intermittent WOT problem on 4.3 OMC

[mlrman]

Hi,

I have one of those weird, intermittent problems that is hard to trouble shoot.

My 4.3 OMC usually runs great and hits about 4600 RPM at WOT with a little trim adjustment. This is the recommended RPM's for this motor.

However, all of a sudden, the RPM's will sometimes drop about 1000 RPM, and I can hear a popping or misfiring noise from the engine, and lose about 8-10 MPH.

Here is what's strange...I have noticed sometimes when this happens, if I shut off the engine and re start it, then it will run fine for a while.....but then it happens again.

Maybe a loose ignition wire or part?

I don't think it's timing, fuel, or compression, otherwise it would always have a low WOT.

The boat is a 1991 Sunbird Corsair CC with a 14.5" x 19 prop and will run almost 50 MPH @ 4600 RPM with just me in the boat....if I can keep it at 4600 RPM's though and thats the problem.

Thanks for any ideas!

 

[superbenk]

Re: intermittent WOT problem on 4.3 OMC

Sounds like a fuel issue to me maybe mixed with timing. There is a thread in the Adults Only section on troubleshooting fuel issues: http://forums.iboats.com/showthread.php?t=283269

 

[searay3]

Re: intermittent WOT problem on 4.3 OMC

Sounds like it could be fuel contamination. Maybe a puddle of water washing around in the tank, gets stirred up and gulped into the fuel pickup. If you have a water separator, dump the filter contents in a clear jar and see whats there. If not, maybe add one...

 

[mlrman]

Re: intermittent WOT problem on 4.3 OMC

Guys, I appreciate your posts, and I'll try both suggestions.

But the fact when it starts running rough, and I restart the engine and it runs fine seems like an electrical issue.

This boat usually runs great, then like hitting a switch, it starts misfiring and loses 1000 RPMs. If I restart the engine, it will run fine again, for awhile.

 

[superbenk]

Re: intermittent WOT problem on 4.3 OMC

Guys, I appreciate your posts, and I'll try both suggestions.

But the fact when it starts running rough, and I restart the engine and it runs fine seems like an electrical issue.

This boat usually runs great, then like hitting a switch, it starts misfiring and loses 1000 RPMs. If I restart the engine, it will run fine again, for awhile.

I recently dealt with a similar issue that turned out to be a sticking anti-siphon valve that was intermittently causing stuttering and bogging issues. I replaced the valve and the boat's been running great since. The boat would run fine for a bit until it started starving for fuel, it'd cough/sputter/bog, then pick up again. Might be worth checking out.

Also check your coil. I think there's some steps in the Adults Only section for troubleshooting your coil and ignition system.

 

[mlrman]

Re: intermittent WOT problem on 4.3 OMC

I recently dealt with a similar issue that turned out to be a sticking anti-siphon valve that was intermittently causing stuttering and bogging issues. I replaced the valve and the boat's been running great since. The boat would run fine for a bit until it started starving for fuel, it'd cough/sputter/bog, then pick up again. Might be worth checking out.

Also check your coil. I think there's some steps in the Adults Only section for troubleshooting your coil and ignition system.

thanks.


I am curious...what led you to isolating that anti siphon valve as the culprit?

 

[superbenk]

Re: intermittent WOT problem on 4.3 OMC

thanks.


I am curious...what led you to isolating that anti siphon valve as the culprit?

Spending lots of money replacing all the other fuel related crap on the boat - it was about the only thing left to check/replace