Ignition related running problem

[bruss]

I have a 1968 plus 2 with a psoitive earth tacho. The car had been converted to run negative earth before I bought it and the tacho runs erratically The car will start and run well for a short while. When the engine is hot the car starts to miss particularly under load and then shortly cuts out completely. After allowing the car to cool it will start and run again untill it is hot then fail again.
As part of diagnosis I have run a wire directly from the battery to the coil, and disconnected the tacho supplied feed. The strange thing is the tacho continues to work although still erratically.

I have thought it was the coil breaking down but have replaced the coil as well as the condenser and the same thing happens.
One other thing to note is that the central graphite 'plunger' contact in the dizzy cap has almost dissapeared- eaten away presumably.

Searching on here I have seen a comment re the ignition switch building up a resistance and breaking down. Is this possible?

Also with the change to negative earth previously, is it possible that I have connected the coil sw and cb sides back to front. If so would the car still run?
Any ideas on what's going on?

Thanks

[neilsjuke]

Next time you get the fault hot wire the coil from the solenoid removing the lucar terminal from the coil you will then have a starting point as to where to look yes the coils are polarity sensitive (this will give faster wear and pitting to the points ) and you would have had to swap the terminals round if you think of the points being earth then with a coil marked - + the neg would go to the dist .you should always change the rotor arm at the same time as a distributor cap .note the condensers made in the east seem to be very poor and are not very stable looking at a scope when running.
Neil

[Ross Robbins]

I suspect the graphite plunger in the dizzy cap. I would check all the things Neil suggests as well, but I had a hot miss in my Elan race car that drove me nuts until I swapped out the cap. It seems the rotor was making intermittent contact with the plunger, Cap swapped problem solved.

[bob_rich]

Hi bruss

I have twice had a rotor arm developed a voltage break down through the rotor arm plastic body to the distributor shaft. Car would start (rich mixture on choke and low power so spark plug fired at lower voltage) but very quickly it would splutter to a halt as the attempts to get higher power/revs and thus breakdown voltage in the cylinder was higher and then the voltage broke down the rotor arm plastic and not the plug.

Tacho of course see a spark so it worked OK but spark is not in the cylinder so engine spluttered to a halt. Voltage breakdown of the type described is to a degree temperature dependent so probably worth changing rotor arm as they are not 2 expensive.

Hope this helps best of luck

Bob

[512BB]

I have recently bought a number of super duper rotor arms from a guy called simonbbc on Ebay. Do a search. Having heard that standard Lucas ones made today are le crap [french] , I thought i would buy some from this chap when I came across them, as previous buyers gave them a good press. I havent used any yet, but they look okay.

Leslie

[billwill]

Are the original type rev-limiting rotor arms still available anywhere?

Just curiousity really.