Rochester Monojet Troubleshooting

Main Air Bleed Plugged?

Rochester Monojet  Main Air Bleed

There was a question about the air bleed the sits on the top of the float bowl. That is the round brass plug with a small hole in the center. This is located above the main circuit delivery passage and the plug should be domed with a small hole in the middle. These are not removable and if damaged just make sure the hole is there and if closed try to open it up with a piece of wire. That is what really matters. 

When plugged the engine won’t idle and if missing, or the hole is too big, then the engine may not gain speed because of too much air going to the main circuit.

Gas Leaks From the Bowl Gasket

He may have set the float level to high or his Nitrophyl float is heavy and is causing the main body to over fill. The Monojet is not prone to warpage like its older brother (the “B”), but it still can if screws are over tightened. If it is a warp problem he can try two gaskets or lightly coat the sealing surface of the gasket (both sides) with a light coat of Permatex #3(aviation form-a-gasket). It is in most parts stores in a small can w/brush. Its unaffected by gas or oil.

Rochester Monojet 1 Barrel Carburetor Parts

What Does the Bowl Vent Attach to?

Rochester Monojet Float Bowl Vent
PVC, or whatever the vehicle uses to recirculate gas vapors.

Won’t start unless I pour gas into the carburetor.

First be sure the electrical system is up to par. Many carburetor problems turn out to be electrical problems.

  • Verify the accelerator pump is squirting gas by pumping the pedal once. Gas should come out of the main discharge. This is necessary to start a cold engine.
  • Is the choke valve closed when cold?
  • Long shot is the gas is bleeding back to the fuel pump. Check all vents.
  • After running, stop the engine and look down the carburetor. Is gas still dribbling out of the main discharge? If so, then try seating the check ball by tapping on it lightly to form a good seal.
  • Is the check ball, then spring, the T in place?

After the Engine Warms Up, the Engine Dies.

  • Make sure the choke valve is opening as it warms up.
  • Once open and the engine dies means there isn’t enough fuel, or in the case of black smoke is flooding.
  • Look for vacuum leaks around the mounting, intake and any other vacuum sources.
  • When it starts back up, if you get black smoke out of the tailpipe, then the carburetor may be flooding out.
  • If the vehicle has been sitting for months then the gas has probably varnished the inside of the carburetor. Gas tank will need to be removed and cleaned, gas lines cleaned.
  • Check float level.
  • Look at the needle & seat. Does the needle come out of the seat when the float drops and not sticking?
  • Test the fuel pump for 4.5 lbs prox.
  • Change fuel filter. Open end toward the fuel inlet. Be sure the filter has the check valve on one end.
  • The float fits tight in the float bowl. Be sure it isn’t rubbing the side of the float bowl.

Why Does My Monojet Flood?

  • Most common problem is the needle & seat leaking. Test it first.
  • Test the fuel pump, especially if it has been replaced. Many new pump put out too much pressure. 4.5 lbs is about right. Refer to your motors manual for exact pressure. If the pressure is too great, the needle will be forced open, letting gas get by.
  • Float might be leaking. Nitrophyl floats (looks like plastic) has to be weighed as they are solid but can over time absorb fuel. 9.5 grams. Brass floats can be shaken to see if gas is inside, or heat up a pan of water and immerse the float. Any leaks will be indicated with bubbles.
  • The float on the Monojet fits tight. Be sure it isn’t rubbing on the bowl sides.
  • Check the float level, however to flood over the top of the carburetor float would need to be excessively off spec.
  • If the engine has been sitting for months, then the gas may have turned and coated the inside of the carburetor with varnish. The carburetor, gas tank and fuel lines will need to be removed and cleaned. Flushing with new gas isn’t going to do it. Varnish coats the walls and will fall into the gas stream at some point. This can cause the carburetor to flood, die, or simply lack power.

Why Doesn’t the Idle Mixture Screws Change RPM?

This indicates there is a problem in the carburetor.

  • Too much fuel – Look at the list above for flooding.
  • Plugged passage – remove the mixture screws and blow out the passage. Don’t forget to plug the mixture hole that leads to the inside of the bore.
  • Are the screws bent, or scored? Replace if they are.
  • Check the passages in the venturi. They are small and easily gets plugged.
  • Try spraying carb clean through the passages and see if the cleaner comes out the other end.
  • Has the car been sitting for several months? If so then the gas may have turned and coated the gas tank and the inside of the carburetor. Adding fresh gas will not correct this.
  • Any dirt on the bottom of the float bowl would indicate a good cleaning is necessary.

Why Can’t I Get My Carburetor To Idle?

It idles too fast even when the idle screw is all the way out.

  • Look at the fast idle cam and see if it is on the low step.
  • A vacuum leak can cause this. Using spray carb cleaner spray around the mounting, where the throttle body meets the flow bowl, any vacuum line & fitting and the intake manifold. Also try covering up the top of the carburetor bore to block off the air. If it doesn’t die then a vacuum leak for sure. Always have a fire extinguisher close by.
  • If the throttle plate was removed for any reason it may not have been put back in the same position causing a gap around the valve. This will cause the RPM to go up.
  • If the carburetor was sitting for months, the gas may have turned varnishing the inside of the carburetor. It will most likely stink of varnish.
  • Another possibility is the linkage may be holding the throttle open. Disconnect the linkage to the carburetor and see if it will idle.
Updated on 05/28/2021

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