On 2007-09-26 11:17:09 -0400, timW said:

On each occasion the same thing has happened. I give the engine a
couple of pumps with the throttle, as instructed by the previous owner,
leave the throttle slightly open, and turn the ignition. After a couple
of tries it starts and all seems well, until I throttle it back to
idle. The engine ticks over for maybe 10 - 20 seconds, then begins to
sound progressively rougher until it cuts out altogether. After that I
can't get it running at all. The first time, on the trailer at home
(yes, I did have water running to it) I managed to coax it to life half
an hour later, and kept it revving for a while before I set it back to
idle. But when we went to launch the boat on the weekend, the damn
thing cut out on me at idle and refused to start up again, even after
we let it sit at the dock for half an hour, trying over and over. I
virtually drained the batteries in my attempts.

Bob's answers are reasonable, but...

For some reason, perhaps the new manifolds, vacuum leak popped into my
head as I read yours. On some old cars (and the marine i/o I tried to
help last weekend), the choke is both vacuum and electrical (actually
heat). From a cold start, the choke is closed, but it opens by itself
even if you only leave the key on.

Standard stuff: Are the plugs wet, are you getting spark, smelling
"flooded"? Does ether (starting fluid) change anything? Are you simply
cranking or getting an occasional "bump"? How's the compression (by
number for each cylinder, not just "stick your thumb over the plug
hole)? Is the advance correct at BOTH target RPMs?

Odd item: Sometimes one of the springs on the centrifugal advance break
or the plate is stuck and can't rotate back easily. At lower RPMs, the
spark is too advanced for a while.

Another odd item: Vapor lock from fuel running too close to a hot item
(like the manifold).

Frankly, I suspect the carb as I pretty much had to rebuild ours every
year when I ran gas engines. That's why I have a diesel inboard and
electric lawn mower & chain saw and....

If the "Car Guys" are still doing their weekend sticht, I might call
them if I were in your shoes.

--
Jere Lull
Tanzer 28 #4 out of Tolchester, MD
Xan's new pages: http://web.mac.com/jerelull/iWeb/Xan/
Our BVI pages: http://homepage.mac.com/jerelull/BVI/