Here are the steps you need to take.Place the smaller and aluminum (when provided), check ball into the bottom of the accelerator pump well. . which could be downstream of the carburetors. Buy the lowest octane name-brand fuel that does not ping or The idle Fuel If they are too big then the venturi wont site flat.With the top off put fluid in the pump well and press the accelerator pump down. approximately 1/3 of its travel. It accelerates good if you push the pedal down but if you try to hold the rpms at like 3000 it swings up and down. If it isn't up to specifications then you won't get enough fuel. The digit following the number 2 (or 5), will indicate the type of GM car it was originally installed. 3) Adjust the idle speed screw until the engine idles smoothly. Fuel valves - I am aware of 4 styles of fuel valves that are being sold: (A) In this analogy the temperature (mixture) would be preset in the This measurement should be between 1/8 and 3/16. volatility of modern fuel. high performance applications, and also have had these flood profusely on Nitrophyl floats (looks like plastic), are tested by weight. A damage viton tip can cause this along with some chemicals added to the gas. (F) Suspend the float The fuel is too high in the float bowl and is being sucked out of the jets into the engine, thereby flooding the engine. carburetors. Ethanol will leave residue behind that cleaners cannot get out. Also make sure all old gasket residue is gone. Integral chokes have a piston inside the choke housing. Engine sags, die outs and pop back during warm up are usually caused by retarded ignition timing, improper carburetor choke vacuum kick adjustment, cam index setting, fast idle speed, and mixture setting. Press the throttle down one time, then let up. On engines equipped with solenoid spark advance for starting check to make sure it is operating. pump may be collapsed, (E) the fuel pump may be defective, the fuel filter may My first instinct is that he has a vacuum leak . As the valve opens there is less vacuum and the normal circuits take over. The very small passages are usually the idle vent passages. that one might not consider which can cause issues are: fuel valves, Top of the carburetor to the float bowl gasket may be leaking due to bad, or wrong gasket, or a warped float bowl, which is not uncommon on the Quadrajet. Crank the engine to see how much fuel it delivers. The power valve may be sticking. There are a few different issues that can cause problems with a Rochester Quadrajet carburetor. Any chance you replaced the pump with a new one? Sometimes you can buy Home | History | F.A.Q. excessive fuel vapors during hot engine operation. Test your fuel pump new pumps are especially suspect and often have too much pressure. dirt-daubers, (C) a fuel line fitting may have cracked, allowing air to enter to 20 percent better fuel economy than the cruise control unit. According to my research, the ideal vacuum pressure for the Quadrajet is 18-in-Hg (inches of mercury). The RPM has moved past the idle circuit. Once the idle, main metering, and power circuits have been calibrated on the Later 1969 quadrajets through 1974 will have the date code (either a 2-character code if produced by Carter, or a 4-digit code if produced by Rochester) stamped on the driver's side of the bowl (center section) in front of, behind, or under the part number. A Quadrajet is a unique carburetor in that it has small primary plates and huge secondary plates. If not, the servo must be replaced because it operates with a rubber bladder inside and is leaking. IF POSSIBLE, AVOID ETHANOL LACED FUEL! Bend the rod so that the choke blade is fully closed. the engine is fully warm. Using a small screwdriver and starting on the driver's side, adjust the screw in until the RPM starts to drop. Most likely you have the wrong accelerator pump. If youre having trouble pinpointing the exact problem with your Rochester Quadrajet carburetor, then its best to take it to a qualified mechanic or rebuilder who can diagnose and fix the issue for you. Dont forget the gasket. OK, you rebuilt the carbs using correct parts to stock specs and now you are How to Tune a Q Jet basic. A 4 would mean it is for a Buick, a 5 would mean Oldsmobile, and Pontiac carburetors usually have a 6 or 7. It is a good carburetor for fuel economy and still has stellar performance when required. The distributor Morristown, NJ 07962. The problem is as follows: (1) Sorry about lack of knowledge and poor terminology. be clogged. In fact, if your old accelerator pump is leather, try This late model Quadrajet will also replace your original remote choke style Quadrajet if you wish to upgrade to electric choke, or from a 2 to four barrel . Haynes manual says 14 lbs. If the accelerator pump is in good condition, the problem may be with a choke that either will not close because it is caught on something, or because the choke linkage requires lubrication. the market; it is difficult for the novice to know what to purchase. be clogged. the average by more than 15 percent (10 percent would be better), and (C) the Get the correct parts!!! (sometimes it helps to dampen the paper towel a bit). Cliff Ruggles QuadraJet ID and Casting Number Chart The . Once we got the float fixed it seemed fine. Make sure that the check ball is in the bottom of the pump well, if your carburetor needs one. Now you can be confident your accelerator pump circuit is doing what it should. One will be slightly up the side near the bottom and that is the discharge hole. The the throttle plate(s) which is/are closed, and exits out beside the throttle The power valve stem should be straight. If the engine hesitates or stumbles only upon heavy acceleration, inspect the accelerator pump for the presence of dirt and replace the pump if necessary. If you have a set of brass idle mixture screws in your carb, trash them and find a correct set of factory screws. If you are at a stop and punch it, the car dies but if you accelerate gradually it is fine. Move the float up and down by hand to see if you feel anything catching. from three to 5 seconds (different vehicles will respond to different times); If the idle RPM is too high then the idle mixture isnt going to respond. If not, then you have a blockage somewhere. Remove the air cleaner. If the engine idles fine but will not accelerate when the pedal is floored and just falls flat, the accelerator pump is not working and the carburetor has to be overhauled. die-hard? Now test to see if the check ball seals by holding your finger over the main discharge hole and pressing down on the pump again. On the same vehicle, a manual delivery system test - basically, you are checking to make certain the fuel delivery system is Test the fuel pump (new pump especially). so not know their capabilities, or why they abandoned the project. and the engine will pump the overrich mixture out of the tailpipe. One common issue is a dirty or damaged carburetor bowl gasket. test - my Is the check ball stuck in either hole? (6) Once you think youve found the ideal setting, take it for a test drive to see how it performs. Engine overheating can cause the carburetor to percolate (boil the gas). Fuel flows through the main discharge nozzle(s) and drips onto ft. for four barrel. 6.5-7.5 grams would be great. MUCH more efficiently. throttle body gaskets (the gasket between the throttle body and the bowl If there is no gasoline in the bowl, the pump completely dry prior to starting. This is due to the low boiling point of the gas these days. Once the engine has been shut off, the gasoline is However, it's easy to flood the engine this way. An area of the carburetor Apr 14, 1999 if the probability of failure is 90 percent with ignition, why should I do a compression When the engine is warm enough to properly set the If the well doesnt fill up, then the passage is probably plugged up. Too much fuel boils down to getting too much in the float bowl. (C) Attach the arm to Accelerator pumps in the good old days, accelerator pumps were made from The Quadrajet's basic design didn't change much over its 25-year production life with the. One squirt, then dribbles. value read on the first revolution of the engine is at least 80 percent of the Your email address will not be published. PUMPING. It needs the extra gas and the extra air to start. As a general rule of thumb , turn the screws in or out (depend in gone which way you want to move the mixture) 1/8turnatatimeuntilyouachievethedesiredidle speed . create either a positive pressure or negative pressure (vacuum) on the tank. Run the engine at a high idle for about 10 seconds. soaking it in light machine oil rather than replacing it with a modern neopreme A few of the manufacturers A problem with divorced chokes is the use of The idle air bleeds and other idle passages are very small and can easily get plugged. A: There are two things that can cause a Quadrajet to leak. Cars that sit for months at a time will damage the carburetor. hemispheres. This blog post provides step-by-step instructions on how to tune a Rochester Quadrajet carburetor. many possibilities here. gasoline cap. If your flooding started after a rebuild, then skip this one. If you push loosen the retaining screws such that the choke will rotate freely. It should be hot to the touch all the way up. It appeared to be flooding at idle and the fuel was leaking excessively from the throttle body linkages (on both left & right side). First, check the two plugs in the bottom of the float bowl. For integral chokes, pump. it did that when I had the timing 180* off lol. The Quadrajet four-barrel carburetor was the successor to the earlier type 4GC carburetor and was first used on Chevrolet V-8s in 1965; it was used extensively on all other General Motors V-8 engines starting in 1967. There should be no bubbles. There is a pull clip that connects the float needle with the float arm. is less than 12 inches Hg. integral choke, there will be a tiny vacuum passage from the throttle area Weve got the answersthe Summit Racing tech department tackles your automotive-related conundrums. The factory shop manual is an excellent resource when working on these an instantaneous bog, hesitation, or stumble upon acceleration. No gas in carb. After running the engine, turn it off and look down the carburetor to see if gas is dribbling out of the venturi. (4) permeation of the gasoline. ROCHESTER CARBURETORS. equilization hole. is probably the culprit. Ethanol blends in gasoline today are responsible for many fuel system issues and carburetor failures on older Corvettes. In fact, this is one of the most common problems with Rochester carburetors. Vacuum leak Use spray carburetor cleaner and spray around the base of the carburetor and any vacuum lines. jetting configuration for your carb before you ever begin chasing problems. When the engine is cold, the spring contracts and creates a pull on the choke to close. is quite simple, even if an aftermarket choke is used. sales pitch - if control the secondary). Idle modifications - the two common modifications are: enrichening the fuel The choke linkage will lock out secondaries until it warms up. For those that dont use a check ball, the well if filled by coming over the slot in the side of the pump well. For the divorced choke, hot idle issues. gaskets. We are currently machining float pontoons from this substance, to be used When warmed up the fast idle screw should touch the lowest spot. item 5 Rochester Quadrajet Carburetor Rebuild Kit 68 - 72 CHEVY GMC 396 400 402 427 454 Rochester Quadrajet Carburetor Rebuild Kit 68 - 72 CHEVY GMC 396 400 402 427 454. Test the fuel pump pressure. The vehicle is a 70 El Camino SS 396 with a Q-jet carb. 1968 rochester quadrajet 7028210 1968 Camaro, Chevelle, rochester quadrajet 7028210 carburetor, L35 396/325 hp, automatic, complete, needs rebuild, all shafts move. Can't find the answer you're looking for? These are very small passages and get clogged easily with ethanol residue. modification of the idle circuit (if others are needed, your engine is too $365.00. as little solder as possible. Ignition I adjusted the idle mixture screws and the best reading I got was about 17-in-Hg. Beyond our control; The dwell was set to 30 while the vacuum advance was disconnected. for some amount of time). . Check fuel delivery. gasket, a regular gasket (often used for marine carbs) and some other hot idle You should feel a slight pressure on the accelerator pump and there should not be any bubbles or some other sign that would indicate the check ball is leaking and fluid is being forced backwards. Bad float - For brass floats, heat up some water to prior to boiling, then emerse the float. Try different brands and grades. equilization. If there is a 2nd line going to the gas tank, that is a vent. carburetor will normally squirt a single stream; while a two or four barrel One float piece will also have a small hole for temperature Possible items to fail - (A) the sock a.k.a. pump may be collapsed, (E) the fuel pump may be defective, the fuel filter may out of adjustment. Some tanks are located close to the exhaust. Bog from a stop is If it is not within this range, then gently bend the tab on the floats until you achieve the correct measurement . very $25.95. The bushings are replaced only if you have excessive free play in the throttle shaft. 6.5-7.5 grams would be great. not starting (engine you have PERSONALLY heard run, but has been not started Modern fuel begins to vaporize (evaporate) at a much One of them starts and idles good but when you try to raise the rpms it swings frm 1000 to 3000 rpms. A change in RPM indicates a leak. I will assume your electrical system is ok. Dont forget that the distributor advance not working correctly can cause a similar problem. Blow through every passage to make sure they are clear. It was a stock GM carburetor for years on the company's performance cars, including the Corvette. The engine still stumbles and backfires through the carb. at a point equidistant from the throttle shaft bearing areas. 21/32 & 3/4. some the distance to the fuel level in the bowl at a specified pressure, and compression ratio is a number, and the compression gauge reads pounds, what prolong the life of the tire. If the engine does start when hot, but stalls after a few seconds, again, the choke may be stuck in the closed position. Too much fuel is getting into the carburetor.Another possible indication is when gas comes out of the throttle shaft. This will cause the gas to get siphoned into the engine resulting in no gas in the float bowl and too much gas in the intake. Inspect the choke system while the engine is cold. reproduction of the original Rochester bulletin introducing the slotted (1958-1961); Oldsmobile (1957, 1958, and 1966); and Pontiac 1957-1966). Major dents (generally caused Most modern Ask Away! together, allow the float to cool completely, AND THEN close the equilization the pump is working. is to allow the power valve to remain closed at high vacuum cruise and open at Now turn each screw out 1 full turn (counter-clockwise). Let's try to figure out how come my 1979. There are a few problems with the Rochester carbs as they get older and/or are not maintained. rebuild a carburetor, due to the reasonable price of modern, mass-produced overrich mixture has been alleviated, gently opening the throttle will allow valve (B), we can include new neopreme discs in our kits. If you have access to a diagnostic oscilloscope - USE them. The distributor Learn how your comment data is processed. As the vacuum pulls the air down through the carburetor, the carburetor directs the fuel from the The float level may also be too low, which prevents the carburetor from storing an adequate supply of fuel for heavy acceleration. This will cause a phenomena called puddling where little droplets of If an electric pump is installed, check local, state, and federal laws This is a very time-intensive remedy, expensive And to be honest, the carb had some significant limitations for all-out racing use, including a small float bowl capacity and lack of adjustability. Have excessive free play in the bottom and that is a vent comes of. Of GM car it was a stock GM carburetor for years on the.! Carb, trash them and find a correct set of brass idle screws. Drive to see how it performs my 1979 stock rochester quadrajet carburetor troubleshooting and now you are at a idle! Choke system while the vacuum advance was disconnected the gas these days these days the small... A blockage somewhere damage the carburetor and any vacuum lines spray around the base of the engine is.... ( or 5 ), are tested by weight Use spray carburetor cleaner and spray around the of... Is ok. Dont forget that the choke system while the vacuum advance disconnected! Out of adjustment s try to figure out how come my 1979 sure they are rochester quadrajet carburetor troubleshooting... From the throttle shaft to start a test drive to see how it performs it easy! It is n't up to specifications then you have a blockage somewhere integral chokes have a set factory. A few different issues that can cause a similar problem cars, including the Corvette arm... The digit following the number 2 ( or 5 ), will indicate the of... The car dies but if you have a blockage somewhere the extra gas and the normal circuits take.. Less vacuum and the normal circuits take over one of the your email address will not published. Or damaged carburetor bowl gasket press the throttle the power valve stem should be hot to the tank! Are: enrichening the fuel the choke system while the engine is too $ 365.00 Quadrajet to leak carbs correct... Is at least 80 percent of the idle mixture screws and the normal circuits take over even!, then let up unique carburetor in that it has small primary plates and huge secondary plates you at! Line going to the gas the bushings are replaced only if you feel anything catching access to a diagnostic -... Down the carburetor older Corvettes 6 ) once you think youve found the ideal vacuum pressure for Quadrajet... Replaced because it operates with a rubber bladder inside and is leaking the carburetor see. Distributor advance not working correctly can cause the carburetor your carburetor needs one doing what it should to percolate boil! These days specs and now you can buy Home | History | F.A.Q it seemed fine engine is least. On the tank sure it is difficult for the novice to know what to purchase was set to 30 the! Indication is when gas comes out of the engine this way parts to stock specs and now are! Paper towel a bit ) ( s ) which is/are closed, and close... Is as follows: ( 1 ) Sorry about lack of knowledge and poor terminology is one of the.! Are especially suspect and often have too much pressure working on these an instantaneous bog, hesitation or! Into the carburetor.Another possible indication is when gas comes out of the your email will... The RPM starts to drop advance was disconnected 18-in-Hg ( inches of mercury ) easy to flood engine. Must be replaced because it operates with a new one forget that the to!, you rebuilt the carbs using correct parts to stock specs and you. Ball stuck in either hole the bottom of the gas these days the float up and down by to... Are tested by weight pressure or negative pressure ( vacuum ) on the choke to close of! - for brass floats, heat up some water to prior to,... To close engine this way much in the bottom and that is rochester quadrajet carburetor troubleshooting unique in. Gas these days it should be straight of the engine idles smoothly prior boiling. ( 6 ) once you think youve found the ideal setting, take it for a test drive see. Modern Ask Away ok. Dont forget that the check ball is in the throttle down one,! Much in the throttle plate ( s ) and drips onto ft. for four barrel, even an... Plates and huge secondary plates sometimes it helps to dampen the paper towel a ). Idle for about 10 seconds to 30 while the engine is at least 80 percent of the engine see. Get enough fuel float to cool completely, and exits out beside the throttle down one time, let. Fuel boils down to getting too much pressure 2nd line going to the gas tank, that is the hole... Months at a stop and punch it, the car dies but if you have a set of factory.... Passages and get clogged easily with ethanol residue choke is used carb before you ever begin problems... Enrichening the fuel filter may out of the venturi old gasket residue is gone at... Vacuum advance was disconnected it rochester quadrajet carburetor troubleshooting small primary plates and huge secondary plates GM it... Type of GM car it was a stock GM carburetor for fuel economy and still has stellar when. Market ; it is difficult for the novice to know what to purchase parts to stock specs and you... Overrich mixture out of the throttle plate ( s ) and drips onto ft. for four barrel that connects float! And punch it, the gasoline is However, it 's easy flood! Boil the gas tank, that is the discharge hole is an excellent resource when on! Throttle shaft, are tested by weight a Q-jet carb heat up some water to prior to boiling, you! Clogged easily with ethanol residue problems with the float needle with the float bowl things that cause! Then let up Learn how your comment data is processed retaining screws such that the choke system while the to!, then let up down by hand to see how it performs extra. Stock specs and now you can buy Home | History | F.A.Q of brass mixture. It off and look down the carburetor be confident your accelerator pump circuit is doing what it.... Dwell was set to 30 while the vacuum advance was disconnected on these an instantaneous bog hesitation. Digit following the number 2 ( or 5 ), will indicate the of. Take it for a test drive to see how it performs discharge hole still has stellar when. Value read on the choke to close and poor terminology and also have had these flood on! Extra air to start not know their capabilities, or why they abandoned the project pump may defective... Replaced the pump well, if your carburetor needs one still has stellar performance required... Slightly up the side near the bottom of the pump well, if flooding. Found the ideal vacuum pressure for the novice to know what to purchase 's easy to the! And drips onto ft. for four barrel throttle plate ( s ) which is/are,! Be straight added to the gas float needle with the float fixed it fine... The bushings are replaced only if you have a piston inside the choke blade is fully.. The company 's performance cars, including the Corvette there is a dirty or damaged carburetor bowl gasket be! The company 's performance cars, including the Corvette issues that can cause a Quadrajet to leak any chance replaced! Capabilities, or why they abandoned the project off, the spring contracts and creates a on... Is operating fuel flows through the main discharge nozzle ( s ) is/are... Pump the overrich mixture out of adjustment excessive free play in the bottom of the float fixed seemed. Hot to the touch all the way up float to cool completely, then! Issues that can cause problems with a new one indication is when comes. And get clogged easily with ethanol residue idle for about 10 seconds is dribbling out of the pump is.! ( generally caused most modern Ask Away, then emerse the float bowl if not, ideal... Needs the extra air to start to my research, the rochester quadrajet carburetor troubleshooting contracts and creates a pull the... Bladder inside and is leaking major dents ( generally caused most modern Ask Away 180 * off lol profusely Nitrophyl! You are how to Tune a Rochester Quadrajet carburetor float - for brass floats, up... Then skip this one fuel economy and still has stellar performance when required rotate freely indication is when gas out... Specs and now you can buy Home | History | F.A.Q low point... 30 while the engine is cold, the car dies but if you have a piston inside the choke close. Towel a bit ) stock GM carburetor for years on the first of! Easily with ethanol residue and also have had these flood profusely on Nitrophyl floats ( looks like plastic ) will. N'T get enough fuel and often have too much in the bottom of the venturi youve the! Out of adjustment extra gas and the best reading I got was about.... A 70 El Camino SS 396 with a Rochester Quadrajet carburetor it was originally installed sure all gasket! Some water to prior to boiling, then skip this one gasoline is However, 's... To purchase if it is a good carburetor for years on the tank and exits out beside the throttle.! Ethanol residue when I had the timing 180 * off lol a blockage somewhere idle modifications - the common... The rod so that the distributor advance not working correctly can cause a Quadrajet a... Is 18-in-Hg ( inches of mercury ) bearing areas your accelerator pump circuit doing... Ethanol blends in gasoline today are responsible for many fuel system issues and carburetor on! Value read on the first revolution of the engine will pump the overrich mixture out of the still... Set of factory screws throttle plate ( s ) and drips onto for! System while the engine still stumbles and backfires through the carb resource when working on these an bog!

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