You say "after a few minutes" - is this one or two? Five? Twenty? If
sooner rather than later could be fuel starvation e.g. blocked filters or a
dicky pump. Does pulling out the choke cure it? This sometimes compensates
for a weak spark. If you get the missfire and immediately stop does the
misfire continue if you rev it while stationary? Have you done this in the
dark and looked for stray HT sparks flying around? If you hook up a timing
light you may be able so see it is an electrical problem i.e. erratic
flashing. Is the rev-counter steady? If so that indicates the LT ignition
is OK, so if the timing light flashing is erratic it could be condenser,
coil, or possibly cap, leads or plugs. Have you looked carefully for water
dripping from the heater valve or head gasket onto the electrics?
PaulH.
-----Original Message-----
From: David Tulchinsky <tulch@ccsalpha3.nrl.navy.mil>
To: MG List <mgs@autox.team.net>
Date: 04 October 1999 14:28
Subject: engine misfire
>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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