Runs does not run....
 

Have a '68 912 that when it is cold, starts right up, but if you run it till it fully warmed up, then shut it off, it stated back up, then 5 minutes later, it will not start. Looks like there is no spark, what would cause this? Now after it has sat over night, it still will not start. Fuel pressure is fine, carbs are fine, we are at a loss...... Bad coil?

"Looks like there is no spark" Did you check? You obviously checked the other variables: fuel and air, so you just need a spark at the right time.

BTW: When was your last valve job?

Otto

Otto-
The engine was rebuilt and heads also, seals, guides, etc, about 2,500 miles ago. There is no problem with fuel delivery, or pressure, just have run out of ideas on why it will not run

Go with your guess. Heat soak and an iffy coil do not mix well. Swap in a new coil and report back. Be sure to check all electrical connections in the starting/running circuits as well, particularly those where wires attach to the coil and distributor using the push on fittings.

It started more or less as a heat problem and now it doesn't start at all. Engine rebuilt, then I follow ejboyd5 and replace the coil and check the wires. Apart from that I would advice you to check the valve adjustment with these 2500 miles after rebuilt.

Did you check the spark from the plug? And if you have a spark, is it on the right moment. In other words, did the dizzy clamp get loose and changed the timing?

Otto

What distributor does it have? The.050 had rotor issues. Seems a resister (or something) in the rotor would become intermittent when hot, cool down and alls good. I had it happen, took a while to figure it out. If you have a extra rotor it's worth a shot, my 69 did exactly what you are describing.

Had a similar problem with VW years ago it was a bad condenser.