David, the symtoms are classic for fuel starvation. Look first for a clogged
fuel
filter. John
David Tulchinsky wrote:
> 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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