Quadrajet Troubleshooting

Rough Idle

This can be caused by several things, but there is one item often missed by the rebuilder and that is the idle tube. These are very small passages and get clogged easily with ethanol residue. Use thin wire to clean out the passages completely. Blow out with compressed air. A whistle with air will indicate a clean passage. If wire doesn’t do it, try heating the outside of the passage and then blow air through. There may be a small air restrictor in the passage that makes getting through with wire difficult.

Carburetor is flooding.
Gas is running out of the top, or you get a strong gas smell? This is a sign of flooding. What is flooding? Simply, there is too much gas getting into the float bowl.

Here is a list of items to check when this happens:

  • Fuel pump pressure – check with your motors manual for the correct pressure. New pumps are especially suspect.
  • Bad float – For brass floats, heat up some water to prior to boiling, then emerse the float. There should be no bubbles. Nitrophyl floats have to be weighed. 6.5-7.5 grams would be great.
  • Check float level
  • Make sure float isn’t binding – move it up and down by hand and see if you feel anything catching.
  • Make sure float doesn’t rubbing somewhere.
  • Does the seat have a gasket?
  • Did all of the old gasket get removed?
  • Look for any cracks around the area the seat screws in.

Engine is Uneven, or Surges

  • Timing, dwell & idle adjustments.
  • Look for any fuel restriction – check for kinks in the fuel line.
  • Replace any fuel filters – There is usually one in the fuel inlet of the carburetor and often there will be an in-line fuel filter.
  • Vacuum leak – Use spray carburetor cleaner and spray around the base of the carburetor and any vacuum lines. If the RPM smooths out, then you found the problem. There could also be a leak at the intake manifold.
  • Check the float level.
  • The power valve may be sticking. It needs to move up and down freely.
  • Dirt in the system – take the top off the carburetor. If you see dirt in the bottom of the fuel bowl, rebuild the carburetor, clean the fuel lines & the gas tank.
  • Main metering jet may be the wrong size. If you have been running the carburetor for awhile, then this won’t be the problem. Jets don’t wear out. Make sure the jets are not loose, or plugged.
  • Secondary throttle valve alignment.

Missing Tubes

Sorry about lack of knowledge and poor terminology. What I was referring to pickup tubes are call the main well air bleed tubes (2 inside tubes ) and secondary accelerating well tubes( the 2 outside tubes) fig. 12 page 14 of the Quadrojet models 4mv,4mc carburetor manual all are connected to the air horn. My carb. doesn’t have the secondary accelerating well tubes. The manual is not specific to marine however your video is and it shows all 4 tubes. Looking through your parts section I found pt # 17-91 Rochester carb. brass tube. It’s description states that there are 2 that stick out from of the bottom of the float bowel top. I don’t have these. My question is should I have the secondary accelerating well tubes and if I do is that the right part # ?

He needs to have all four tubes, 17-91 is one of those numbers.

Quadrajet Carburetor Kits


Idle Compensator is Missing
The Hot Idle Compensator, when used on 4MC and 4MV Quadrajet carburetors (View C) is located in a chamber at therear of the carburetor float bowl adjacent to the secondary bores. Its purpose is to offset the enriching effects caused byexcessive fuel vapors during hot engine operation.
In a perfect world the Idle Compensator should be installed on those applications that need it. Since the compensator part is not being produced you don’t have any choice but to run without it. You won’t have any problem unless you get into a high heat situation, or use your air conditioner.
NOTE: Even though your carburetor may have a place for the compensator to sit, it doesn’t mean it came that way. It was used on vehicles with air conditioning.


Adjusting Idle Mixture Doesn’t Change Anything
This is usually indicative of a problem with either too much air, or not enough fuel (in the idle circuit).

  • First of all be sure you are adjusting the idle mixture at the idle RPM. If the RPM is too high, the idle circuit will be bypassed.
  • Be sure the throttle body gasket isn’t leaking. Spray carb cleaner around the outside of the gasket. If the RPM changes then you found your problem.
  • The idle air bleeds and other idle passages are very small and can easily get plugged. If the carburetor was sitting for many months, then the ethanol residue could be plugging things up.
  • Our Quadrajet manual has an illustration that shows where the air bleeds and other passages are. You need to follow each one with compressed air to be sure they are open.
  • Take the idle mixture screws out and see if they are plugged. 
  • See above for the ‘rough idle’ article.

Quadrajet High Speed Lack of Power

Lack of power is often caused by electrical, or engine wear. We won’t discuss here, instead we will concentrate on what problems the Quadrajet may have.

  • Fuel filters are dirty, or plugged – replace any in-line filter and the filter at the carburetor fuel inlet.
  • Float level may be too low – set to specifications.
  • Float isn’t dropping far enough to open the needle fully.
  • Power valve may be dirty, or sticking. Make sure the power valve moved up and down easily and there is a spring under the valve.
  • Float is catching when moving up and down. The float pin may be worn. Move the float up and down by hand to see if you feel anything catching.
  • Needle is sticking in the seat. A damage viton tip can cause this along with some chemicals added to the gas. NOTE: Do not apply any pressure to the needle when adjusting the float.

Engine Hesitates

This can be caused by one of several things, including electrical system problems.

  • Timing, dwell & idle adjustments.
  • Accelerator pump itself may be bad. The cup curled, or is damaged. Some chemicals added to the fuel, or left in the carburetor can cause this.
  • Do a quick test of the accelerator pump circuit.
  • The pump passage way to the main discharge may be plugged.
  • 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.
  • Check the float level.
  • Carburetor mounting bolts are loose.
  • Secondary throttle valve alignment.

Polish the pump well with crocus cloth, or 2000+ sand paper. Apply a couple of dabs of white lithium grease. Today’s gas has little lubrication. A little grease will help.

What to look for when your Rochester Quadrajet is flooding.

  • The number one problem to look for is leaks around the needle and/or seat. Many rebuilds are ruined by a dirty fuel tank, or filter. Also make sure all old gasket residue is gone.
  • New viton tip needles sometimes are covered with residue, causing it to not seal until started several times. Wiping with mineral spirits can help.
  • Check the float for leaks – For brass floats, heat up some water and just prior to boiling immerse the float. The float will heat up inside and if there is a hole, air will bubble out.
  • Nitrophyl floats (looks like plastic), are tested by weight. 6.5 – 8 grams, depending on which float it is.
  • Move the float up and down by hand – Do you feel anything catching?
  • Test your fuel pump – new pumps are especially suspect and often have too much pressure. Consult your repair manual for the specification, but 4 to 5 lbs is average for most carbs. If your flooding started after a rebuild, then skip this one.
  • Can you see gas dribbling out of the main discharge, or venturi while at idle? This would indicate the main discharge, possibly the check ball, not sealing. This is more of an over rich condition than flooding, but flooding to one person may be different to another person.
  • If your carburetor hasn’t been rebuilt recently, then considering doing so. Most carb problems are attributed to dirt. A carburetor that has been sitting for 6 or more months can also be coated inside with gas that has turned to varnish.
  • There is a pull clip that connects the float needle with the float arm. Be sure the clip is not routed in one of the small holes. It should be around the outside of the arm.

Protect your engine with ETHANOL DEFENSE

And yet another troubleshooting guide here.

I have a Rochester Quadrajet Marine Carburetor #17059286. It appeared to be flooding at idle and the fuel was leaking excessively from the throttle body linkages (on both left & right side). I assumed the carb needed to be rebuilt and the leaking was normal (from a drain in the carb) when flooded. I rebuilt it using a kit I ordered from you. I reinstalled the carb and I’m having the same issue of fuel leaking from the throttle body linkages. In your youtube video (Rochester Marine Carburetor rebuild part 3 of 3) at 4:00 you mention not usually having to take apart the thottle body to replace bushings. My question is: Do you think I should replace the thottle body linkage bushings? And if so, do you carry the kit? I think this is what I need to do. However, I read a post online that said the fuel at the thottlebody should already be atomized, and should therefore not leak out the linkage,and that would indicate that the carb is flooding due to the float not being adjusted properly. I set the float to 3/16 like the instructions stated.

Your problem is not bushings. The carburetor is simply puking out too much fuel. The bushings are replaced only if you have excessive free play in the throttle shaft. Flooding fuel will leak out anywhere it can and onto the throttle valves and out the shaft is the 1st place fuel likes to go.

Too much fuel boils down to getting too much in the float bowl. Did you test the (brass) float for leaks? Immerse the float in hot water and if there are any leaks, it will bubble. If you have a Nitrophyl float (black plastic kind of thing), replace it. They absorb fuel over time.

Did you put the fiber washer where the seat screws into the float bowl.

Many times after rebuilding a carburetor and after re-starting the engine, crud from the tank and the fuel lines will rush up to the carburetor and into the needle & seat causing it to stick open. Take the carburetor apart and wash out the needle & seat and try again.

Manually move the float up and down and watch for any kind of rubbing, or catching. Also watch the needle to make sure it is move straight up and down and not getting caught up.

Too much fuel pump pressure could be holding the needle open. Any chance you replaced the pump with a new one? New pumps are always suspect because they come from China and they don’t seem to care about specifications very much yet.

These are just a few ideas, but bottom line is that too much fuel is getting into the float bowl.

HARD STARTING WHEN COLD

First lets start by learning how to start a classic car with a carburetor.

  • Press the throttle down one time, then let up. The accelerator pump will squirt an extra bit of gas into the intake. The engine needs a bit of help when cold. You can check this with the engine off by looking down the carburetor and pump the throttle once to see the squirt.
  • The choke valve will close once you step on the gas. Make sure it is closing when cold.
  • It may take a few seconds of cranking to fill the carburetor (if low).

Now what to look for if you are having problems.

  • As mentioned above make sure the choke valve is closing. Some will open just slightly so it doesn’t choke out.
  • Is the fast idle sitting on the higher step of the cam. If the fast idle isn’t working then the engine may die after starting. It needs the extra gas and the extra air to start.
  • If the carburetor is losing too much gas overnight, then it may be percolation. See this page.
  • Test the fuel pump (new pump especially). You want to make sure the pressure is where it should be. There is also a volume test if you can find the spec for that.
  • With the engine hot and off, look down the carburetor after 1st turning the engine off. Do you see gas dribbling out of the main discharge. If so, then the check may be leaking. 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.
  • The gas cap should be vented. A non vented gas cap on a system that needs a vent will cause gas to get siphoned out of the float bowl. Simple to test by loosening the gas cap after turning the engine off.
  • If you are using a fuel additive that contains alcohol, then that will cause gas to evaporate more. We recommend using Ethanol Defense.

Hard Starting

  • The fuel bowl may be draining while sitting. Make sure the fuel filter has the check valve installed.
  • After running the engine, turn it off and look down the carburetor to see if gas is dribbling out of the venturi. This would indicate the discharge check is leaking.
  • If there is a 2nd line going to the gas tank, that is a vent. Be sure you can blow through the passage.
  • See if the vent on the carburetor is cut off for some reason.
  • Engine overheating can cause the carburetor to percolate (boil the gas).
  • Cold starting
    • Depress accelerator pedal to the floor and release. Leave foot off accelerator pedal and pause a moment.

Stalls After Start

  • Timing, dwell & idle adjustments
  • Choke Rod
  • Vacuum break adjustment
  • Float level

Engine Loss of Power or Stalling in Hot Weather

Determine whether carburetor is spewing or vapor locking. This can be done by performing the following road tests in 70 to 80 degree weather.

  • Let the car idle in drive with the air conditioning on for 10 mins.
  • Shut the engine off and let sit for 10 mins.
  • Start car and accelerate at 3/4 throttle to 60 mph. If condition is going to occur, it will occur before 60 mhp.
  • Observe either loss of power and/or stalling; note the color of the exhaust smoke.
    • If exhaust smoke is black, carburetor is spewing.
    • If exhaust smoke is colorless, carburetor is vapor locking.

Cold Sags, Die Outs & Pop Backs

For all drivability complaints, check all vacuum hoses to make sure they are properly connected. A disconnected hose can allow air leaks which give lean mixture as well as problems of improper action of the component operated by the hose. Also ignition contact point gap and ignition timing should be checked on al drivability complaints. 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.

Retarded basic ignition timing causes pop back during starting. Adjust to specification on vehicle emission control label +_ 2 1/2 deg for best engine operation.

On engines equipped with solenoid spark advance for starting check to make sure it is operating. To check disconnect leak to solenoid at bullet connector and energize solenoid momentarily with a jumper. The engine will speed up approximately 50 RPM and timing advance 7 1/2 deg if solenoid is operating.

Updated on 01/10/2021

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