I agree with the fuel starvation theory, I have had su pumps do this in the
past. Also check for bad or dirty connections at the pump. With a Weber I
don't believe that you would have to add a fuel pressure regulator when
adding an aftermarket pump, none of my weber buddies seem to have one. YMMV
Good Luck! Don Walton
73 74 77 B's
10:12 AM 10/4/99 -0500, you wrote:
>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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