In a message dated 6/8/99 12:10:33 PM Pacific Daylight Time,
paul.hunt1@virgin.net writes:
> With it running-on, measure the voltage at the coil and try pulling wires to
> see if you can find one that makes it stop. On the other hand, the
> anti-overrun valve may stop the problem, even though I believe its main
> purpose is to stop Dieseling.
diseling and run are the same thing they refer to the engine wanting to
continue to turn after the key has been turned off. Assuming that no
rewiring of the car was done recently I think that the clues are in the
original post. (IMHO)
> >
> >I am having a problem with my 1977 MGB retrofitted with Dual SU HIF-4's.
> >About a month ago, I gave my car a good tune up: new NGK plugs, wires, cap
> >and rotor, the carburetors where running a little rich, so I leaned them
> out
> >just a little bit. The car runs beautifully, 23+ MPG,
lean mixtures run hotter combustion temptures
> > but with the coming of
> >hot weather the car has started to run on very badly.
Lean misfire and or preignition. If some of the carbon in the combustion
chamber gets red hot it can cause the fuel to preignite (like over advanced
timing) even the tip of the spark plug can do this.
> > I am running 87 octane
> >right now (due to price) but the gas here in Bakersfield, CA is of fairly
> >good quality, and the car has not been overheating.
The difference between regular and the higher grades is that premimum is more
knock resistant, actually it doesn't burn as fast. Also, combustion
temptures do not directly relate to coolant temp.
> >When I turn off the key, the ignition light glows very faintly (as opposed
> to
> >glowing very brightly as it did before) and the car continues to run...I
> >don't mean dieseling, I mean a slight miss, as if I could still drive it
> >around the block. If I rev the engine, the car misses but revs easily
> >(sometimes the engine will stall while I am revving, and then the engine
> will
> >diesel very badly) ...I have to let the clutch out with the tranny in gear
> in
What I suspect is happening here is something inside the combustion chamber
is getting super hot from the lean mixture (a chunk of carbon, the tip of the
plugs) and it is acting like a glow plug that as long as there is fuel and
air coming into the cylinder will ignite it. Anti-run on valves work by
shutting off the fuel, without fuel cars don't run on or diesel or what ever
you wish to call this. Since you have retro the car to SU I doubt that you
have a anti-run on valve.
> >order to stop the engine, and only then does the ignition light glow
> >brightly.
OK now I'm confused, In your last paragraph you said the key was off now the
ignition light is on. Does it stay on all night? does the battery go down?
To fix your problem I would try enriching the mixture a tad and try a tank of
high test. Also if your timing is advanced beyond factory specs set it back
and see if that helps. If those items fix the problem then go back to your
original setting one at a time and see what you can get away with. Run on is
not a good thing to have, it plays hell with rod bearings If those items
don't fix the problem they you may have to remove the head to get the carbon
out. Verify that you have the correct plugs for your car (not the super
extended tip ones)
I have had problems with run on in the summer months also these remidies have
worked for me.
Rick Ewald
67 MGB
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