Troubleshooting 4 Jet



Using Ethanol Defense will help eliminate your ethanol problems.
Ethanol Fuel Additive
Tip for cleaning out passages.

Wire from twist ties work well for the small holes. Cleaners and compressed air will not remove the ethanol deposits left behind.
 
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.


Rochester 4 Jet Lack of Fuel
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?
 

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.

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.

Starts with Starting Fluid Only

With engine off, look down the carburetor and see if you get 2 good squirts of gas when pumping the accelerator.
If not, then there may be a problem with the main discharge, or accelerator pump circuit.
  • Remove the carburetor
  • Take the top off
  • Fill the float bowl with mineral spirits
  • Pull up the accelerator pump to the top and see if the fluid fills the well. If not, the the intake is plugged depending on what type you have.
  • With the well filled press the pump down. You should see fluid come out of the main discharge holes. If not, then remove the venturi and try again. If it comes out now, then there is a stoppage in the venturi (small holes)
  • From here test the pressure of the fuel pump. Should be 4.5 lbs prox.
  • Move the float up/down to make sure it isn't binding.
  • Test the needle & seat. It might be stuck closed. Blow through the inlet with the float down. You should be able to blow through.
  • If the accelerator pump cup is leather then it might be dried out and brittle. Replace it. If not, then put 2 drops of oil on the cup and give it some time to expand.
Watch a video about the throttle body gasket and looking for vacuum leaks.


Question:
Can one of the emulsion tubes be missing from the venturi? Answer: Sometimes those fall out. I would simply try to find another 4G and pirate an emulsion tube from its booster cluster. Compare the size/number/placement of holes with the remaining tube and modify as necessary. Install new tube to same depth as old one with a light coating of Permatex Anaerobic Gasket Maker on the press-fit surface. Question: When I took apart my 4gc (7024121) there was no aluminum check ball in the pump intake cavity. Does it take the large one or the small one that is in my kit?

Answer:

1st you have to be sure you need a check ball. Carburetors that use a check ball in the accelerator pump well will have two holes at the bottom. One for intake and the other for out flow. When there is only one hole, then no check ball is needed since the hole is the out flow and if you placed a check ball there, the flow to the main discharge would be cut off. When you have two holes, then use the smaller of the check balls and these are often made of aluminum instead of stainless steel. Aluminum is used to make the check ball lighter since only the weight of the fuel lifts the check ball off of the hole to allow fuel to enter.

Won't Idle

There are several reasons why your Rochester 4 Jet carburetor may not be idling properly. Here are a few common issues to consider assuming the engine is at operating temperature:

  1. Clogged idle jets: Over time, the small passages in the idle jets can become clogged with debris, preventing the carburetor from delivering the proper amount of fuel at idle. This can cause the engine to stall or run rough at low speeds.

    Carefully check each passage in the venturi and the passage coming up from the idle mixture screw. These are very small and easily get plugged. I like to spray air, or spray carb cleaner through each passage to make sure it is clear.

  2. Vacuum leaks: Any leaks in the vacuum system can disrupt the fuel-to-air ratio, leading to rough idling or stalling. Check for cracks or loose connections in the hoses and gaskets.

  3. Incorrect adjustment: The idle mixture screws on the carburetor may need to be adjusted to provide the correct fuel-to-air ratio. This adjustment should be made while the engine is running and warmed up.

  4. The idle may be set below manufactures recommendation. When the idle is too low, the throttle valves are closed too much and will smother the engine.

  5. The main discharge may be leaking. At idle, or just after turning off the engine, look down the carburetor and see if gas is dripping from the discharge. If so, the the main discharge ball may be leaking. Seat the ball by tapping on it lightly. Be sure the spring is above the check ball.

To diagnose the issue with your carburetor, start by checking for any obvious signs of damage or wear. Next, inspect the idle jets and vacuum system for clogs or leaks. If these components appear to be functioning properly, check the idle mixture screws and adjust as necessary. If the issue persists, it may be time to bring your vehicle to a professional mechanic for further diagnosis and repair.





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