Boat Stalls at Certain Speed (1986 GM 140 Sterndrive Inline 4cylinder)

[rlambert]

Hi,

I've posted before but since have done several tests to rule out certain problems.

The problem is that at a certain speed/rpm (7mph) the motor stumbles and will stall if you continue to push forward the throttle. It usually backfires through the carb when it stalls. The problem only happens under load. In neutral the engine rev's up fine with a little lag in response.

I understand that this is usually a fuel lean situation in the engine. I did check compression and have 135-140psi straight across. I have already rebuilt the carburetor with special attention to the accelerator pump and power piston and valve, changed the distributor, plugs and wires. I recently changed the fuel filters. The only thing I've yet to confirm is the fuel pressure being delivered by the fuel pump and the manifold vacuum but I can check that once I buy the proper equipment.

I first noticed the problem a year ago but you could usually get past the rough spot after a few tries. The problem has progressively gotten worse and now it never gets past the rough spot. Other than the fuel pressure and vacuum is there anything else I'm missing here. The engine is mechanical advance on the distributor but I replaced it so that should rule out that. Can the carburetor still be bad even though I rebuilt it and it looks very clean. Power piston works fine when I apply vacuum to the port. Any help????

 

[cr2k]

Re: Boat Stalls at Certain Speed (1986 GM 140 Sterndrive Inline 4cylinder)

If you replaced the distributor with one with a vacuum advance DON'T RUN YOUR BOAT. You have violated USCG Laws and put yourself in danger.

If you have a vac port on the carb for the dist, that is probably not a marine carb either. This will cause that problem.
If you used manifold vac for a distributor that will cause that problem too.

To my knowledge there are NO Marine approved (read spark sealed) distributors that have vacuum advance.

Take the centrifugal one apart and clean and lube the weights and check the springs.

Did you use the new plunger assy in the carb kit? If so I would check the spring in it against the old one. Alot of the newer after market kits use a softer spring, which can enhance that problem.

You did completely clean out all the accelerator pump circuits and put the correct check balls (2 one aluminum and one SS) in the proper places? Also does the engine respond to needle valve adjustments? If so try it a little richer. You are stumbling at the transition from idle circuits to main jets.

 

[rlambert]

Re: Boat Stalls at Certain Speed (1986 GM 140 Sterndrive Inline 4cylinder)

Thanks for the response Cr2k. I did not replace the distributor with a vacuum advance unit. They are both mechanical advance only. It is the proper Sierra replacement part. The problems have occured before and after the following part replacements:

1. Distributor Replacement
2. Carb Rebuild
3. Fuel Filter Replacement

Nothing I have replaced has made the problem happen better or worse. That is why I'm ruling them out. Carburetor changed accelerator pump and spring and everything else. It's all in the right spot. Boat runs great other than this problem. It will idle all day just fine. Could this be the fuel pump, blocked fuel line or something in the carburetor that doesn't come with the standard rebuild kit?? The vacuum leak I was talking about that I would test for would just be in the manifold so that the vacuum that draws in the fuel through the carb would be comprimised. I've heard that this can be an issue.

 

[Don S]

Re: Boat Stalls at Certain Speed (1986 GM 140 Sterndrive Inline 4cylinder)

(1986 GM 140 Sterndrive Inline 4cylinder)

Mercruiser or OMC. There are differences.

Have you checked for water in the fuel?

Might want to test your fuel system as described in this thread. Sounds like you are starving it for fuel.

http://forums.iboats.com/showthread.php?t=283269

 

[rlambert]

Re: Boat Stalls at Certain Speed (1986 GM 140 Sterndrive Inline 4cylinder)

All parts are mercruiser except for the distributor which is the replacement Sierra part. The old distributor was original and I thought the weights might be bad and the cam was quite worn but changing it really did not make a difference in the mentioned problem. I think you are right. There is a fuel supply issue. It may be my fuel pump or some part of the "boat side" fuel system. I will conduct the tests described in the article and let you know how it turns out. Thanks for the link.

 

[cr2k]

Re: Boat Stalls at Certain Speed (1986 GM 140 Sterndrive Inline 4cylinder)

Often the check valve will get corroded. The one right on the out line of the tank. The USCG mandated one. I have seen many that are aluminum.

Get one of those vacuum pumps like for bleeding brakes, then you can see if it is drawing and how much vac it takes.

To start with you can take an old OB tank or even a small container of gas and just plumb it to your fuel pump and see if that helps. Then you will know what side of the pump the problem is on.