kit. These springs are also available in their "Race Calibration Kit" that contains a selection of main jets and rods, secondary rods, hangars, PP springs, high flow needle and seat and an accelerator pump. There are also other suppliers of PP springs and other various tune up and repair parts; see the links below this section. |
− | *GM power piston spring p/n 7037305 is set for a tip in point of ~6 in/Hg, and would work well with a high vacuum, smooth idling and/or wide LSA type cam. | + | *GM power piston spring p/n 7037305 is set for a tip in point of ~6 in/Hg, and would work well with a high vacuum, smooth idling and/or wide LSA-type cam. |
*GM PP spring p/n 7036019 has a tip in of about 8 in/Hg. | *GM PP spring p/n 7036019 has a tip in of about 8 in/Hg. | ||
*GM part #7029922 is a rather weak spring that was OE for HO Pontiac engines from the early ‘70s. It delays enrichment until vacuum drops to ~3 in/Hg. This spring will work with durations >/= 230 degrees @ 0.050” lift. | *GM part #7029922 is a rather weak spring that was OE for HO Pontiac engines from the early ‘70s. It delays enrichment until vacuum drops to ~3 in/Hg. This spring will work with durations >/= 230 degrees @ 0.050” lift. | ||
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===Adjustable part throttle (APT) setting=== | ===Adjustable part throttle (APT) setting=== | ||
− | Another adjustment that can be made to a Q-Jet is the | + | Another seldom-mentioned adjustment that can be made to a Q-Jet, is the travel limiter for the PP. It can be adjusted (screwed) up or down to allow the primary needles to sit higher or lower at max vacuum, and is another tuning aid when dealing with long duration or overlap cams. This adjustment is referred to as the APT or "adjustable part throttle" setting. |
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There are two common types of APT adjustment locations: later carbs use a plugged hole in the airhorn, earlier carbs use a plugged hole in the baseplate. | There are two common types of APT adjustment locations: later carbs use a plugged hole in the airhorn, earlier carbs use a plugged hole in the baseplate. | ||
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+ | Before removing the adjustment screw from the carb body (airhorn-adjusted APT), count the turns it takes to lightly bottom the adjustment screw and write it down before removing the screw- if it needs to be removed; often it can just be left in place. Use this setting as a baseline at reassembly. | ||
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+ | ====Airhorn APT adjustment location==== | ||
+ | It is shown with a red arrow pointing to it, below. The yellow arrow points to the hole where the power piston is located. | ||
[[File:Apt screw details.jpg]] <br style="clear:both"/> | [[File:Apt screw details.jpg]] <br style="clear:both"/> | ||
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This adjustment is called the "adjustable part throttle" setting, or just "APT" for short. Using those terms will help you find additional info on the subject should you want it. The setting procedures will be for a Quadrajet 4-barrel- but they are basically the same for the Rochester Dualjet 2-barrel as well, which is the primary side of a Q-jet carb, anyway. | This adjustment is called the "adjustable part throttle" setting, or just "APT" for short. Using those terms will help you find additional info on the subject should you want it. The setting procedures will be for a Quadrajet 4-barrel- but they are basically the same for the Rochester Dualjet 2-barrel as well, which is the primary side of a Q-jet carb, anyway. | ||
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If you don't have the tool that fits the adjuster screw, carefully slot the screw yourself so you can use a small straight blade screwdriver to adjust it. | If you don't have the tool that fits the adjuster screw, carefully slot the screw yourself so you can use a small straight blade screwdriver to adjust it. | ||
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You may read that 2 to as much as 3-1/2 turns CCW from lightly bottoming the adjustment screw is a starting point. What may also be done for a starting point is for the top of the adjustment screw to be just a small amount above the carb body casting without a gasket in place. The carb [I]may[/I] be richer than needed in that position (or not, depending. But in that position the engine can be run safely and further fine tuning can be done, and with the following modification to the airhorn to allow easy access to the adjustment, it can be quickly and easily adjusted. | You may read that 2 to as much as 3-1/2 turns CCW from lightly bottoming the adjustment screw is a starting point. What may also be done for a starting point is for the top of the adjustment screw to be just a small amount above the carb body casting without a gasket in place. The carb [I]may[/I] be richer than needed in that position (or not, depending. But in that position the engine can be run safely and further fine tuning can be done, and with the following modification to the airhorn to allow easy access to the adjustment, it can be quickly and easily adjusted. | ||
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<br style="clear:both"/> | <br style="clear:both"/> | ||
− | Now, you can use the choke or your hand hand to partially block some of the air | + | Now, you can use the choke or your hand hand to partially block some of the air into the carb. Set the APT to where there is a slight increase in RPM as the carb is partially choked, while running at about 2000 RPM. Don't choke it so much that it stumbles or tries to stall, just enough to see if- and how- the RPM changes. The slight increase in rpm indicates the ATP is set just at the edge of being lean. Further adjustments can be made as needed, be sure the engine is always fully warmed up and that the idle mixture screws are spot on before adjusting the APT, and readjust the idle mixtrure screws after each APT adjustment as well. |
− | Another method (from SMI, IIRC): " | + | Another method (from SMI, IIRC): "simply turn the screw DOWN (CW) 1/2 turn at a time until you experience a "lean surge". (It will feel like someone is moving the throttle, when their not, or a hesitation on light throttle acceleration). Then turn the screw UP, until the lean surge goes away. |
− | If you are experiencing an "off-idle stumble", you can turn the screw up 1/4-1/2 turn at a time until it is eliminated. But be sure to adjust your idle mixture screws properly first, as they can also cause an off idle stumble. | + | If you are experiencing an "off-idle stumble", you can turn the screw up at 1/4-1/2 turn at a time until it is eliminated. But be sure to adjust your idle mixture screws properly first, as they can also cause an off idle stumble. |
====Baseplate APT adjustment location==== | ====Baseplate APT adjustment location==== | ||
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====Aneroid or spool location==== | ====Aneroid or spool location==== | ||
− | There were also additional Q-jet designs used mainly in 1975 | + | There were also additional Q-jet designs used mainly in 1975 only, that used an auxiliary enrichment system in addition to (or in some cases instead of) the primary PP system. One has an accordion-shaped aneroid "bellows" that lifted/lowered a single metering rod in a jet; it used no vacuum or power piston to operate it. The aneroid bellows changed the air/fuel ratio in accordance with the barometric pressure. Another was identical to the aneroid except the aneroid was deleted and was replaced by a filler spool that did not respond to barometric pressure; instead it was adjusted to give the correct air/fuel ratio according to ambient conditions only. These systems was found to be lacking, so were discontinued. |
+ | |||
+ | The other system uses a vacuum operated PP like the primary PP, but it differs by using only one metering rod and one fixed jet to feed both sides of the primaries. These were abandoned soon after they were released; it was found the primary PP system could be tailored to meet the requirements without the added complexity of the auxiliary system. | ||
{| | {| | ||
|[[File:Aneroid.jpg|thumb|center|350px|Red arrow points to aneroid adjustment screw through hole in airhorn. Hole is plugged with a cup plug.]] | |[[File:Aneroid.jpg|thumb|center|350px|Red arrow points to aneroid adjustment screw through hole in airhorn. Hole is plugged with a cup plug.]] | ||
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|} | |} | ||
[[File:ANEROID OR FILLER SPOOL DETAILS.jpg|thumb|left|600px|Early APT, ca. 1975]] <br style="clear:both"/> | [[File:ANEROID OR FILLER SPOOL DETAILS.jpg|thumb|left|600px|Early APT, ca. 1975]] <br style="clear:both"/> | ||
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===Secondary tuning=== | ===Secondary tuning=== | ||
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The AV shouldn't open when you jazz the throttle in PARK/neutral. Only when under a load, and at sufficient RPM do you want the AV to open. Also, there shouldn't be a noticeable transition into the secondaries. Many guys think the carb should "bang" into the secondaries, but what's happening is a bog when the secondaries open, followed by the engine 'catching up' to the carb- this gives the impression of acceleration. A timing slip will show otherwise. | The AV shouldn't open when you jazz the throttle in PARK/neutral. Only when under a load, and at sufficient RPM do you want the AV to open. Also, there shouldn't be a noticeable transition into the secondaries. Many guys think the carb should "bang" into the secondaries, but what's happening is a bog when the secondaries open, followed by the engine 'catching up' to the carb- this gives the impression of acceleration. A timing slip will show otherwise. | ||
− | You can get a 'feel' for how it's adjusted by pushing the AV open (engine OFF) | + | You can get a 'feel' for how it's adjusted by pushing the AV open (engine OFF) w/a finger. It should shut w/o hesitation when you let off pressure. After you've been working w/the Q-jet for a while, you will get a feel for how the AV is adjusted just by doing this. |
There is an adjustment for the AV spring. Should you want to try different adjustments on the air valve, use a 3/32" allen wrench to loosen the lock screw under the AV spring adjustment. 3/4 of a turn from first contact of the spring is the base setting, changing the setting 1/8 to 1/4 turn at a time will get it dialed in. | There is an adjustment for the AV spring. Should you want to try different adjustments on the air valve, use a 3/32" allen wrench to loosen the lock screw under the AV spring adjustment. 3/4 of a turn from first contact of the spring is the base setting, changing the setting 1/8 to 1/4 turn at a time will get it dialed in. | ||
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The next thing is to check to see that you're getting full opening of the secondary air valve. Some carbs had the amount of the air valve opening set to a lot less than it's capable of. The fix is easy, just look where the red arrow (left) is pointing, that's the stop. It can be carefully bent to set the opening amount. | The next thing is to check to see that you're getting full opening of the secondary air valve. Some carbs had the amount of the air valve opening set to a lot less than it's capable of. The fix is easy, just look where the red arrow (left) is pointing, that's the stop. It can be carefully bent to set the opening amount. | ||
− | I will use a Dremel to slot the linkage to make bending it easier- I prefer that over grinding the stop in case it needs further adjustment to lessen the opening amount, because the idea is to increase the opening until you either reach the max it will open or to where it doesn't help performance any more- in which case you'd decrease the opening back to where it was best. The linkage can be bent several times | + | I will use a Dremel to slot the linkage to make bending it easier- I prefer that over grinding the stop in case it needs further adjustment to lessen the opening amount, because the idea is to increase the opening until you either reach the max it will open or to where it doesn't help performance any more- in which case you'd decrease the opening back to where it was best. The linkage can be bent several times w/o breaking as long as the cut isn't made so there's not enough metal, and it's not bent too many times. |
<br style="clear:both"/> | <br style="clear:both"/> | ||
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Also, everywhere the casting is ground on or polished removes the protective finish from the zinc of the carb body, and that can lead to the dreaded attack of the "white stuff" that's mentioned anytime old carbs are talked about. | Also, everywhere the casting is ground on or polished removes the protective finish from the zinc of the carb body, and that can lead to the dreaded attack of the "white stuff" that's mentioned anytime old carbs are talked about. | ||
− | == | + | ===Changes to calibrations=== |
− | + | As far as jetting and calibrations, they have to be optimized for the engine anyway- and without knowing whether the carb was spot-on, lean or rich to begin with- and what the smoothing, etc. to the castings did to the flow and air/fuel ratio, there's no way anyone can say you will need to add or remove fuel, nor what circuits may be affected, or even if one circuit may want more fuel while another circuit wants less. So instead of worrying about any changes caused by the work you're contemplating doing, plan on taking care of all of it when the carb is set up for the engine and vehicle. | |
− | + | Lastly, there's a lot more to be had in setting up the APT, idle/transition and main circuits, and the secondary tip-in than there is in dickering with the castings. | |
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==Quadrajet tuning and repair parts== | ==Quadrajet tuning and repair parts== | ||
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*[http://www.carbkitsource.com/ The Carburetor Doctor] | *[http://www.carbkitsource.com/ The Carburetor Doctor] | ||
*[http://www.edelbrock.com/ Edelbrock] | *[http://www.edelbrock.com/ Edelbrock] | ||
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==Recommended Resources== | ==Recommended Resources== | ||
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===A. Wiki articles and Hotrodders Knowledge Base links=== | ===A. Wiki articles and Hotrodders Knowledge Base links=== | ||
---- | ---- | ||
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:Discusses sudden flooding and float issues. | :Discusses sudden flooding and float issues. | ||
− | ;[http://www.hotrodders.com/forum/q-jet-power-piston-springs-160724.html '''''Q- | + | ;[http://www.hotrodders.com/forum/q-jet-power-piston-springs-160724.html '''''Q-Jet Power Piston Springs'''''] |
:Thread offers information on power piston springs in relation to tuning a Rochester Quadrajet. Information provided on spring ratings, manufacturers and part numbers*. | :Thread offers information on power piston springs in relation to tuning a Rochester Quadrajet. Information provided on spring ratings, manufacturers and part numbers*. | ||
− | ;[http://www.hotrodders.com/forum/4mv-quadrajet-4bbl-choke-linkage-missing-need-diagram-showing-how-188110.html '''''4MV Quadrajet: 4bbl choke linkage missing need diagram showing how to mount on carb'''''] | + | ;[http://www.hotrodders.com/forum/4mv-quadrajet-4bbl-choke-linkage-missing-need-diagram-showing-how-188110.html '''''4MV Quadrajet :4bbl choke linkage missing need diagram showing how to mount on carb'''''] |
− | ;[http://www.hotrodders.com/forum/rockester-quadrajet-tuning-tips-200058.html?highlight=quadrajet+4mv ''''' | + | ;[http://www.hotrodders.com/forum/rockester-quadrajet-tuning-tips-200058.html?highlight=quadrajet+4mv '''''Rockester QuadraJet Tuning Tips'''''] |
:Covers adjusting carburetor and timing settings for optimal performance. | :Covers adjusting carburetor and timing settings for optimal performance. | ||
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;[http://home.earthlink.net/~quadrajets/quadrajet_airflow_ratings.htm '''''Quadrajet Air Flow Ratings'''''] by GRZ | ;[http://home.earthlink.net/~quadrajets/quadrajet_airflow_ratings.htm '''''Quadrajet Air Flow Ratings'''''] by GRZ | ||
− | :Discusses the Quadrajet's air flow ratings from | + | :Discusses the Quadrajet's air flow ratings from factory, and tips on modifying or replacing the air valve shaft, the secondary metering rods and hangar, and the cam follower for maximum air flow. |
;[http://www.arkansaspontiacs.org/techstories/RockyQ-jets.htm '''''How Well Do You Know Your Rochester Quadrajet'''''] by Rocky Rotella | ;[http://www.arkansaspontiacs.org/techstories/RockyQ-jets.htm '''''How Well Do You Know Your Rochester Quadrajet'''''] by Rocky Rotella | ||
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:''[http://www.recarbco.com/technical/rochester/qjet.html Recarbo Fuel Systems]'' - ''"This guide is just that: a guide. It is not set in stone, because it is a documented fact that Rochester didn't follow these rules 100%."'' | :''[http://www.recarbco.com/technical/rochester/qjet.html Recarbo Fuel Systems]'' - ''"This guide is just that: a guide. It is not set in stone, because it is a documented fact that Rochester didn't follow these rules 100%."'' | ||
− | :''[http://www.carbkitsource.com/carbs/numbers/Rochester/Quadrajet/index.html Rochester Quadrajet Carburetor Numerical Index]'' - From [http://www.carbkitsource.com/ "The Carburetor Doctor"]. | + | :''[http://www.carbkitsource.com/carbs/numbers/Rochester/Quadrajet/index.html Rochester Quadrajet Carburetor Numerical Index]'' - From [http://www.carbkitsource.com/ "The Carburetor Doctor"]. |
==References== | ==References== | ||
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[[File:Q-JET FILTER HOUSING GASKETS.jpg|thumb|300px|left|The two most used carb filter housing gaskets. Earliest was 7/8" (and are rare), later were all 1", early had gasket over the threads, late style had white gasket on end of housing before threads]] | [[File:Q-JET FILTER HOUSING GASKETS.jpg|thumb|300px|left|The two most used carb filter housing gaskets. Earliest was 7/8" (and are rare), later were all 1", early had gasket over the threads, late style had white gasket on end of housing before threads]] | ||
[[File:REBUILT CARB TELL-TAIL PIN TO BLOCK INCORRECT AIR CLEANER STUD INSTALLATION.jpg|thumb|300px|left|Dead give-away of a rebuilt carb: Arrow points to tell-tail pin installed by rebuilders to keep the air cleaner stud from being inserted into the wrong hole]] | [[File:REBUILT CARB TELL-TAIL PIN TO BLOCK INCORRECT AIR CLEANER STUD INSTALLATION.jpg|thumb|300px|left|Dead give-away of a rebuilt carb: Arrow points to tell-tail pin installed by rebuilders to keep the air cleaner stud from being inserted into the wrong hole]] | ||
+ | [[File:TYPICAL MANUAL TRANS Q-JET THROTTLE ARM.jpg|thumb|300px|left|Manual or TH400 trans throttle arm that's missing the portion below the throttle shaft centerline where the detent/TV cable would be attached]] | ||
[[File:Secondary rods 002.jpg|thumb|300px|left|Selection of secondary metering rods: Lean left, rich middle, medium right]] | [[File:Secondary rods 002.jpg|thumb|300px|left|Selection of secondary metering rods: Lean left, rich middle, medium right]] | ||
[[File:CHOKE BIMETAL COILS 2.jpg|thumb|300px|left|Several types of hot air choke thermostats]] | [[File:CHOKE BIMETAL COILS 2.jpg|thumb|300px|left|Several types of hot air choke thermostats]] | ||
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[[Category:Engine]] | [[Category:Engine]] | ||
[[Category:Carburetors]] | [[Category:Carburetors]] | ||
+ | [[Category:Good articles]] | ||
+ | [[Category:GM]] |