How to rebuild a Rochester Quadrajet 4MV carburetor
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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 cite sections and passages from related books to help the reader follow along with ease. | 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 cite sections and passages from related books to help the reader follow along with ease. | ||
| − | Although the article specifically deals with the Rochester Quadrajet (aka "Q-jet, qjet, etc.) 4MV model, the information found here is useful for working on other Rochester Q-jet models as well. | + | Although the article specifically deals with the Rochester Quadrajet (aka "Q-jet, qjet, etc.) 4MV model, the information found here is useful for working on other Rochester Q-jet models (including the Dualjet (which is the front half of a Quadrajet) 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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==Other considerations== | ==Other considerations== | ||
| − | *When getting a carb from another vehicle, whenever possible, get the factory cable bracket. Even if you're using an aftermarket intake, it might come in handy for making up your own bracket. And regardless if using an aftermarket intake or not, the bracket can be used to measure the correct cable/carb relationship for setting up whatever bracket that's used. | + | *When getting a carb from another vehicle, whenever possible, get the factory cable bracket. Even if you're using an aftermarket intake, it might come in handy for making up your own bracket. And regardless if using an aftermarket intake or not, the bracket can be used to measure the correct cable/carb relationship for setting up whatever bracket that's used. The throttle linkage are specific to the manufacturer (GM products including 1971 Ford Mustangs with the 429 Cobra Jet and mid-1980s Mopars have used the Quadrajet). |
| − | *Make note of the engine displacement/engine VIN code. These carbs were used on 4.3L V6 through 7.4L V8 engines, so knowing what it came from gives a leg up on what may potentially need changed. | + | *Make note of the engine displacement/engine VIN code. These carbs were used on 4.3L V6 through 7.4L V8 engines (besides its use with GM products the Quadrajet was found in some obscure applications e.g. 1971 Ford Mustangs with the 429 Cobra Jet and 1983-88 Chrysler vehicles), so knowing what it came from gives a leg up on what may potentially need changed. |
*If the Q-jet is to be used with an automatic transmission that uses a detent (kick down) or TV (throttle valve) cable, be sure the throttle arm is correct (below, right). Many manual transmission-equipped vehicles are missing the lower part of the throttle arm (below, left); this is where the detent/TV cable ordinarily attaches. | *If the Q-jet is to be used with an automatic transmission that uses a detent (kick down) or TV (throttle valve) cable, be sure the throttle arm is correct (below, right). Many manual transmission-equipped vehicles are missing the lower part of the throttle arm (below, left); this is where the detent/TV cable ordinarily attaches. | ||
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=====Leaking float bowl well plugs===== | =====Leaking float bowl well plugs===== | ||
| − | A common problem on the early (1968-back) Q-jets especially, is leaky | + | A common problem (mostly) on the early (1968-back) Q-jets especially, is leaky wells. The early leak-prone plugs can be identified by being brass cup-type plugs as opposed to aluminum. The late-type plugs are shown below, left- front plugs arrows, rear plugs circled. The 'cure' for this has long been to use JB Weld, etc. to seal them over. This is just a stop-gap measure, the epoxy will degrade and quit sealing. |
The well plug stop-leak pads that come in many rebuild kits are a joke (below, center and right). They will seal the leaking rear plugs for a few heat cycles before shrinking down to a fraction of their former thickness. The result is the carb will initially run as it should, ''quickly'' followed by the same symptoms of a leaky plug returning. Do not waste your time with them. If you have a leaking plug, repair it right. | The well plug stop-leak pads that come in many rebuild kits are a joke (below, center and right). They will seal the leaking rear plugs for a few heat cycles before shrinking down to a fraction of their former thickness. The result is the carb will initially run as it should, ''quickly'' followed by the same symptoms of a leaky plug returning. Do not waste your time with them. If you have a leaking plug, repair it right. | ||
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[[File:Q jet well plugs.jpg|400px]][[File:WELL PLUG 1.jpg|295px]][[File:WELL PLUG 2.jpg]] | [[File:Q jet well plugs.jpg|400px]][[File:WELL PLUG 1.jpg|295px]][[File:WELL PLUG 2.jpg]] | ||
| − | FWIW, there is an epoxy by Devcon (made in the USA, look for "Devcon Home"). It comes in the older 2 tube packages and the newer double plunger syringe type. On the back it clearly states it <u>*RESISTS UNLEADED FUEL*</u>, salt, gasoline, oil etc. Full set time 8-12 hrs/2500 psi. However, be aware there is no empirical evidence that this epoxy will work for the life of the carb without any delamination or degradation | + | The tell-tail signs of leaking well plugs is if the carb float bowl empties overnight. This will usually leave a small hit of fuel in the bottom of the accelerator pump well, but unless the engine starts and runs well when cold (or has an electric fuel pump), the engine will often stall after starting, then will take a lot of cranking for the mechanical pump to refill the bowl and the engine to restart. |
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| + | On the post ~1968 Q-jets, the [B]secondary[/B] plugs are spun in then peened, so to speak. The [I]main[/I] plugs are pressed and sealed w/a soft metal around them- and they can and do leak on some carbs, regardless of the year. | ||
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| + | Epoxy is not the way to fix a leaking main well plug- tapping them for a machine screw works much longer- as in forever. Epoxy will eventually delaminate from the metal and the leak will resume.Ruggles mentions using Marine Tex epoxy to seal the threads of replacement well plugs, but he states that using epoxy alone to repair a leaking well plug is akin to "cutting your arm off and putting a Band-Aid on it". | ||
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| + | FWIW, there is an epoxy by Devcon (made in the USA, look for "Devcon Home"). It comes in the older 2 tube packages and the newer double plunger syringe type. On the back it clearly states it <u>*RESISTS UNLEADED FUEL*</u>, salt, gasoline, oil etc. Full set time 8-12 hrs/2500 psi. However, be aware there is no empirical evidence that this epoxy will work for the life of the carb without any delamination or degradation. | ||
So, 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 plugs that will seal them for good. The primary wells seldom leak, the secondary wells on the early (1968-back) carbs do leak much more often than the later carbs. | So, 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 plugs that will seal them for good. The primary wells seldom leak, the secondary wells on the early (1968-back) carbs do leak much more often than the later carbs. | ||
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=====Soaking the parts===== | =====Soaking the parts===== | ||
{{Caution}}According to [http://www.hotrodders.com Hotrodders] forum member '''''trees''''': | {{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 | + | <blockquote>''"Soaking all metallic parts is the way to go but using small brushes lightly is a good idea. Do not soak non-metallic items." ^''</blockquote> |
=====Cleaning with brushes and wires===== | =====Cleaning with brushes and wires===== | ||
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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. | + | {{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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| − | Edelbrock has four different PP springs: gold (4"/Hg), orange (5"/Hg), black (6"/Hg), and yellow (8"/Hg) in their p/n | + | Edelbrock has four different PP springs: gold (4"/Hg), orange (5"/Hg), black (6"/Hg), and yellow (8"/Hg) in their p/n 1994 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. | ||
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==Recommended Resources== | ==Recommended Resources== | ||
| + | *[http://www.florida4x4.com/tech/quadrajet/index.php Many good links] to various tuning and rebuilding info across the internet | ||
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===A. Wiki articles and Hotrodders Knowledge Base links=== | ===A. Wiki articles and Hotrodders Knowledge Base links=== | ||
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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:CHOKE HOT AIR SEVERAL TYPES.jpg|thumb|300px|left|Several types of hot air choke housings]] | [[File:CHOKE HOT AIR SEVERAL TYPES.jpg|thumb|300px|left|Several types of hot air choke housings]] | ||
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[[Category:Engine]] | [[Category:Engine]] | ||
[[Category:Carburetors]] | [[Category:Carburetors]] | ||
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