TIA to all.

1978 Chrysler 318 V-8 gasoline engine, www.ctow.ca/Trojan26, hour-meter
coming up to 1000 hours, about half of them in the four years I've owned the
boat.

Recently the engine began to run rough, stumbling quite badly. This happened
several times about twenty minutes out of port, trolling along in the
1200-1400 rpm range. Twice, I have then put the boat up on plane to scoot
home (3200-3400 rpm), only to find that half-way home the whole problem
disappears quite suddenly and dramatically, the engine smooths out, revs
pick up and I have to pull back on the throttle.

One other observation: when misbehaving the port exhaust manifold (I think -
the thing with the risers leading to exhaust hoses, muffler...) stays cold.
Once the problem clears up, both manifolds get warm.

My mechanic found that some of the exhaust manifold plugs had corroded out,
so he retapped and replaced them, cleaning up the engine compartment air
(dangerous to be exhausting CO into engine room, I know), and he said that
likely explained my problem: that after a while the engine area filled with
exhaust which didn't support combustion well enough. Running on plane
scooped more fresh air in through the vents and so it cleared up. (Didn't
explain the cool port manifold however...)

The engine did sound much better with the plugs replaced, but the stumbling
after twenty minutes of low-speed operation recurred. I checked both the
water-separating and inline fuel filters, and found a trace of sediment in
the water-separating one, but no water. I cleaned the air filter, and
brightened and tightened the battery connections. The mechanic checked the
distributor by feeling for shocks, but got none. Of course, when I took him
out to demonstrate the problem, it didn't occur, although we went right upon
plane rather than reproducing the experimental conditions exactly.

So, yesterday I changed the spark plugs (reminding me yet again of the joys
of standing on my head for hours in a tiny space in the bilge). The old ones
were blackened, and I checked a few gaps but didn't really dwell on this (no
pun) and they seemed OK. The new plugs were in the 0.032" range. The old
ones, especially at the aft end of the engine which sits lower due to the
engine not being level in the boat (straight-shaft inboard), were oily on
the threads, and I think that the further aft one had oil further into the
block on the threads ... which seems strange to me. They're very hard to get
at with a spark plug wrench, and I'm wondering if they were never quite
tightened in enough. (I was careful not to overtighten, however - having
stripped threads once many years ago...)

I was going to change the leads, too, just for thoroughness, but couldn't
find the right marine set late on a Saturday afternoon. Will have another
look Monday. The distributor cap and electronics control module were new two
summers ago (about 200 engine hours ago), as were the plugs and leads.

Then, I took the boat out for a prolonged low-speed cruise - and everything
worked perfectly. The weather was a bit cooler than it has been here (26C
instead of 32C) and a bit windier, so again my original conditions could not
be exactly replicated, but I'm hoping the problem had something to do with
the plugs.

Summary: was running rough after twenty minutes of low-speed operation, port
exhaust manifold cool, cleared up after a few minutes at planing speed.

Anything obvious that I'm missing?

Thanks again.

Charles

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Charles T. Low
www.boatdocking.com

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