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.
  
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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.
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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==
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*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.
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*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). 
  
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*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.
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*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=====
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A common problem on the early (1968-back) Q-jets especially, is leaky main 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.   
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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]]
  
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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. Ruggles also 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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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''''':
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<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>
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<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=====
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{{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.  
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{{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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===G. Re-installation===
 
===G. Re-installation===
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====Carb gaskets====
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=====Early factory intake with exposed heat crossover=====
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[[File:Early qjetintake1.jpg|thumb|right|350px|Arrows indicate heat passage holes which can be blocked if desired]]
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On early (about 1969-back) Chevy BBC and SBC Q-jet intakes, the manifold was equipped with an exposed heat crossover below the primary side of the carb. This intake manifold requires a specific gasket and heat shield (below left) be used to prevent vacuum leaks and to resist the heat.
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[[File:Carb gasketsearly q-jet.gif|thumb|350px|left|Metal heat shield goes against carb; fiber insulator goes against intake]]
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The heat crossover can be blocked either at the intake gasket, or by blocking off the ports that feed heat to the "U"-shaped channel in the intake with 1/4" pipe plugs. This mod will prevent the base plate from possibly warping and should help provide a denser air/fuel mixture due to the cooler temperature. Be aware that this can mean longer warm up times, and may require some changes to the calibrations. <br style="clear:both"/>
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{{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}}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.
  
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{{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.
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=====Later factory and aftermarket intakes=====
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In most cases a thick fiber insulator-type gasket will provide good sealing and help to keep the carb temperature down. Heed the caution above regarding not overtightening the carb.
  
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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.
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====Baseplate vacuum fittings====
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Not all carbs have a tapped vacuum port in the rear of the baseplate. When it is present, this port can be used for the power brake booster. Don't use it for the PCV system; that port is located in the front of the baseplate (primary side). The port is threaded with pipe threads, either 1/8" or 1/4".
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{{Caution}}Do not overtighten any vacuum fitting or plug into the baseplate! The aluminum casting will crack if the male pipe threads of the fitting are overtightened.
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Seal the plug or 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.
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{{Note1}}Do not use teflon tape on any fuel line fittings. The filaments of teflon tape that is left over after a fitting is removed is hard to completely remove from the threads, and it will clog the needle and seat and/or jets if it finds its way into the carb.
  
 
==Tuning==
 
==Tuning==
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There are different locations for the orifices, but they're all adjacent to the primary throttle bores of the baseplate. The sizes vary from no opening at all to around 1/8". Measure them using a drill bit.
 
There are different locations for the orifices, but they're all adjacent to the primary throttle bores of the baseplate. The sizes vary from no opening at all to around 1/8". Measure them using a drill bit.
  
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Carbs from bulk rebuilders will often have the bypass channels or orifices blocked off with lead shot tapped into the casting. This can be fairly easily removed if it's encountered. They block them in various places, sometimes the baseplate, other times in the carb body. If this ever comes up, follow the channels from the baseplate into the body and you'll find these bypass holes. Often these rebuilt carbs will use cheap unmarked, unplated brass metering rods and might even have unmarked jets.
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Carbs from bulk rebuilders will often have the bypass channels or orifices blocked off with lead shot tapped into the casting channels/orifices, to make the carb "generic" as opposed to being tuned for a specific, exact application. Generally speaking, there's no harm in doing this as long as it has been done correctly. Unfortunately, trying the carb on your running engine is about the only way you can know for sure, unless the desired orifice sizes are known ahead of time. Fortunately if need be, the lead can be fairly easily removed, or drilled to give bypass air.  
  
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The older 'tunnel' type (circles):
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The rebuilders block them in various places: sometimes the baseplate, other times in the carb body. If this ever comes up, follow the channels from the baseplate into the body and you'll find these bypass holes. Often these rebuilt carbs will use cheap unmarked, unplated brass metering rods and might even have unmarked jets.
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The older 'tunnel' type bypass air channels (circles):
  
 
[[File:Bypass 1.jpg]]
 
[[File:Bypass 1.jpg]]
  
  
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The later 'tab' type, most look like the one here: one orifice is in the left tab, the other orifice is just inside the bore (arrow on right). Some have two orifices like on the left (right tab would be where the circle is):
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The later 'tab' type bypass air orifices, most look like the one shown below: one orifice is in the left tab, the other orifice is just inside the bore (arrow on right). Some have two orifices like on the left (right tab would be where the circle is).
  
 
[[File:Bypass2.jpg]]
 
[[File:Bypass2.jpg]]
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|}
 
|}
  
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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 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.
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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 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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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.
  
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==Changes to calibrations==
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==Carb gaskets==
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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.
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[[File:Early qjetintake1.jpg|thumb|right|350px|Arrows indicate heat passage holes which can be blocked if desired]]On early (about 1969-back) Chevy BBC and SBC Q-jet intakes, the manifold was equipped with an exposed heat crossover below the primary side of the carb. This intake manifold requires a specific gasket and heat shield (below left) be used to prevent vacuum leaks and to resist the heat.
  
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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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[[File:Carb gasketsearly q-jet.gif|thumb|350px|left|Metal heat shield goes against carb; fiber insulator goes against intake]]
  
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===Drilling jets===
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The heat crossover can be blocked either at the intake gasket, or by blocking off the ports that feed heat to the "U"-shaped channel in the intake with 1/4" pipe plugs. This mod will prevent the base plate from possibly warping and should help provide a denser air/fuel mixture due to the cooler temperature. Be aware that this can mean longer warm up times, and may require some changes to the calibrations. <br style="clear:both"/>
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The flow through a jet is determined by the surface finish (small consideration as long as it’s very smooth), the length of the orifice (moderately important, depending on how much the difference is), the entrance and exit angles and their finish, and the size of the orifice (major consideration).
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When a jet is drilled oversize, all of these things are changed except the angles of exit/entrance. The number stamped on a jet (Rochester, Holley, etc.) is an indication of the actual flow, NOT the orifice diameter. The manufacturers flow the jets to see what number they get stamped on them. The same orifice diameter jet may be stamped w/different numbers. This tells you they do not flow the same, even though the orifice diameter is the same. Subtle differences in the above parameters account for the different flows.
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−
 
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The difference between the correct and too rich/lean jet sizes may only be 0.002”-0.004”. There are no readily available drill bits that are that close in size to one another. So at best, the changes made to jets by hand drilling are going to be in steps decided by the availability of drill bits. Because drill bits are often available in 1/64” increments (the small numbered bits- which have a finer increment between bits- are too small for drilling the average carb metering jet), this will be on the order of a 0.0156” change between drill bits- which is HUGE (~40%) change in metering area when in the 0.070” jet orifice range. 
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It is for these reasons there is no practical way for a person to casually drill a jet and have any way of knowing what the flow is going to be. Drilling out jets is a holdover from the dark days when guys were struggling to make “high performance” engines live, and if they got within 10% of ‘right’ they were at the top of the heap. Nowadays, thankfully, we all (should) know better and leave the drilling of jets and other dubious practices to the desperate, the uneducated, and the poverty-stricken.
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==Quadrajet tuning and repair parts==
 
==Quadrajet tuning and repair parts==
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==Recommended Resources==
 
==Recommended Resources==
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*[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%."''   
  
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:''[http://www.carbkitsource.com/carbs/numbers/Rochester/Quadrajet/index.html Rochester Quadrajet Carburetor Numerical Index]'' - From [http://www.carbkitsource.com/ "The Carburetor Doctor"].  
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:''[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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{{youcanedit}}
 
  
 
[[Category:Engine]]
 
[[Category:Engine]]
 
[[Category:Carburetors]]
 
[[Category:Carburetors]]
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[[Category:Good articles]]
 
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[[Category:GM]]
 

Latest revision as of 19:40, 10 September 2023

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