How to rebuild a Rochester Quadrajet 4MV carburetor
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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=== | ||
| − | + | {{Note1}}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==== | ||
| − | + | {{Caution}}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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---- | ---- | ||
=====Soaking the parts===== | =====Soaking the parts===== | ||
| − | + | {{Caution}}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 ideal. Do not soak non-metallic items." ^''</blockquote> | <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===== | ||
| − | + | {{Caution}}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" oxidation deposit build up===== |
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''''']. | 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''''']. | ||
=====Drying the parts===== | =====Drying the parts===== | ||
| − | + | {{Caution}}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===== | ||
| − | + | {{Note1}}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 | ||
| − | + | {{Note1}}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=== | ||
---- | ---- | ||
| − | + | {{Caution}}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. In severe enough cases the baseplate can even crack. | |
| + | |||
| + | {{Caution}}Do not overtighten the vacuum fitting used for the power brake booster (or any other application requiring a high flow, manifold vacuum source). The casting can crack if the male pipe threads of the fitting are overtightened. | ||
| − | + | Seal the fitting with teflon tape or a good thread sealant. Either of these will also act as a lubricant, making overtightening a lot easier than if the threads were dry, so be careful. | |
| − | + | ||
==Tuning== | ==Tuning== | ||
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====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. Usually about 0.090" is sufficient. |
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). | ||
===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 are seldom mentioned when Q- | + | 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. |
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. | 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. | ||
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| − | Edelbrock has some Q- | + | 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 | + | *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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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) with a finger. It should shut without hesitation when you let off pressure. After you've been working with 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 without breaking as long as the cut isn't made so there's not enough metal, and it's not bent too many times. |
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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 '''''Rochester 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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[[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 | + | [[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 center line 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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