As I mentioned before, I’ve been having a problem with my MGB. It runs fine and strong, but after a bit it starts ‘missing’. Last week this happened and as I was limping home it quit altogether. I got it started again, made the two miles back to the house, and it died in the driveway.
At first I thought it might be a loose spark plug wire. If it was running on three, that seemed logical. But when it quit entirely I began to think the problem might lie elsewhere. So: Spark, fuel, or oxygen? I never considered the air cleaners. They’re K&Ns as new as the rest of the engine (a couple thousand miles since a complete rebuild). The car was backfiring when it was missing. I talked to the guy who put the car together and he said this implies that fuel is getting into the system; so we ruled out a failure of the (new) fuel pump. It occurred to me that it’s been raining recently, and that even on sunny days the car has quite a lot of dew on it in the mornings. Condensation in the distributor cap? (Incidentally, it’s a new cap with a new rubber grommet and silicone wires that are supposed to be better in damp climates than stock ones.)
Today I popped off the distributor cap and saw that it was dusty inside. I wiped it out with a paper towel and went for a drive. I guess I did about ten miles, which is farther than I got last week. Ran like a champ. Admittedly I did not go very far, and when I brought this up a few months ago it started acting up after 20 miles. But there is this: When I started it today it ran typically rough when cold. It was not warmed to operating temperature, but to about 160ºF and it died. After wiping the distributor cap it started right up and idled normally.
The question: Is is likely that the dust in the distributor cap is what caused the problems?
This is not a dusty part of the country. The rubber grommet is new, as are the Pertronix electronic ignition, Lucas coil, and wires. Whether or not the dust is the culprit, how did it get in?