Reply With QuoteYour timing could be off.
I have had a problem recently of my starboard motor surging at around 2800 rpms. This only happens under load (when I am running the boat).The boat will refuse to go past 3000rpms. When the boat is in neutral it revs to 6200 rpms without a hiccup.
The engine in a 350 small block with mercury racing 700cfm carb, mechanical fuel pump.
So far I have checked distributor, replaced coil, rebuilt carb, checked fuel pressure from fuel pump..all this to no avail.
These engines have not given me any problems before this.
Any help appreciated as I running out of possibilities.
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Your timing could be off.
My first thought would be some fuel delivery problem but you seem to have checked most of those bases. A bad vac. leak should show up worst at idle. Yea, I'd turn to possible ignition troubles, maybe advance stuck not advancing?
Hate to throw a bad thought in there ............ does it run OK below 3K? Slip a timing chain set one tooth maybe? I've never had it happen so I'm unsure of just what the result would be.
It's a world different just whinging a motor free as opposed to loading it to work.
Sounds like you've checked the obvious things. If the fuel pressure/flow is up to snuff, and the distributor is advancing properly it's time to start looking at hard parts. I.E. broken valve springs, bent pushrods, or worst yet: a cam gone flat. Pull the valve covers and inspect the valvetrain carefully. Then roll the motor over with a breaker bar and watch the valves move. If one or more moves less than the others you've got a cam lobe issue. A flat cam will rev higher unloaded than loaded and sometimes cause a surge due to the funky vacuum signal created by the bad lobe. I don't recommend free revving your motor to 6200 rpm tho. Also, do you have water separators on both engines, or do they share one common from the tank?
Good luck
fog
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