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MG MGA - MGA mid range misfire

I am struggling with a persistent but intermittent mid range misfire on my 1956 Roadster (rough running under load when hot and at mid range revs). This a car I have had since the 60's and was restored by me, so I know it and its many quirks quite well. It is fairly original so there may always be items to replace....

Have tried most of the obvious solutions, new vacuum lines and diaphragm, distributor serviced, points, cleaned up fuel line, checked plug colour etc. No indication that any cylinder is misbehaving or that 1&2 are out of balance with 2&3. A wide range of timings has been tried, with an occasional (temporary) improvement. When the misfire occurs it is like an ignition problem with 1/2 cylinders or a fuel block on one SU - but that is just a guess.

Car runs well otherwise but this is an infuriating recurring problem, which can come back at any time - when throttle opens suddenly. Head is leaded - LRP is being used. engine was rebuilt 20 years ago - does 1K/year. Problem often goes away when I change something, only to come back, often when hot.

I have a tankful of last years LRP - and am wondering if I should add additives or top up with leaded.

I guess I need to start looking at fuel line dirt, replacing capacitor, distributor cap, rotor, coil etc Testing compression (head gasket??).

Any suggestions? I feel like I am missing the cause while staring at it...

Mark
M Scibor-Rylski

Scibor
The symptoms you describe sound like a distributer cap with a hairline crack failing under load but also randomly. Sometimes the crack connot be seen
John
J H Cole

Thanks John - I may as well start with replacing that, will report results.. Could not see any tracking when running in the dark - but I guess the tracking may be inside.


Mark
M Scibor-Rylski

I would possibily change out the coil. Your other choice is to take your car and have it put on an dyno to see what the car is doing under load and how much voltage is going to each wire. The ocillascope will tell you the whole story of how your fuel and electric system is doing. No more guessing that way!!!
JEFF BECKER

I agree with Jeff. My car ran poorly under load, but not all the time. It finally got worse, and I tried a spare coil to see if it made any difference. It was cured! I had been fooling around with points, carburetors, and timing with out any improvement. Now I had to go back and re-tune all the things I had messed with. Too bad a coil doesn't change color when it goes bad, a new one and a bad one look alike.
Ed Bell

Hi Mark. It could also be a defective distributor rotor. Sometimes they crack, and then leak high voltage current. Cheers! Glenn
Glenn

You mentioned the points, but did you replace the condenser?

M.D.
'57 Coupe
M. D.

I've had both a hair-line cap crack and a bad rotor. Both times everything looked "fine".

Larry
Larry Gray

Thanks everyone, really appreciate the input - I am eliminating items as I go! Have changer the rotor arm, and put in new plugs, checked the vacuum advance retard, and now chasing a new sports coil and a condenser replacement (in reverse order). Then its distributor cap... Meanwhile problem persists. Intermittent to boot...

Watch this space!
M Scibor-Rylski

After a 5 year re-build and only covered 50 miles so far I had a breakdown last Sunday,I could not find the fault, engine was miss firing badly, called out the AA, it was the rotor arm breaking down. It was about the only thing I did not replace during the re-build.
Dave
Dave

After cleaning my plug wires thoroughly with WD40, and rerouting one wire that looked a bit worn on the outside, my similar problem seems to have disappeared. I have heard of guys putting the plug wires in the dishwasher, never tried that. I also didn't try my mechanic friend's trick of misting water around the wires with a windex bottle (in a fairly dark place). But I saw it done, (spray spray, snap snap) and it saved the owner of the car the price of 7 plug wires. (Not saying you shouldn't replace them all, as they are probably all the same age.) Just relaying a technique.
Tom

This thread was discussed between 29/05/2005 and 10/06/2005

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