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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For decoding the carb number, please see the following article for further information and assistance: '''[[Rochester Quadrajet carburetors: Identification and decoding]]'''
 
For decoding the carb number, please see the following article for further information and assistance: '''[[Rochester Quadrajet carburetors: Identification and decoding]]'''
 
[[File:Carburetor_Identification_Number.jpg|thumb|center|750x250px|Location of 1968-up Q-jet carb number. Before this the carb number was stamped into a round disc located on the driver side of the carb, towards the front]]
 
[[File:Carburetor_Identification_Number.jpg|thumb|center|750x250px|Location of 1968-up Q-jet carb number. Before this the carb number was stamped into a round disc located on the driver side of the carb, towards the front]]
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==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. 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 (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.
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*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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{|
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|[[File:TYPICAL MANUAL TRANS Q-JET THROTTLE ARM1.jpg|250px|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]]
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|[[File:LINKAGE TH350 CARB WITH AT.jpg|215px|Automatic trans throttle arm]]
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|}<br>
  
 
===B. Parts===
 
===B. Parts===
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*Jets
 
*Jets
 
*Fuel Filter Housing
 
*Fuel Filter Housing
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=====Needle and seat=====
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Be sure the metal seal under the old needle seat has been removed before a new seal is installed.
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Be sure to hook the needle clip as shown below:
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[[File:Float clip1.jpg]]
  
 
====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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=====Accelerator pump assembly=====
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If the accelerator pump wasn't working correctly, be sure to attach the linkage to the pump arm hole nearest the pivot to get the biggest pump shot. You can try the smaller shot and if that works well, use it.
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There are several different accelerator pump rod lengths used depending on the carb, if it's the wrong length this can cause the pump to work poorly.
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If the springs are reversed and the return spring that should be under the pump assembly is used as the override spring on the shaft, and the override spring is used under the assembly, that can cause it to work poorly. Sometimes the fuel filter spring gets mixed up with the accelerator pump springs as well.
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If the piston seal is wrong it can have a hard time pushing fuel and/or allowing the pump to refill.
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There's a ball that acts as a check valve under a screw in the float bowl, driver side. Be sure the ball is in place and sealing as it should. The check ball seat can be re-formed by carefully tapping an old check ball against the seat using a small punch and light mallet/hammer.
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Sometimes the accelerator pump bore gets glazed where it doesn't seal against the piston very well. The bore can be lightly honed using 600 grit wet or dry. I use a wooden dowel w/a piece of cloth wrapped around the dowel and the emery paper wrapped around the cloth. Then make like a boy scout starting a fire or like you're lapping a valve. You don't want to remove the finish, just break the glaze ever so lightly and to smooth the bore.
  
 
=====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=====
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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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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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The secondary throttle shaft rarely ever need bushings; even brand new there's quite a bit of clearance between the shaft and throttle plate. If the secondary shaft/throttle plate bore so bad that bushings are needed, the rest of the carb is likely worn very badly; another core or at a least another throttle plate should be considered.
  
 
=====No drilling/Teflon bushing fix=====
 
=====No drilling/Teflon bushing fix=====
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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. The original screw thread size is 3-48, however the holes can be opened up enough to use a larger screw like a 6-32 thread if need be.  
 
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. The original screw thread size is 3-48, however the holes can be opened up enough to use a larger screw like a 6-32 thread if need be.  
  
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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.
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=====Throttle blade screw replacement=====
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As mentioned above, the screws can be enlarged to 6-32 if need be. The factory stakes or peens the screws to retain them from falling into the engine, you should do the same (the end of the screw has to protrude past the shaft to do this).
  
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To do the staking, 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; the blue formula works good. Be careful to not hit the ends of the screws unless the head of the screw is supported against something solid. You don't want to bend the shaft.
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[[File:Vise grip.jpg|right]]
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Alternatively, you can use a pair of clamping vise grips to get the jaws on both ends of the screw. This can disrupt the ends of the screws enough to keep them from falling into the engine.
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If the ends of the screws look too long, they can be ground down with a dremel or small file to the approximate length of the stock screws before peening them.
 
If the ends of the screws look too long, they can be ground down with a dremel or small file to the approximate length of the stock screws before peening them.
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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.
 
  
 
=====Cleaning orifices=====
 
=====Cleaning orifices=====
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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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;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#post1374844 Rochester Quadrajet 4MV Carburetor: Removal, Disassembly, Rebuild (Rookie Level)]''.
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One area that needs special attention are the secondary well fill orifices. Most first-time rebuilders don't even realize they exist. The location is shown below.
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[[File:Sec wells1.jpg]]
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Do not enlarge the orifices unless you know what you're getting yourself into. If these orifices are made too large the secondaries can run too rich.
  
 
=====Throttle blades=====
 
=====Throttle blades=====
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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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[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
  
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===Idle problems===
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===Idle circuit===
 
*[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
  
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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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====Idle bypass air====
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Similar to drilling the primary throttle blades, the idle air bypass orifices introduce air below the primary throttle blades. These orifices can be enlarged/reduced to help some engines idle better.
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The idle air bypass orifices are almost always in the base plate. There are holes above the baseplate connected to the orifices, but they are usually much larger than the orifices so the orifices in the baseplate are all that need to be enlarged or made smaller.
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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.
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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 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):
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[[File:Bypass 1.jpg]]
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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).
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[[File:Bypass2.jpg]]
  
 
===Power piston===
 
===Power piston===
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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.
  
−
==Changes to calibrations==
+
==Carb gaskets==
−
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.
+
[[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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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: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]]
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[[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]]
 
[[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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