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MG MGB Technical - Misfiring between 2000-3000 revs okay above/below

I have a 1973 MBG roadster. I started getting a problem with misfiring above 2000 revs. I originally thought it was the fuel filter, which with some help, I cleaned. I then noticed that when it started misfiring, it is like it is almost cutting out, if I kept reving it it would then pick up okay over 3000 revs and was okay then. It has new points, distributor installed by a MGB garage. I replaced all of the HT leads and the Coil, and I also tried driving with the air filters off and no difference. I dont know what to try next. Has anybody got any suggestions ??
Cheers
Peter
Peter Boyle

Peter, Have checked the oil in the carburetor dampers? Low oil level in the dampers can cause a miss on acceleration. Clifton
Clifton Gordon

It it's just a "cough" or small intermitant miss, you may wish to check the timing - a small advance 2 or 3 degrees helped when my engine experienced a similar problem.

rn
RN Lipow

Does the ignition light flash when it misfires?

I experienced a very similar misfire to yours which turned out to be a loose connection in the brown wire to the starter. I guess at that rpm range it would hit a certain harmonic and lose contact.
Rob Edwards

Peter,

On the fuel side you may need to look at the carburettor needle valve/s being faulty, or dirty. Secondly, might be the carburettor piston/s sticking with correct oil in damper/s.

Regards,

L.C. '74 B/GT
Larry C.

Peter,
I had a similar problem last summer. Mine would miss only under load at about 3200 RPMs at normal temp. I went to my local MG repair shop and he handed me five brand new condensers and told me to try until I found one that would work. The first one I tried did the same thing only at 4000 RPMs. The second one has worked just fine. I took the bad one and the three untried back to him. He said that he had a bad batch of condensers and showed me a box with at least two dozen new, but bad condensers. He guessed that one out of five is bad right out of the box. So if you have recently replaced your condenser that might be your problem.

Dave
Dave Kalp

Watch the flashing of a timing light connected to each plug lead in turn. If it is almost cutting out and is caused by an ignition problem you should clearly be able to see this on the timing light. If the flashing remains constant it will be fuel/air leak.
Paul Hunt

Thanks for all of the comments on this problem. Dave Kalp got the problem correct and it was a faulty condenser. There was also a second poroblem which may have been caused by the first. The distributor cap centre conductor had been worn away and was burning the top of the rotor arm. All have been replaced (and very cheaply as Lucas has a sale on) anbd the car is fine now.
Cheers
Peter
Peter Boyle

This thread was discussed between 23/02/2003 and 26/02/2003

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