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MG MGB Technical - engine miss

Just acquired a '70 GT and the engine misses at a sustained speed, low or high speed, it doesn't matter. It seems to accelerate fine and smooth. I adjusted the fuel mix so it runs richer. That seemed to help. I put in new plugs, checked the points gap, new air filters, cleaned up the carbs. Haven't checked the timing. Carbs out of sync? Any other simple things to check?
Thanks
tim
tim

Does the tach flick with the missfire? If so it is an ignition LT problem. Can you reproduce it on the driveway? If so clip on a timing light to the coil lead and each plug lead. Any irregularity of the flashing in time with the missfire shows an HT problem. If the irrgularity is on the plug leads but not the coil lead the cap/rotor are the problem. Use an 12v inductive pick-up type as this will also show plugs breaking down. As well as the timing check the points dwell, not just the gap. Any irregularity in the timing or dwell indicates ignition problems. Remember, "90% of SU carb problems are due to Lucas electrics". Carbs out of sync would not normally cause an irregular missfire, nor would weak or rich mixture. But if they are an unknown quantity they should be set-up from scratch - after the valve clearances and any ignition problems have been resolved.
Paul Hunt

Paul points you to the ignition circuit, and I agree: I've had your problem on 2 different cars and both times it was in the ignition circuit. I was getting misses at about 2400 rmp, regardless of vehicle speed.

What I'll add and what Paul didn't explicity state is to check the plug leads. After fiddling with timing, condensers, points, dwell meters, plug gaps, valve clearances, and carb settings I was stumped. I realized I had overlooked the easiest thing to check.
Larry Bailey

This thread was discussed on 22/08/2003

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