Hi,
I'm writing this for a neighbor w/ a '72B.
Here is the problem. Driving around town below 45 mph the
car runs fine. On the highway, after a few minutes at >55mph
the engine misfires and speed quickly drops off, car slows down
to ~30mph. After a few minutes at ~30 mph power comes back, car speeds
up and the whole process starts all over again.
Kind of frustrating, to say the least. Also note, it is not so much the speed
as the engine RPM. It occurs in lower gear as well, but as he doesn't hang
out in them as much he doen't notice much there.
He says it sounds like a valve mistiming and or a sticky valve.
Car specifics:
1972 MGB.
18V engine that is not original (I don't know engine number, but
could find out if important.)
Weber down draft carb.
No catalytic converter.
Things that have been checked and replaced.
-Spark plugs and leads (several times).
-Points (several times).
-Carb rebuilt.
-New double row timing chain
-Head rebuilt (valves relapped, new springs, etc.)
-Distributor replaced.
-At one point a mechanic mentioned that is might be sludge in the
gas tank so that was removed this weekend. A bit of rust, but
little sludge. (We were really hoping for sludge.)
My neighbor is really frustrated w/ the car at this point. Would love to
drive it, but it is a bit dangerous. He says the only thing that helps at
all is to put a some valve cleaner in the tank, and this allows him
to get up to 60 mph before the trouble begins.
So what could it be??
Might the distributor be incorrect? Should we check the advance curve
to make sure the advance is properly functioning?
Maybe a bad float valve in Carb causing fuel starvation at high speed?
I believe it has a SU fuel pump, so pump pressure should not be a problem.
What else could it be. I think it would be easier to track down if every
time it hit 3000 rpm it misfired and when backed off runs fine. But,
it continues to misfire for a while there after.
Any help as to what to check next would be appreciated.
Thanks
-Dave
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