Yep, had exactly the same problem with a 4.3. Not being any sort of expert
I spent days trying all sorts of things. I was about to check the timing
advance with a old cheap neon strobe light I had knocking around, the type
that plugs between the spark lead cap and the spark plug. I noticed the
flashes from the light were very erratic on all cylinders after the engine
had warmed up. Cleaning and de-crudding the distributor and rotor arm
contacts resulted in strong regular flashes and a smooth running engine. It
worked for me, but I don't quite know why.

Steve

"sudsy" wrote in message
om...
need help. my 1990 mercruiser 3.0L runs fine for the first 1/2 hour
or so. then after it sits while we swim or change skiers it starts
and revs fine, but runs very rough and even stalls when put into gear
and given gas. Not a fuel or fuel system problem since it always runs
good for the 1/2 hour or so. this problem is driving me crazy. any
suggestions welcome.