Well, my vote, based on the description of the problem, goes to a
sticking valve. If the problem cleared up immediately on return to
lower engine load and speed, I'd go with a fuel pump/line problem,
but if it essentially goes away as the engine cools a bit, I'd say
valve. One thing to check is whether the valve clatter increases as
it loses power--greater valve lash as the valve sticks open. Also,
the fact that a gasoline detergent valve cleaner seems to help would
point in this direction. Even with a fairly recent valve job, things
can go wrong. I had an engine with Rimflo valves and iron guides (I
know, I shouldda used good bronze ones, but my machinist, a hot
rodder, liked the wear properties of iron) which built up enough crud
to stick badly in about 3,000 mi. If you can get it really hot to
the point of missfire, and then do a compression test, this might be
conclusive.
It might be worth getting a low pressure air gun and blowing out the
fuel line back to the tank, running the fuel pump with a hose in a
graduated container for a time to check volume, maybe checking the
rubber hoses for deterioration. But I go with valves...
Chris Attias
'64 MGB
'84 Alfa Romeo GTV-6
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