MG Car Repair: MG Midget 1500 intermittent spark at low revs, electronic ignition system, distributor caps

Question
I have a 1978 MG midget 1500 which will not spark consistently at low revs.

The engine has been fitted with a fast road cam, webers and used to idle perfectly at 650 rpm.
A
few months back it blew the head gasket. Since the rebuild I've been unable to get it to idle consistently - it misfires lots below 1000rpm, above this speed there is a noticable improvement. (the misfire mostly affects number 1 cylinder but does seem to move between cylinders)

I've fitted a pertronix electronic ignition system after struggling to set the dwell angle on the original points distributor. This did not improve matters.

I've tried swapping HT leads around to see if they were at fault. I also tried a few distributor caps.

I've done lots of reading on the web but haven't found a solution. Electrics aren't my strong point. Could you suggest some things i could test.

Thanks

Mark

Answer
Hi Mark,
I learned the hard way working in dealerships that you can not assume anything. So if your car had come to me I would first run a compression test even though it has just had engine work. I would want to see at least 135 to 180 PSI and very little difference between any cylinder. (Throttle open and at least 4 or 5 revolutions on each)
Since you have noted it seems to be more on one cylinder then another so you need to check the air gap on each cylinder to see if the shaft is straight.
If all else fails you need to find a shop with a scope. It can look at both the primary and secondary of the ignition system.
Howard