Rochester 2 Jet Troubleshooting

Flooding is when gas is coming over the top of the carburetor. This simply means there is too much gas getting into the float bowl. Here is a list of things that can cause this. Not in any order.

  • Test the needle & seat.
  • Check fuel pump pressure. You will need to check your motors manual for the correct setting, but should be about 4 – 4.5 lbs.
  • Look at the needle viton tip. They can get damaged if too much pressure are put on them adjusting the float.
  • Clean the residue off the viton tip with mineral spirits.
  • Does the seat have 2 gaskets by mistake, or are the gaskets damaged, or is the float bowl cracked around the seat. Any of these problems would allow the the fuel to leak past the needle & seat with nothing to stop it?
  • Holding down the needle into the seat gently, try blowing a small amount of air in the inlet. Is it holding?
  • Does the power valve have a gasket under it, only 1 gasket and it is not damaged, or cracked?
  • Move the float up & down to see if it is hanging up somewhere. Make sure it isn’t rubbing on the side of the bowl, or the bowl gasket.
  • Most of the 2 jets use a clip to clip the float to the needle. Make sure the float pulls the needle straight and that it doesn’t bind the needle.
  • Is there dirt in the needle. Dirt may be coming up for the gas tank, regardless of the filter.
  • Power valve – Your power valve should move up/down easily. It would flood if the power valve was stuck open due to the pin staying down, or the piston staying down. Take the power valve out and blow through it while pushing the stem in & out. You shouldn’t be able to blow through when the pin is up. Follow the vacuum hole going to the power piston to make sure it is clear.
  • When the vehicle has sat for 6 or more months, the inside of the carburetor has probably varnished up. Once the fuel turns not only does the carburetor have to be cleaned, but also the gas tank and fuel lines must be cleaned, otherwise crud will get back into the carburetor and you will start all over.
    • Once this happens you will need to make sure every passage is clean. Clean the carburetor by soaking in simple green for 2 hours. Wash out with hot water if possible and blow out all passages. Spray some spray carburetor cleaner through the passages. This will break down the varnish. Test each passage to make sure it goes all the way through.
Rochester 2 Jet Parts
2 Jet Parts

Below you will find some troubleshooting guides. Keep in mind that most problems in a carburetor can be caused by multiple parts. A carburetor problem can rarely be attributed to one thing only.

Hesitation off Idle

My carb is on a ’66 Impala. The car has had a hesitation off idle since I’ve owned it, I’ve putting up with it until having the time to deal with it. . It has a good stream of gas coming from the accelerator pump. Other than the main vacuum port in the back that feeds the brakes and trans there is only one port that is for the vacuum advance. For some reason there is full vacuum, not metered vacuum. This doesn’t seem right to me, is there something that plugs up in these carbs that would make this port go to full vacuum ? i have only owned the car approx 6 months.

Look at the transition passage. There is a hole, or slot just above the idle mixture screw hole in the bore. Follow this up to the venturi by blowing through it. Make sure it is clear. This passage gets you additional air/fuel mixture from idle up to the accelerator pump circuit. The other thing to look at would be the distributor.

Emulsion Tubes

Emulsion tubes are used to emulsify the gas. While you should use them when you have them, don’t worry about them if missing. Many re-builders leave then out anyhow.

Gas Dribbling From Venturi at Idle

This can be caused by carburetor vacuum at the venturi drawing fuel from the main discharge. The main discharge has a check ball and spring on top of the check ball. Gas can leak by the check ball if it isn’t sealing. Try tapping on the check ball lightly to seat it. A full tutorial of testing the check ball and accelerator pump circuit can be found here.

The gasket under the venturi can be holding up the venturi. Make sure it’s sitting flat and it may need to be trimmed a bit. Does it have the correct gasket. It should be covering any holes.

If this is happening at idle, then take a look at the idle passages, the idle vent tubes especially. When these are clogged, the dribbling can occur. Take a look at the idle circuit guide here.

Rochester 2G, 2GC, 2GV, 2 Jet Carburetor Questions & Answers.
User post questions to us in a variety of ways.

Here is where we post the questions and problems and the possible solutions.

When my car sits for a few days after driving, it will not turn over unless I squirt gas into the card. I have replaced the fuel pump and installed a check value between the fuel pump and the carb, but the problem persists. Any suggestions on what could be the issue or how to troubleshoot it? 

This could be caused by a couple of things.

Percolation – gas has a lower boiling point these days. After getting the engine hot and turning it off, the gas boils out of the carburetor. This is called percolation. Check this by feeling the carburetor after running the vehicle and getting the engine hot. Feel the carburetor to see if it is hot to the touch. If it is then the problem is probably percolation. This is a tough thing to correct, but cooler gas is the answer. Make sure the fuel lines are away from the exhaust manifold. You somehow need to get the heat away from the carburetor.

Add Ethanol Defender to your gas. They don’t claim it will help the heat problem, but it does help with the ethanol problem.

Make sure all of the small passages around the venturi and on top of the carburetor are clear. Run thin wire down all passages.

Make sure any vents on the top of the carburetor are clear. Without a vent you can get a siphoning action.

After turning off the engine look down the carburetor. Do you see gas dribbling down the throat? If so then your main discharge may be leaking, or the venturi gasket may not be sitting flat. These gaskets sometimes needs to be trimmed to sit correctly.

There is always a chance that gas is leaking out of the bottom of the carburetor. Remove the carburetor, fill with gas and let it sit overnight on a piece of paper. Any leaks will easily show up.

If the car has been sitting for a long time, the gas could have varnished the inside of the carburetor including the needle & seat.

A long shot would be the fuel pump. You can eliminate this by disconnecting the fuel line after stopping, then hook it back up when you want to start the engine. Obviously if the problem goes away, the issue is with either the fuel pump, or gas tank. Some gas tanks are vented.

The wrong gas cap could cause a non vent action. Loosen the cap and see if this cures the problem.

Fuel may be simply evaporating through the top vent. This is due to the lousy gas we now have. There isn’t anything you can do, but crank the engine until the fuel bowl fills again. Don’t pump the pedal at 1st. That will only make the accelerator pump squirt some fuel, which will not be enough.

My copper float has a round hole and a
slotted hole in the float lever assy. Which hole does the wire clip on the needle hook into on the copper
float?

Doesn’t matter. Just be sure the needle is pulled straight up and down, or it will bind.
I have a 1967 chevy C-20, 283, 2g carb.. i think. The accelerator cup keeps tearing or rupturing.. I need to order a new rebuild kit however i would love to solve this problem first. Is it ethanol? I cant feel any burrs in the chamber.. its about my 4th or 5th accel pump since my last rebuild and i have damaged the the gaskets taking it apart as many times.. Any help would be much appreciated. thanks 

1st of all add Ethanol Defender to your gas. This will eliminate a lot of your problems.

Polish the accelerator well so that the cup glides smoothly using crocus cloth.

Wash all chemicals out of the carburetor after cleaning with hot water. Almost all chemicals react with Ethanol.

Even ethanol ready rubber will fail if the ethanol is strong enough. Ethanol separates from gas when it sits resulting in more that 15% concentration. The kit contains multiple check balls. Which goes where?

The smaller and usually aluminum check ball resides in the bottom of the accelerator pump well. The larger stainless steel resides in the main discharge tube along with the brass colored spring and the T retainer. Keep in mind that not all 2G carburetors use a check ball in the bottom of the well. A well with only 1 hole at the bottom will not use a ball. Those with 2 holes (1 slightly on the side) will use a ball.

Should I clean my carburetor using the dip from parts stores?


Yes it is, but follow the manufactures instructions. Some cleaners will discolor your carburetor if left soaking too long. Due to restrictions cleaners these days won’t do much more than remove the oil and grease.

After soaking wash with hot water, blow out passages with compressed air, then run thin wire down all passages. Ethanol leaves deposits behind. The small holes are especially prone to plugging. We used to recommend against the wire cleaning because it could result in making holes bigger, but because of ethanol, it is the lesser of 2 evils.

My carburetor backfires and the car will only run with the choke on 3/4

Most likely this is due to too much air, or not enough fuel. Look for a vacuum leak. Check all connections that might have vacuum. Also check any vacuum hoses. You can use a spray carburetor cleaner and spray around the connections. If the idle smooths out, then you probably found the problem.

Next look for reasons the carburetor isn’t getting enough fuel.
Check fuel pump pressure (compare the number with your motors manual).
Check the float level.
The needle may be sticking in the seat, not letting enough fuel in.
You could have plugged passages. Ethanol will leave deposits behind that cleaners and compressed air will not remove. Use a thin wire to run down all passages.
Are the idle mixture screws way out of adjustment?
If the carburetor hasn’t been rebuilt, then do that if you suspect dirt is your problem.

More from my friend Jim:

My first thought is a “BUNA” black rubber accelerator pump cup that has enlarged due to alcohol contamination. With little or no accelerator pump function there would be a lean pop or backfire or sever stumble. When there is a vacuum leak, unwanted air enters the fuel mix stream and causes either high idle or rough idle and stalling. Closing the choke butterfly eliminates some of the normal air flow and the fuel mixture becomes more normal. I would remove the air horn to inspect the accelerator pump cup. I would also to see if an inlet pump check is required and/or present.(some “2G” carbs don’t require a check ball and refill from a slot inside the main body). I would remove the Venturi cluster and blow the cluster and the passages in the main body. If its an early “2G” I would check to make sure the carb has the correct flange gasket. Some early “2G” carbs used exhaust crossover to preheat the cast iron base and if the incorrect gasket is used it will inject exhaust gases into the fuel stream.

UPDATE: Customer found the holes leading from the carburetor throat and idle mixture screw was plugged. This would mean little to no fuel from the idle circuit.

My carburetors are running rich at idle 

Here are a few things to check. Not in any particular order.

Float level.
Bad float. For brass floats, heat some water and immerse the float. Any bubbles indicate a leak and the float should be replaced. Nitrophyl floats should be replaced. The only test is to weight them.
Fuel pump pressure.
Power valve is not seating, allowing fuel past it when in non power mode.
Fuel is leaking past needle & seat. Be sure you cleaned the old gasket material off completely.
Main discharge check ball is not seating. Hold the ball down with a brass drift punch. Put fluid (like mineral spirits) in the pump well. Press the accelerator pump down. You should feel some pressure. If the ball is not seating, fluid will be siphoned at idle. You an seat the ball by hitting it gently with brass drift punch and hammer.
Run thin wire down all small passages, especially the venturi.
Seat was damaged when installing. Too much pressure on the seat when adjusting the float is a common problem. Inspect the viton tip closely for any marks.
Should I use a sealant on the gaskets 

NO, nothing. There are a few cases where we use Permatex Anarobic, but that is only for extreme cases where we need to get a leak stopped. Tri

Power running too rich 

I have a set of 3 Rochester carbs on my Model A Coupe and they were burning VERY rich. I removed front and & carbs, installed block off plates and eventually determined that it was the front carb that was affecting fuel mixture. Other than the accelerator pump laying in pieces in the pump well, nothing else seemed out of order. I used the pump from your kit to replace it (damaged pump in carb was the wrong pump) and remounted the carb but fuel mixture was still way too rich. After several days it dawned on me that it was most likely the idle circuit in the front 2G that responsible for extra fuel in the manifold. I again removed the front carb and checked the throttle block and sure enough the Idle circuit was not blocked off. So I taped the block 6-32 and installed set screws into the idle ports and sealed the ports with Chemical resistance JB weld. Coupe runs like a top now. By the way, your video’s really helped my understand carb operation; which eventually led me to the solution of my dilemma.

Power valve adjustment

This is the power valve adjustment. Leave it alone and it should be plugged.

Power Valve

The Emulsion tubes are missing from my carburetor 

In some 2-bore carburetor applications main well inserts are used. They are larger than the main well tubes and set in a recess beneath the venturi cluster. The main well inserts surround the main well tubes and have calibrated holes that are similar to those in the main well tubes. The purpose of the main well inserts is to break up vapor bubbles which may form in the main well during hot engine operation. This prevents the vapor bubbles in the fuel, caused by heat, from disrupting carburetor metering and provides even fuel flow from the main well tubes and discharge nozzles. The main well inserts are used only on applications where engine heat causes excessive vapor bubbles in the main well area.

Not all 2 jets had these tubes. Sorry we don’t know which did and which didn’t, but they seem to work ok without them when missing. These are not being produced so you will only be able to get used ones if you can get any at all.

There is a small hole next to the power jet. 

This extra hole is a bleed hole for the power jet. Make sure it is clean and leave it alone.
Mike,
Thanks for your quick response. I have re-adjusted my floats on all three carbs. The check balls are in the main discharge (new check ball, spring and keeper). I ran wire through small passages in the venturi cluster. After putting everything back together, I ran engine and only the front secondary carb dripped during idle and shut off. I pulled the top off again, checked float and replaced the venturi cluster with another one. I started the engine and no drips were visible. After three or four starts the front venturi cluster started dripping on one side. When I removed my fuel lines again, there seemed to be a lot of line pressure at the front carb. I have a Mr. Gasket fuel pressure regulator inline set at 3 lbs. Everything that has been done to this carb also has been done to the rear carb and it does not drip. Any suggestions would be greatly appreciated. Very frustrating.

A carburetor that drips from a venturi cluster or booster when the engine is idling is caused by a vacuum leak or clogged air bleeds. The carb is not going into full idle transition so fuel will drip from the booster. Cleaning the booster assembly with air pressure and carb cleaner usually does the job. On the other hand dripping after the engine is turned off sounds like high fuel pressure! Mr. Gasket Fuel Pressure Regulator is the worst on the market. The pressure setting are incorrect and they often leak fuel. Napa discontinued the sale of this brand for the above reason. I installed one on a 1953 chevy PU, set for 4lbs to correct a overfill problem on a Rochester “B”. and still had the same problem. Checked the fuel pressure again and had 8lbs, no matter where I set the dial it remained at 8lbs!! The second regulator leaked fuel from under the pressure knob. Get a different brand and then check fuel pressure after the regulator with a pressure gauge. Thanks JIM L

Also check the venturi gasket to be sure it is laying flat. They sometimes are made a bit too big and need to be trimmed.

Car spits and sputters when I accelerate .

There could be multiple problems. Assuming it runs ok once the vehicle is powered up would mean there is a problem in the accelerator pump circuit.
Look down the carburetor bore and pump the throttle. You should see a strong squirt of gas coming out of the venturi.

Little or no gas indicates you have a stoppage, or something is installed incorrectly. To troubleshoot the pump check out the Rochester 2G, 2GC Accelerator Pump article.

Now if you are having problems on power after accelerating, then you problem is elsewhere, probably not enough fuel.

My marine 2GC doesn’t have a power valve

This is normal on some carburetors. The power circuit uses the jets and vacuum to supply extra fuel on power. Hi. First of all thank you for your videos I found them to be very helpful.
We have a 1968 Chevy Caprice with the 307 and the noted carburetor above. Car was hesitating and been sitting for a while so I got a rebuild kit cleaned it out and put in the new parts. It starts great, choke is working fine and has good power but the engine stumbles and the accelerator pump is not working correctly. I took it apart a couple of times and verified the orifices were pen (first try they were blocked and cleaned them out). Now when I push the accelerator the fuels kind of trickles out of the orifices. I disconnected the accelerator pump rod and when I manually pushed it down into the chamber farther it seems to squirt the fuel correctly?? Carb number is 7028103. Any suggestions would be greatly appreciated. After removing and trying different things and cleaning it numerous times I am about ready to give up. Thank you!!!

1st of all, the distributor can cause the same symptom.

Did you test the check balls. Don’t know if you have one in the pump well or not, but they need to be tested.
Here are some pages with this information: Check Balls

Accelerator Pump.

Resulting solution
I found out that the rebuild kit had an accelerator pump that was too small.
Buying my next carb parts from you guys!
Hey i have a rochester 2 barrel marine carb and the issue im having is we cant keep it running with out holding the choke butterly in a certain position. As soon as the choke fully opens it dies even if we have the throttle at say 2 thousand rpm any insite on this.

You could have a vacuum leak somewhere.
Using carburetor cleaner spray around the carburetor mounting gasket and around all vacuum lines and the intake manifold gasket. If this help the RPM then you have found the problem.

More likely you are not getting enough fuel. Passages may be getting plugged up with dirt and/or ethanol residue. Marine engine tend to sit around a lot, which allows for ethanol build up in the carburetor. The needle & seat may be sticking shut not allow enough fuel into the float bowl.

This is assuming the carburetor has not been rebuilt recently.

If it has been rebuilt then you have other problems. For example maybe the fuel pump isn’t putting out enough pressure. Venturi passages are very small and may be plugged. Clean out with thin wire. The float level may be incorrect not allowing the needle to open. Fuel filter may be plugged not allowing fuel to flow into the carburetor. Along with the in line filter, there is probably a filter inside the carburetor .

The bottom line is that you are getting too much air (vacuum leak), or not enough fuel. It’s hard to be any more specific not knowing the history of the carburetor.

Venturi Drips Gas

Fuel dripping from the booster at idle is either a vacuum leak or plugged idle circuit. Check carb flange gasket for correct fit. Remove air mix needle and blow out that circuit. Remove the venture booster and blow out all air bleeds in the booster and the main body. If that does not correct the problem then the carb could be over filling. Check the fuel pressure (like to see 4.5 to 5.5lbs max) and float level. Sometimes a back flush before doing any of the above might help. Start the engine and bring up to temp. Rev up the RPM to 2500 or so and cover the air intake with a gloved hand or rag while leaving the at same 2500 or so position. The engine will begin to die, but just before it dies let it recover. Repeat this several times. What happens is fuel goes through air/vacuum circuits and visa versa. If the idle circuits are dirty this often flushes them out!

Plugged fuel tank or float bowl vent plugged will also cause this.

Emulsion Tubes are Missing

Not all 2 jets use emulsion tubes, especially the marine.

Gas dribbles out of the throttle shaft when I turn the engine off.

If this happens only when the engine is hot, then it’s probably percolation.

For other times: Look down the carburetor after turning the engine off. Is gas dribbling out of the main discharge, or venturi? Check the main discharge check ball. It might not be sealing. This is done by testing the accelerator pump circuit.
It may be flooding and there could be many reasons for that.

#1 problem would be dirt in the carburetor.

Leaky needle & seat.

Leaky float.

Car bogs or hesitates

Can be caused by one of the below:

  • Electrical not up to par (especially distributor advance)
  • Accelerator pump cup damaged, or worn.
  • Plugged accelerator pump circuit.
  • Off Idle circuit plugged.
Updated on 01/01/2021

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