Idle and Tach problem - OMC800/351 Ford

[emsisson]

Over the past few weeks I've noticed when starting, my tach jumps all over the place, but the engine seeems to be idling fine. The engine will be running about 700RPM but the tach will indicate 1500, then drop to 500 then zoom up to 2000. I've also had a problem with running at idle, the boat runs fine for 5 or 10 minutes then the idle suddenly drops off and the engine dies.

I've also had a problem re-starting after the engine is warm, the engine acts as if the battery is weak, but it's a new battery with a full charge.

I have this little voice in the back of my head saying that maybe some of this is related to failing exhaust manifolds, at least the hard starting part. Can a leaking exhaust manifold affect starting?

The boat is a 1978, which I completely rebuilt in 1997-1999.
I'm starting to think about repowering. Has anyone switched from an OMC800 to a Volvo Penta Duoprop?

 

[Don S]

Re: Idle and Tach problem - OMC800/351 Ford

You may just need a tuneup and plug wires. Arching from the coil tower to the + terminal can cause the tach to act funny, and bad wires can cause the arching due to high resistance in one of the plug wires or sparkplugs.
Water can cause starting problems if water gets into the cylinders. Too much water and you get a hydrolock, a little water kills the spark. But water doesn't effect the tach.
Repowering to anything will requier a transom rebuild, NOT just a fill the hole. The transom is what supports most stern drives, the stingers like you have now are not supported by the transom.