Dust in the distributor cap

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?

If this “dust” did not/could not get in from the top because of spiffy new seals on the wires and cap-to-distributor-body, it got there either from the bottom - which is even more probematic, obviously, or was generated internally.
Next Q: just what is this “dust”? The voltage involved should penetrate most anything which would come up, but if bits of the cap are being scraped off, that dust would certainly be a formidable insulatior.

The engine was completely torn down and professionally rebuilt. Non-rebuildable parts were replaced with new. I don’t think anything is coming up from the engine.

I didn’t look for scraping of the cap (which is new), but there was nothing obvious. The cap is made for the car and it fits very well. Maybe the rotor and electrodes, being new, are too close and the dust is actually metal? It would be conductive, so maybe it’s messing up the timing? Or is the rotor supposed to touch the electrodes?

I’d hate to be stranded, but that’s why I have a AAA card. I think a longer trip might be in order to see if wiping the cap really worked or if there’s an intermittent glitch instead.

The rotor should not touch the cap terminals. If it did, there should be evidence of abrasion on both the rotor tip and the terminals inside the cap. If this is the case, find a slightly shorter rotor, a different cap, or file down the rotor tip a bit. If only one (or some, but not all) of the distributor terminals shows abrasion, that suggests play in the distributor shaft, which would require replacing or rebuilding the distributor to fix properly.

My first thought was that the dust may have come from the graphite brush at the center of the cap. This would be conductive and might have been causing the poor running. If this is the case, it might be due to a poor quality cap, or the part of the rotor that meets the brush might be rough. If the problem recurs, I’d be inclined to replace the cap and rotor with a different brand.

I might be inclined to replace the condenser, considering the cost, which is small, this would eliminate one cause for concern.

Check your air filter too.

Replacement caps and rotors for old British cars are of terrible quality. I am on a couple of mailing lists for old British cars and I don’t think a week goes by without a cap/rotor problem.
The dust is probably from the carbon button in the center of the cap. The source of the failure is most likely a bad rotor. When it gets hot the insulation in the rotor fails, and the rotor starts to fire to ground rather than fire the spark plug.
Try replacing the rotor, and make sure it sits all the way down on the shaft.

I’ve never used a Pertronix electronic ignition before. Is the rotor just a rotor? Or is it a proprietary piece?

As far as I know the rotor is just a rotor. The electronics replace just the points and condenser.

ETA: If you pull the rotor off the shaft and look inside, you might be able to see where the spark has been jumping to ground.

Thanks, Rick. I’ll have a look. If I’m lucky, there’s a spare rotor in a box in the car. If not, I suspect a rotor is going to be hard to find locally and I’ll have to order one from Moss.

Vickie Brits is closer to you (Victoria British)

Send my money to a Red State (Kansas)? :eek: :wink:

I did buy a couple of things from them. Moss rejected the windshields from their supplier because the were the wrong shape, and so they were unavailable. Victoria British had ones that fit, so that’s what’s on the car. But I’ve been buying from Moss since my first MGB and they’re just down in California. (And they have a nicer catalogue.)

IIRC (I’ve never used one, but I have looked into them), the Pertonix parts replace the points and cam. The cap and rotor are not replaced.