1. Home
  2. Technical
  3. MERCARB
  4. Mercarb Marine Troubleshooting
Mercarb Marine Troubleshooting

Mercarb Carburetor Flooding at Idle

  • Engine is running rough due to ignition problems - we will not address the various causes.
  • Choke valve isn't opening completely when the engine is at operating temperature.
  • Idle mixture incorrectly adjusted.
  • Needle & seat is damaged - putting too much pressure on the needle can damage the viton tip.
  • Inspect the seat closely to be sure the gasket is there and not cracked.
  • Check the seat gasket surface area to make sure old gasket material is present.
  • Check closely for cracks around the seat.
  • Float level , or float drop is off.
  • Float is bad. Mercarbs have Nitrophyl floats and over time they will absorb fuel. Did you replace it when rebuilding?
  • Fuel pump is putting out too much pressure. Check the pressure. Consult your engine manual for proper pressure. New fuel pumps are especially suspect.
  • Check the site tube (clear plastic tube) that runs from the fuel pump to the carburetor for kinks, or collapsing.
  • Make sure power piston is moving up and down freely. Any sticking may cause too much gas.
  • Be sure the gasket under the power valve is there, not duplicated, or cracked.
  • Look down the carburetor while idling. Is there gas dribbling out of the venturi? If so then perhaps the main discharge check ball is leaking and fuel is being siphoned.
  • Follow the Idle mixture hole in the bore up to the top of the venturi. Blow through both (idle & off idle port) and see if air is getting through the venturi. Might be plugged.

Mercarb Accelerator Pump Cup Failing Prematurely

In almost every case this is caused by the ethanol being at a high level. This occurs when the carburetor sits for weeks at a time and the ethanol separates from the gas. Even ethanol ready pump cups will fail if the ethanol concentration is too high.

  • Before installing a new accelerator pump polish the pump well so that the cup glides smoothly.
  • Carefully install the new pump making sure the cup does not curl over.
  • Keep all chemicals away from your carburetor. When using carburetor cleaner to clean the carburetor, wash with hot water to remove all chemicals.
  • Buy non ethanol gasoline whenever possible. Most marina fuel is ethanol free.
  • Use Startron in your gas. This has been demonstrated to be an excellent solution to the ethanol problem.

Mercarb Low RPM at Wide Open Throttle

Carburetor runs fine except at wide open throttle.
Assuming the engine compression is OK, the plugs are good and other ignition parts are good I suspect a lean condition.
  • Test the fuel pump pressure - around 5 lbs, but check you motors manual to be sure.
  • Look at the float level. It should be level when you have the top turned upside down.
  • Check the float drop. Around 1" depending on the application.
  • Move the float up and down to make sure it isn't catching somewhere. It doesn't take much.
  • Check the power piston. It should move up and down freely.
  • Does the power valve pin move up and down freely? Also make sure it lines up with the power piston plunger.
  • Look under the power valve for any foreign material.
  • Check the power valve itself for foreign material.
  • Do you have the correct carburetor on the engine. There are some differences between the Mercarb carburetors that could affect power.

Gas Dripping From Venturi
2 Jet Venturi
Mercarb is a copy of the Rochester 2GC and the problems encountered are similar. 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! Good Luck









Doesn't run unless I close the choke. Engine may backfire.


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.
Check the idle mixture transition slot. As the throttle is opened more of this slot is exposed allowing additional fuel/air to be introduced. This gets the engine over the space between idle and the accelerator pump. Follow this slot all the way up to the venturi to made sure the passage is open.
Make sure all the small passages in the venturi are clear. They are small and easily plugged.
Make sure the gasket under the venturi is flat. They sometimes need to be trimmed.

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 "Mercarb" 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.

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.
How the Mercarb Choke Works
been watching this several times and rebuilt mine. however I am running into fuel flooding my intake...so I am getting a no start (marine... mercruiser 3.7 165. I have taken my carb apart several times to check things... notice the power piston refuses to stay in its place. I can peen it in place...but just wondering what is causing fuel to just flow into my intake. accel pump works great. the float bowl is only half full each check...float is in spec. (using the spring needle and seat setup)

Staking the power piston is the correct thing to do. Just make sure it move up and down freely.

Did you replace the float? Also be sure to run thin wire through all of the small passages. A plugged vent would cause the gas to siphon off at idle. Also test your main discharge. If it isn't correct, that would also siphon fuel. Check ball, then spring, then T. You can also test the check ball to make sure it seals. Hold the check ball down with a brass drift punch, put some liquid in the pump well and press down on the pump. There should be a little pressure if the check ball is sealing.
I have a 1990 Mercuriser 4.3 with a two barrel carburetor. I am getting fuel all the way up to the filter on the carburetor I pull the fuel line off and turn the motor over and the fuel pump is working .But can get fuel into the carburetor would this possibly be that the float is stuck up ?

Could be a stuck needle & seat, but your test of the pump isn't good enough. You need to use a fuel pressure tester. There is also a volume test you can perform, but I don't know what the measurement should be. Fuel pump pressure is around 7 lbs. Refer to your motors manual to be sure.

Another good possibility is the carburetor is dirty and needs to be rebuilt. Boats sit around quite a bit and that is very hard on carburetors. Ethanol seperates from the gas and then attacks rubber. Corrosion also builds up.

Hesitation when accelerating

Please note that this can be caused by a distributor advance problem.

Asside from the distributor the problem is probably in the accelerator pump circuit.
Look down the bore (with engine stopped). Pump the accelerator pump. You should see a strong stream from 2 different locations. If you don't, then run thin wire down the small passages.
Check the main discharge. There should be a ball, then spring, then a T. While holding the ball down with a brass drift punch try to pump the accelerator pump a bit. You should get some pressure. If not, then tap on the brass bunch a few times to seat the ball. Tap easily. Also be sure you have the bigger check ball here. Too small and it will get stuck in the hole.
With the ball, spring and T in place, pump the accelerator pump. You should get fluid out of the main discharge. (we use mineral spirits to test).
Inspect the pump to make sure the rubber cup didn't curl up when installing.
Some of the Mercarbs have a check ball in the bottom of the pump well (aluminum smaller ball). When pressing down on the pump, fluid should go into the main discharge and not back into the float bowl. If the check ball leaks, then fuel will return to the float bowl. NOT all Mercarbs use this check ball. If it does, there will be 2 holes near the bottom of the carburetor.

What spring do you use for the pump return?
There isn't a direct replacement but we've found this one to work.

Do all Mercarbs have an unloader?
No, on many of them this function is built into the torque and deflection of the choke.

This Baffle Around the Float Valve is Missing

This is necessary to operate at 100%, but can be used without it.

Can the Power Piston Be Cleaned Without Removing?

Better to remove it so it can be cleaned, but yes, as long as the piston moves up/down easily. There is a relief hole in the throat next to the piston. Spray silicon spray lubricant through this hole.

Engine dies when letting up on the throttle

  • The most likely suspect is a dirty carburetor - The Mercarb is sitting on an engine that sits more than it is used and over time gas can turn into varnish and coat the inside of the carburetor and the gas tank. This plugs the small passages necessary to mix the fuel and air at idle.
  • Flooding - Look into the carburetor when it dies. Does there seem to be too much fuel. If so, then check the article above about flooding.
  • Check the RPM at idle. If the idle screw is turned too far out, then the throttle valve may be closing and cutting off the air. The throttle valve will always be open a bit even at idle.

RPM won't return to normal after throttling up.

Did you have the throttle valve out? They may be in wrong and not closing all the way. Linkage bent not allowing throttle to return. Remove the throttle to take it out of the equation and operate by hand. If it doesn't have the problem then obviously it's the throttle cable.
Still does it? Look carefully at the fast idle cam. Is it binding, or staying on the high step.

Only runs when the choke valve is wide open

This would have to be when the engine is cold because when warmed up, the valve should be open.
So assuming it is cold:

This tells me that when the choke valve is closed there is too much gas being pulled into the engine.
  • Check the fast idle. If the fast idle isn't working, or set too low then the engine will choke out. The fast idle opens the throttle valve so that more air is used.
  • At idle look at the venturi to see if gas is dribbling out of it with the valve open. That would indicate gas is getting siphoned from the main discharge maybe due to the check ball not seating, or spring above the check ball missing. Also test the check ball to make sure it is seating.
  • Is there a vent hose venting the float bowl to the spark arrestor? If so, make sure it is clear and not kinked.

Engine will not start after running for awhile

1st make sure the electrical system is in good working order. For example a bad coil can cause problem when it gets hot.

Here are things you can look at (not in any order)
  • Look down the carburetor after turning the engine off. Is gas dribbling out of the venturi? If so, then you probably have a problem with the main discharge not sealing, That is where the checkball, spring & T reside. Also plugged idle vent passages will cause gas to be siphoned out.
  • Fuel is getting hot and percolating out of the carburetor. Look for fuel lines near the exhaust.
  • Spark arrestor should be clean.
  • Carburetor vent hose kinked, or perhaps plugged off.
  • Make sure none of the ventilation is blocked off, letting the engine get too hot.
  • If you suspect percolation, or vapor lock, wrap the fuel line with pipe insulation and metal duck tape.
  • Try starting with throttle wide open. If it starts right away then you have a flooding situation. Opening the throttle gives is more air, less fuel.
  • When starting warm move to 1/4 throttle.
  • If you suspect it's getting too hot. Remove the engine cover after turning the engine off, then try again. If it starts then you have a heating problem.
  • Are you getting too much gas? Pull a plug and make sure it's gray or tan. Black indicates flooding.
Can't find what you need? Contact Us
TOP