blueosprey90

Have I found the smoking gun?

MGA race car. Mallory Dual Point Distributor, part # #2332001H

I've had a high speed misfire for two years at 4,000 rpm that I have not been able to resolve. I've swapped parts, swapped distributors, ignition wires, coil from my spares, all without resolution. I've even swapped the head on the theory that my valves were sticking. I was trying to swap the exhaust, and was looking to pulling the engine to do so. So lazy, however, and am now wanting to avoid.

So, looking at it again this morning before the Druthers Meet and Eat, I found this condition on the distributor rotor that is in the car now. The underside appears to have surface cracking, maybe some carbon tracking near one side of the tip, and the "socket" where it mounts on the shaft is oily and dirty. I attribute that, however, to too much oil on the felt wear pieces that run on the shaft.










This is the condition of the rotor that is on my spare distributor. Similar underside cracking, just not as dirty. I'm sure this distributor got swapped in, probably with this rotor.








And this is the condition of a 3rd rotor. It's used, but I'm not sure that this one was ever tried, as it looks pretty clean. None of the cracking shows up in this one.







The cracking on the underside of the rotors is quite similar. Is it a normal manufacturing artifact, or does it reflect damage from use? On the bench, one of my distributors has a lot of shaft end float and I'm wondering if the rotors have contacted the underside of the distributor cap and cracked.

Is what i see the dreaded carbon arching? What are the recommendations? Have I found my problem?


Attachment 141295

A 2

That's a great pic Jeff of you on the outside of the 911

blueosprey90

Mallory Rotor Test: I found these instructions on line for testing the Mallory dual points distributor rotor.

"To test, remove the distributor cap. Remove the coil wire that leads into the cap. Hold this coil wire one-half inch above the rotor. Crank the engine with the key on; no sparks should be obtained. If the rotor is defective, the high voltage spark will pinpoint the exact location of the defect. Replace if necessary."

As noted in the prior post, I had several rotors with surface cracks that I suspected of being the cause of my persistent high speed misfire. As such, I tested the Mallory rotors in accordance with the procedure described above. They all tested OK.

But in performing the test, I had what I would describe as a "dirty spark" at the leading set of points. I wanted to take a video of it, but my camera needed to be charges, so a fail there. In subdued light, the spark looked to be about 3/8" across and looked like a kid's 4th of July sparkler. Not knowing if that was what it should look like, and thinking that maybe there was a short at the points, I pulled the distributor. The points looked pitted, so I attempted to file them smooth. I also cleaned up the internal surfaces in the distributor. I was not 100% satisfied that I had cleaned up the points sufficiently, so I decided to install two new sets of points.

I installed the distributor and retested. Now the spark at the points was virtually non existent. I wasn't sure if I even had spark, so I put the cap back on and verified by testing with a grounded spark plug at the engine head.

I'm starting to be optimistic.

A 2

I’m hoping that solves the issue and the car runs well.

I really admire your resolve to find the issue without throwing in the towel. I’m not sure I would keep going faced with issues like this.

blueosprey90

All that talk about going to The Glen in July has me pumped up a bit.

A 2

Have you signed up yet?

s2000sccaracer

Jeff, when I used a Mallory distributor in my TR3, I had problems with the condensers. Replaced them a number of times. Heard that they had some quality issues. A bad condenser will cause points to wear and pit prematurely. Might want to try a new condenser.

blueosprey90

Thanks Joe.

What I don't understand is how three distributor set up can all have the same problem. I have two distributors and I borrowed a third from Chris Meyers, although I used my own rotor and cap. They all have their own points and condensers. I thought it might be my new exhaust which has a larger diameter (say 2" versus the first 30 inches at 1.75"), and I started to take that apart, but it looked good - and I'd have to pull the engine to install the old system.

I looked at the points on my second distributor at lunch time today - and they don't look that bad.

Usually when I've had a condenser failure, I've had a spitting misfire at all rpms. My misfire is now at 4,000 rpm and above.

s2000sccaracer

Jeff, at one time the advance limiting stop broke in my Mallory dizzy. Also the bushings that the advance weights pivoted on wore out, making the advance weights erratic. I gave up on the Mallory and changed to a crank fire ignition, but that is probably not an option for you. Some of the other Triumph racers went back to Lucas distributors, but had them professionally rebuilt. I have a Pertronix dizzy in my TR4, but its a street car. I don't know how well the Pertronix holds up on the track.

blueosprey90

Joe, over the winter I had an old Lucas 25D4 distributor off of a 62 MGB rebuilt by Advanced Distributor as a back up.