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Mercarb Marine Troubleshooting

Mercarb Accelerator Pump

Accelerator Pump Cup Isn’t Lasting

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.

We recommend you use Ethanol Defense in your gasoline.

Ethanol Defense


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.

I recently rebuilt a 2 barrel Mercarb on a Mercruiser 470 with a kit from you. It starts and idles well, but after I accelerate, it will not return to a low idle. When I shut off the engine for a few seconds, it will be at low idle when it starts up again. It also often has a hesitation going to full throttle from idle. Could this all be accelerator pump related? It seems like the accelerator pump linkage does not return all the way when I back off the throttle. I can push on the linkage by hand and return the engine to low idle. I am stumped on this. I have removed, cleaned, and re- adjusted the carb 3 times after trips to the lake and I still have the same problem. 

What you are describing is most likely the accelerator pump action.

Start by disconnecting the throttle cable to eliminate a cable problem and pump the carburetor to see if the pump action returns OK.

At the same time look down the carburetor to see if there are 2 good squirts of gas coming out of the venturi when you pump the gas.

Is the pump return spring installed below the accelerator pump?

It is possible the pump was connected in the wrong direction. Looking at the pump stem (top). The end goes toward the inside of the carburetor, then the clip installed.

This page has information about the pump adjustment and also how to test the accelerator pump circuit before putting back on the engine.

Idles Erratic or Idles High

  • A vacuum leak can cause this. Spray carb cleaner around the mounting, any vacuum lines and the intake manifold. If the idle smooths out, or speeds up then you found the problem.
  • Check the idle mixture screw for bends, or scoring on the tapered end. This is caused by people tightening the screw too tight.
  • Unhook the throttle to eliminate that possibility.
  • Look down the carburetor at idle. If gas is dribbling out of the venturi then you have a leak at the main discharge. Take the top off and remove the T and spring. Tap the check ball lightly to form a better seal. Oh and make sure the spring and T are there. Also the biggest check ball should be there. After doing this check the accelerator pump circuit on the bench.
  • Look at the throttle plates with carburetor off of the engine. Turn the carburetor upside down to see if the plates (valves) are fully closing. Any opening will cause a high idle and you won’t be able to adjust the idle down. The valves may have been put in upside down, or backwards.
  • Of course adjust the idle mixture to get the smoothest idle.
  • There is an outside chance the fuel pump is getting weak. If it runs OK at high RPM then this isn’t the problem.

Doesn’t Start After Turning Off

  • The 1st thing you need to figure out is if there is too much gas in the intake, or not enough.
  • After turning off the engine, look down the carburetor. If you see gas dribbling out of the venturi then you have a leaky main discharge. Take the T & spring out of the discharge and tap lightly on the check ball to make a good seal.
  • If the engine is cold and won’t start:
    • Engine off, look down the carb and pump the gas once. You should see a good 2 squirts of gas coming out of the venturi. Engines need an extra bit of gas when cold to start.
    • Is the choke valve closed? It’s possible the electric choke thermostat is bad.
  • The gas may be siphoning back after turning off. Disconnect the fuel line after stopping, then before starting connect the fuel line. If it starts then the fuel is being siphoned by a bad fuel pump, or a vent is plugged off.
  • It is possible the float level is so far off that the fuel bowl isn’t filling up.
  • Test the fuel pump for proper pressure. Check the motors manual but 4.5 lbs would be about right.
  • Check the hose that runs from the fuel pump to the carburetor. If there is gas inside the hose then you have a fuel pump problem.
Updated on 06/10/2021

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