OMC 350 dying

OMC 350 runs good for about an hour then quits....slight hesitation then dead. Slow cranking and if it DOES start will only idle back to the marina. I have replaced the coil, battery and alternator...all new today. Same thing happened after about an hour and a half. Engine temp is only about 170 degrees. Getting fuel like it should. Any ideas where to look next? I'm stumped.

 

Ignoring the slow cranking, this sounds like a fuel problem: low pressure in the fuel line due to clogged vent, fuel filter or debris in the tank.

The slow cranking points in a different, more expensive direction: high friction somewhere in the engine.
I'd remove the plugs, do a compression test and try to rotate the engine by hand with a spanner. If results are inconclusive, remove the valve covers and look for any discoloration on the cam shafts or damaged cams.

 

WHAT ELSE HAS BEEN DONE TO THE MOTOR RECENTLY ??
RINGS AND OR BEARINGS ???
DO YOU HAVE A OIL PRESSURE GAUGE ?

 

Turn the engine over by hand when it's cool and *best as you can* make note of how hard it is to rotate. Do the same thing again once it's exhibiting the symptoms you're seeing. Slow cranking when warmed up can mean, (but certainly doesn't necessarily mean), a friction issue. Starter motors typically crank more slowly when the engine is warm, if the starter itself is beginning to fail.

What is the voltage at the battery with the engine off, engine running, and engine cranking? During the slow cranking episodes, does either the cable itself or any of the connections along the way get warm or hot?

How have you verified that the engine is getting adequate fuel?

What about ignition? This engine's ignition system probably doesn't have separate start/run circuits, (I don't have a schematic to verify), but it's worth investigating. On some older stuff, I've seen the "run" circuit, (feeding the ignition through a ballast resistor), fail when the resistor itself opens after getting hot, or.....the connections on the resistor breaking down after being exposed to multiple heating/cooling cycles, dropping the output voltage to a couple of volts lower than it's supposed to be.

The amount of time it takes for the problem to show up is what's strange here.....

 

How did you determine it is getting fuel as it should?

 

I am with gonzo on this question. I know a number of folks who have had this complaint ended up cleaning or replacing the anti-siphon valve at the tank outlet...