Rochester 4 Jet Troubleshooting

I purchased a 4G float from you. It worked well. Thanks. The carb seems to have a “dead spot” I was hoping you might have some suggestions. The car runs and starts right up. If you accelerate slowly, the car is fine. If you punch it, the car will sometimes die or it will almost die then rush back to life. If you are at a stop and punch it, the car dies but if you accelerate gradually it is fine. You can even floor it after you are moving some and it seems okay. My brother says it might be the accelerator pump. The carb is newly rebuilt and is very clean. Once we got the float fixed it seemed fine. I ran it for a couple of days and this “dead spot” showed up. The carb seems to be adjusted properly and the timing is correct for the engine. Can you help?

I will assume your electrical system is ok. Don’t forget that the distributor advance not working correctly can cause a similar problem.

Your friend is probably correct. When you step on the gas an extra squirt of gas is needed and that is provided by the accelerator pump circuit. There is where I would concentrate your efforts. Make sure that the check ball is in the bottom of the pump well, if your carburetor needs one. The way to tell is to look for two holes. One will be slightly up the side near the bottom and that is the discharge hole. The other will be on the bottom and that would be the intake. The check ball that resides here will be the smaller check ball and it is often aluminum. 

Test the check ball by taking the top off. Fill the float bowl. The check ball will lift up by the weight of the gas and allow fuel to flow into the well. For those that don’t use a check ball, the well if filled by coming over the slot in the side of the pump well. In other words it splashes over from the float bowl. If the well doesn’t fill up, then the passage is probably plugged up. Run thin wire through the passage. Ethanol will leave residue behind that cleaners cannot get out.

Take the venturi off, remove the main discharge T, spring and check ball (bigger check ball). Fill the pump well, then press the accelerator pump down to the bottom. The pump well check ball should seal the intake hole and fuel will be forced out to the main discharge. If fuel comes out of the main discharge, then the check ball and passage way is probably ok.

Replace the main discharge check ball and using a brass drift punch, hold the check ball down so that the hole seals. You should now get some resistance when pushing the accelerator pump down. If you get fuel out of the main discharge while holding down the check ball, then use a brass drift punch to gently tap on the check ball. This will reform the check ball hole. Don’t hit too hard, or the check ball might get stuck.

Replace the main discharge ball, spring and T. Run thin wire through all of the passages in the venturi cluster and then replace it. Don’t forget the gasket.

Now when you press the accelerator pump to the bottom fuel should squirt out of the venturi cluster holes.

Using Ethanol Defense will help eliminate your ethanol problems.

Installing Jets
The bigger orifice jet goes in the primary side.
A piece of paper towel folded over the screwdriver blade will hold the part, or a screw, until it gets started in the hole. (sometimes it helps to dampen the paper towel a bit)

I soaked and cleaned my 2 carb both the 1 inch for a chev . One of them starts and idles good but when you try to raise the rpms it swings frm 1000 to 3000 rpms. 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. Ive set the pump and metering rods per your specs and cant make it any better.

My first instinct is that he has a vacuum leak . . . which could be downstream of the carburetors. Also, are the secondary throttles opening and closing completely? Or is there some sort of interference, including from an incorrect base gasket? The Accelerator pump cup gets stuck at the bottom of the well and comes off the pump stem. Most likely you have the wrong accelerator pump. Did you match your carburetor number and is the carburetor number tag on the correct carburetor? If the carburetor number doesn’t find the correct pump then find a pump shorter than what you have and try again.

Only Runs With Choke Valve Partially Closed

So your engine seems to crap out once the engine gets warmed up.
A vacuum leak can cause this. Check around the intake manifold, carburetor base and any vacuum lines attached. A vacuum leak can go away when the engine metal gets hot and expands.When cold your choke valve is closed and the vacuum is pulling in extra fuel. As the valve opens there is less vacuum and the normal circuits take over. What this probably means is there is a lack of fuel with the valve open.
Float level incorrect

Float drop not dropping enough, which will keep the needle from opening. Set to specifications. Don’t fudge.

Fuel pump pressure is lot. Test the pressure with a fuel pump pressure tester. Around 4 – 4.5 lbs will do it.

A passage may be plugged. Especially the small passages such as the idle vent passage. Make sure you can blow through each passage. Include the small passages on the venturi. Simply soaking a carburetor in a cleaner will not unplug a passage. Dried gas leaves residue behind, which will plug small passages.

Bogs, or Hesitates

When you step on the throttle, the engine seems to bog down and in some cases die.
a bad distributor advance can cause hesitation.
Stepping on the throttle is where the accelerator pump circuit comes into play. When the pump moves up (let up on throttle), the accelerator pump well is filled with fuel. Some get the fuel by filling over the top of the well. There will be a slot in the side of the well for this, others use a intake at the bottom of the well which is shut off with a check ball.
In most 4 jets the small aluminum check ball goes in the pump well. 
With the engine off pump the throttle while looking down the carburetor throat. You should see 2 strong squirts coming out of the venturi. If not, then you have a blockage somewhere. 2 & 4 barrels will have 2 squirts, 1 barrel will have 1 squirt.

The small holes in the venturi can be blocked with ethanol residue. run thin wire down the holes. Don’t enlarge the holes.Check the gasket under the venturi, they don’t always fit well and have to be trimmed. If they are too big then the venturi won’t site flat.
With the top off put fluid in the pump well and press the accelerator pump down. Fluid should come out of the main discharge.
The main discharge should have a check ball, spring, then a T to hold it all in.
Look into the bottom of the accelerator pump well. If there are two holes, then there should be a small aluminum check ball on the bottom.

Blow air through the passage way at the bottom of the accelerator pump well. Air should come out of the main discharge.

Is your accelerator pump cup getting stuck in the well? Most wells are tapered so that the pump gets tighter as it goes down. If ethanol has damaged the pump cup then the cup may be swollen.

Does your accelerator pump have a delayer spring (spring over the stem) and a return spring under the pump?
Testing the accelerator pump circuit on a Rochester 4 Jet Carburetor

This article only refers to the type of 4 jet that uses a check ball in the bottom of the pump well. Not all carburetors do.

When rebuilding your 4 jet you may want to test the accelerator pump circuit before closing up the carburetor. 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. Not all 4 jets will have this check ball. Fill the float bowl and the pump well with mineral spirits.

Put the pump in the well and push it down toward the bottom of the well. The check ball will seal the hole, keeping the fluid from going back into the float bowl and the fluid will be forced through the 2nd hole through the main discharge. You should see fluid come out of the main discharge.

The main discharge should have the bigger stainless steel check ball, then the gold colored spring (don’t worry if it is bent), then the T. Stake the T so that it doesn’t pop out. 

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. 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.

The larger check ball in the main discharge tube must also be tested. Hold the check ball down with a brass drift punch and push the accelerator pump down. You might get but a slight amount of fluid past the check ball, but you should also feel a slight pressure.

Replace the venturi cluster and test again by pushing the pump down. You should get a strong squirt out of both sides of the cluster. If not run thin wire through the small passages. They may be plugged with ethanol residue and cleaners will not remove the residue.

To re seat a check ball, using a brass drift punch, hold the check ball down into the hole with the drift punch and tap a few times with a hammer. Don’t get carried away, or you will get the check ball stuck. The hole in the carburetor you are trying to form is soft metal and doesn’t take much to change.

Now you can be confident your accelerator pump circuit is doing what it should.

Is the check ball stuck in either hole? Turn the carburetor over and while heating the bottom where the check ball resides tap the carburetor bowl on the bench. It will most likely fall out when hot enough.

Adjusting Idle Mixture Doesn’t Change RPM

  • If the idle RPM is too high then the idle mixture isn’t going to respond. The RPM has moved past the idle circuit. Adjust he idle as per manufactures specs.
  • Too much air, or too much fuel? Too much fuel will usually show up with a gassy smell.
  • You may have a restriction somewhere. Remove the idle mixture screws and make sure they blow through to the bore and then to the venturi. The very small passages are usually the idle vent passages. Inspect the throttle body gasket carefully to make sure it isn’t covering up a vacuum passage. The 4 jets used multiple types of gaskets and they are easily mixed up. Blow through every passage to make sure they are clear.
  • Check around for vacuum leaks, intake, carburetor mount, vacuum lines coming off the carburetor, or intake. Use spray carburetor cleaner to spray around seams. A change in RPM indicates a leak.
  • Too much fuel – too much fuel entering the float bowl can cause air/fuel mix problems. Check the float level.
  • Look down the carburetor while at idle RPM. If you see gas dribbling out of the venturi then you have a problem with the discharge. Make sure the main discharge has the check ball and it seats. There should also be the spring and the T installed above the check ball.

Bottom line is that the idle mixture is telling you there is a problem with the carburetor, either getting too much fuel, or too much air.

Choke Thermostat is Slow to Open
This is reference to the hot air type of choke.

  • Bad thermostat – replace the thermostat – test by heating water and immersing the thermostat. The spring should move instantly at least 1/4″
  • Hot air tube is plugged. It should be hot to the touch all the way up.
  • No vacuum in the housing – vacuum supplied to the choke housing is used to suck up the hot air from the hot air tube. Take the thermostat off and with the engine running, blow a little smoke into the choke housing. It should get sucked up through the vacuum passage.

I soaked and cleaned my 2 carb both the 1 inch for a chev . One of them starts and idles good but when you try to raise the rpms it swings frm 1000 to 3000 rpms. 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. Ive set the pump and metering rods per yourspecs and cant make it any better.

My first instinct is that he has a vacuum leak . . . which could be downstream of the carburetors. Also, are the secondary throttles opening and closing completely? Or is there some sort of interference, including from an incorrect base gasket?


Flooding is when gas is coming out of the top of the carburetor. Too much fuel is getting into the carburetor.
Another possible indication is when gas comes out of the throttle shaft.

Rochester 4 Jet flooding can be caused by a list of things:

1. Bad float – getting fuel inside the float which makes it too heavy.
2. Fuel pump pressure too high. Check your motors manual for correct pressure, probably around 4.5 lbs
3. Needle & seat leaking – the viton tip on the needle may have gotten damaged when installing. They are easily damaged. Look for any lines, or other abnormality’s on the viton tip.
4. Is the gasket on the seat.
5. Was all the old gasket removed from the needle & seat?
6. Look for any cracks around the seat. Look for any chance of the gas going around the seat.
7. Check the float for any rubbing. The pontoons should be centered in the float bowl.
8. Make sure the needle is being pulled straight up by the float. It’s possible the needle is getting stuck at an angle. This could be caused by the needle to float clip being installed incorrectly. It need to pull straight up.
9. You may be percolating. Engine gets hot, turn off key and the gas boils over. This is due to the low boiling point of the gas these days. Try different brands and grades. Make sure the fuel lines are away from the exhaust.
10. Fuel could be leaking around the power valve. Make sure the power piston moves up and down freely. Any sticking will leave the power valve open. The power valve stem should be straight. Make sure there is a gasket on the power valve and that there isn’t any old gasket left behind.

How Do I Know I have the Correct Accelerator Pump?

With the engine off, look down the carburetor and pump the gas. You should be getting 2 strong squirts of fuel from the venturi. If that is happening then you are good to go.

As far as size the 4 jet there are 2 sizes of pump cup. 21/32 & 3/4. The 3/4 would probably get stuck in the well when pushing down to the bottom. The 21/32 will fit loose until you get closer to the bottom of the well. Most wells are tapered so the pump might not get tight until it gets close to the bottom.

Updated on 05/03/2021

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