Sam, it was nice talking to you yesterday! It has become painstakingly clear to me that TR3s are hard on distributors! I guess most of it can be written off to age, and the hundreds of thousands (millions) of advance cycles the distributor go through in a lifetime.
I find the best upgrade to a TR3 distributor isn't electronics, rather upgrading the advance assembly. The TR4 advance assembly (entire shaft assembly) is an easy, undetectable upgrade that offers tremendous reliability and consistency, when compared to even a brand new TR3 DM2 distributor. It also allows me to tailor the ignition curve to best suit your TR3 using modern fuel.
Its also true that the aluminum advance weights in the DM2 distributors wear relatively fast when compared to the steel weights in the 25D distributors like the TR4. Reducing excess slop and getting rid of the toggle-type advance mechanism which uses an extra pivot point (less pivot points means less frition and a more linear advance and retard operation) will give you the consistnecy that's key to a good distributor.
I'll also add that the 25D assembly changes the position of the advance springs to give a more direct "pull" when at low speed, which allows the advance weights to pull back to center easier under low rpms. This means that your idle speed will be much more consistent and can be tailored to be rock steady, as you could never do when they were new. You could get very close, but not like this! The original setup had the advance springs canted at a relatvely steep angle, so the pulling force near idle was very weak. This commonly allows for a timing variance of 2-8 degrees at idle, which can change your idle speed dramatically.
Sorry that I rant on about this tuff, but its a passion!
Maintenance Issues
