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Q & A: Hesitation, Electrical Problems

Hello John
I have a 1978 MGB, and after having done some reading and checking
serial numbers, etc., I learned it actually has a 1974 engine. I
recently had a new set of SU HIF carbs installed and I noted right away
that the engine seemed to have a slight hesitation while running at a
steady speed. It is like one of the cylinders doesn't fire occasionally,
but the slack is then taken up as it continues running. It is most
noticeable between 2000 and 3000 rpm, in all gears. It is less
noticeable in 4th and overdrive, but is quite noticeable in 2nd and
3rd. It accelerates and idles just fine, however - at least there is no
noticeable hesitation, if there is any at all. There have been three or
four occasions where the engine simply quit, as if the ignition had been
turned off. It did start again after a few tries and I was able to
continue driving. Also, on a few other occasions the engine almost
quit, but kept going for several minutes and the problem cleared up
enough to continue driving. I checked the fuel pump flow and it seems
to be ok. The distributor had a frayed low tension lead inside, which I
repaired, and I cleaned the distributor and all the contacts on the
ignition wires and coil, but that did not seem to help the hesitation.
I suspect the cause is not the new carbs, but some electrical fault. My
feeling is perhaps the coil or distributor. Also today, I noted the
tachometer needle suddenly dropped to 0 for several seconds and the left
turn signal, which was flashing at the time, also quit working. After
several seconds, they both stared working again. There was no
noticeable change in the engine speed at this time. Since I was waiting
to make a left turn, I revved the engine a few times and it seemed to be
ok, and I was able to drive off again normally. There also have been a
number of other incidents where the tachometer needle suddenly dropped
to 0 and returned to normal in a second or two. I understand voltage
pulses from the coil are converted into tachometer rpm, which leads me
to think the coil is involved. It is a 16C6 (6 volt) coil, which I
understand is ballasted, and installed on 1975 and later units. The
wiring diagram indicates a "resistive cable" on a white/light green wire
from the distributor, and a "resistor for distributor" (which i presume
is the ballast - correct me if I am wrong) connected on the other end
with a white wire. I can't seem to locate either of these items. Where
would they be located (they could have been removed, also)? The engine,
as I indicated above, is a 1974 model and some of the wiring and hoses
have been rerouted to fit the layout of the 1978 body, so it is a bit
confusing trying to work with the wiring diagram. Would the engine
still run with a 6 volt coil if the ballast has been bypassed or
removed? Should I replace the coil with the earlier HA12 (12 volt) and,
if so, how would I go about making the corresponding changes in the
wiring to accommodate it? Any thoughts on where the problem may be
originating, so I can make a start on finding the fault.
I apologize for the length of my explanation, but I felt you should
be given all the information instead of me simply asking something like
"my engine quit, any ideas?"

GEOFF!

Sorry for the delay in answering.

You have three problems: dirty fusebox, a lean carb condition, and
an unknown condition. The dirty fusebox is allowing the WHITE/GREEN fuse to
occasional... read more