I've been fighting a nagging ignition problem for several years, with the latest fix being the installation of a Pertronix ignition system. That move seemed to be the cure to the malady. The old MG started easily and had more go to its' get up. Then, one Sunday morning as I was going out for a drive, the car started running on three cylinders. I made a few half-hearted attempts to find the problem (I suspected an issue with the Pertronix system), but I mothballed her until I could find time to focus on the problem.
I was off work this week (a 'use or lose it' policy at the job) and with the gentle prodding of Richard Cunningham and Mike Darby, I seriously began to look into the problem. Richard dropped by the house and we started looking at the usual suspects.
The Pertronix system is dead simple. They told me via email that the only way one cylinder would misfire would be if one of the magnets that trigger the Hall Effect sensor was missing. All present and accounted for.
We swapped ignition leads with no apparent good result. The number three spark plug was wet, but that was to be expected in a dead cylinder.
The distributor cap appeared to be in good order - no burning of the contacts or obvious cracks could be found.
We decided to break for lunch and afterwards we dropped in on the Wizard of Silverhill, Mike Darby with the cap and leads. Mike took the diagnosis a bit deeper and pulled the ignition leads off the cap. Two of them were out of contact with the cap - the copper lead ends were close enough to conduct the current, but it was weak. The arcing burned the cap and melted the lead ends into the rubber boot. Problem found?
Mike rummaged around his stock of used-but-serviceable parts and found a good dizzy cap with tested leads. He made me a bargain-basement deal and Richard and I departed to try out the new-old parts.
Back at the McDonald garage, we installed the cap, sorted out the leads and started the old girl up. Three cylinders. Richard had an appointment to keep and, on departure, suggested that I swap leads around to see if I could isolate the problem. I did - no joy. Feeling bummed out, I went in the house, bypassed the liquor cabinet and headed to the computer to see if I could find similar stories but with happier endings.
Mike called to see how things were progressing. On hearing the tale of woe, he asked if I had taken the next step and swapped spark plugs around to see if the misfire followed the plug. Erm, no, I hadn't.
Taking his advice, I moved number three plug (the apparent dead cylinder) to number four and vice versa. Started her up and bingo - number four is now misfiring. It is a bad spark plug. I called Mike to relay the good news and then left to run errands with my long-suffering wife. We dropped by a local auto parts store and left with four new sparkies.
I waited until later in the evening and went to gap and install the plugs. Two of the four were missing gaskets. Damn.
The next morning, B&E, I went back to the same parts store with all four plugs and showed them the problem. The young lady at the counter opened two more spark plug boxes and removed the gaskets for me. Anyone wanna bet where the two now - gasketless spark plugs went?
Back under the bonnet of the MGB, the spark plugs were snugged home and I started her up. The little four banger was again banging on all four and I took her out for a drive through the neighborhood streets. I found that my old friend was just as happy to be on the road as I was.
The lesson here is to diagnose fully. Start at the beginning and work your way methodically through the system proving every component along the way.
Thanks, Richard and Mike for your advice and expertise! Now to adjust the Mini's brakes again.
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