Failing electronics? Yes, certainly a strong possibility. But failing or already failed? I could rescue this - at least for now - by adding a single, simply MOSFET stage to the amplifier's output. If I choose a FET with enough gate capacitance, it should remove all the tiny pulses, thereby restoring a proper pulse train PLUS, by hooking the FET's drain up to the car battery, I'd get the max 9 volts under cranking (instead of the current 4.5V) to enable the coil to generate healthy sparks. Tempting.....

What coil & dizzy you using then?
Early 1s had an amplifier seperate to the dizzy. If these broke down that was the problem.
On later 1s there was a small amp on the dizzy body

If the above you have said with the electronics work I would do that,easier to repair in the long term

Hicap phill said:

What coil & dizzy you using then?
Early 1s had an amplifier seperate to the dizzy. If these broke down that was the problem.
On later 1s there was a small amp on the dizzy body

If the above you have said with the electronics work I would do that,easier to repair in the long term

Click to expand...

Phill, I believe it's the original distributor and amp from the look of it. Obviously it's had at least one new cap and leads in the last 36 years. This is the model with the amp on the side of the dizzy. I just put a new coil on it. Supposedly OEM, but the original had a primary resistance of 1 ohm whereas the new one is nearly 3 times that much for some reason. Maybe there were some shorted turns on the primary of the original coil, I don't know. I'm very tempted to rig up my idea just to see if it fixes things....

Check your battery make sure its fully charged or not ancient, when my 3.9 had been standing for a while it is sometimes difficult to start, there appeared to be only intermittent spark. I replaced coil , amp, distributer, leads and plugs(seeing as i'd changed everything else) still no good. Took dizzy out again and spun by hand and generated lots of sparks. Starter was stealing all power ...no spark. Replaced battery. What showed up was a few times when I released the key and engine still turning it started! I couldn't measure volts while trying to start as I was alone.

Yes, when an engine frustratingly tries to fire-up at the end of a cranking it's a pretty clear pointer to a battery issue. That's not the problem here, though as the motor spins like a top and continues to do so over many minutes of intermittent cranking (to save it from overheating). Not being able to see what's going on without a pal to turn the key is a major obstacle to effective fault tracing I fully agree.