I had an engine do a similar thing once, it turned out the harmonic balancer had delaminated the rubber and had rotated, so what the timing light read had no relation to where the timing actually was. Only noticed when putting the car away one night, I left it idling outside with the bonnet up and came back 3 or 4 minutes later to cherry red headers, a sign of a massively retarded engine. The thing is, it didn't drive all that bad considering but also wouldn't rev.

 

That's a possibility Aussie, I had the balancer come apart on a 5.0 I owned.

Jim, there is usually a metal spring clip on the shaft or inside the rotor, if it's the old-fashioned kind, that goes onto the flat on the shaft. If that fell off when you removed the rotor, it would let the rotor rotate on the shaft.

 

Ross,
I enjoy the edit feature too!

There's no spring clip on this dizzy. It's a duraspark and if I can't track down the proper rotor and cap today I may see if I can re-engineer the current rotor to work.

Jim

 

Okay...I'm not as dumb as I feared. After messing with things today I found that the reluctor pin on the back side from the rotor notch had come up when the reluctor was removed. The rotor base was hitting the reluctor pin. Drove the pin down to its former location and the rotor fit down perfectly.

The vacuum advance pot is still junk, so I've got one of those coming tomorrow, but that still doesn't sort my stumble issue.

The mechanical advance springs were still in place, but I wonder if they are wearing out?

I'm also holding the ICM suspect. The tach says the motor runs up to 1500 and drops to 900, before building back up to 1200. The timing light says the mechanical advance kicks in at about 1000rpm and advances about 8* as the rpm rise to 1500. It holds there +/-2* while the rpm drop to 900, and as it climbs up to 1200, the advance moves ahead another 8-10* to 26* total advance.

The motor sounds like it's accelerating from idle to 1500rpm, stumbles and sputters, but maintains rpm, and then begins to climb to maybe 2800-3000rpm? It sounds like it's racing a heck of a lot faster than 1500 rpm.

Anybody know how to test an ICM? The dizzy seems to match a 1975 F100 Duraspark...will verify based on vacuum advance pot tomorrow.

I'm wondering if the ICM isn't keeping up with the RPM...not providing ground to the coil for spark past the 1500 mark. If it falls off, then the tach will fall off...but the advance should fall off too if rpm is actually falling off.

Still thinking things through.

Jim

 

Durasparks usually just die, I haven't heard of one that just gets "confused". Springs do get weak, tho, and I've seen advance mechanisms that get sticky.

 

Check rotor and cap for arc trails. Could be contaminated. My suggestion would be to change them both, not a huge investment. Also, timing light check of each cylinder can show misfires, as stated above.

 

All good suggestions and if I may add another check your total timing to be sure that the distributor is advancing completely. You probably should be seeing 36-38 degrees. Good luck on this one

 

If I understand the process required to find your total timing it's impossible for me to reach. I can't get the truck up to 3k or 3500rpm.

I can check other cylinders for misses. It runs great to that 1500 mark...then I'm fairly certain it misses...but I will check that out tomorrow.

We'll see what I can figure out tomorrow.

Jim

 

Got her fixed and running right!

I replaced the cap and rotor, and replaced the vacuum advance pot (thought I knew that wouldn't fix it...it was shot...you could drink through it!).

That didn't work.

So I talked to the guys at the part shop and they had an ICM for $33. It was worth the gamble, because it paid off!

Revs up to 2500 without a hitch! I haven't gotten it any further than that. Unfortunately I ran out of time just as I got the ICM hooked up...but in a couple hours I'm going out on the highway.

Thanks guys for all your help and input. In the end it was a process of elimination, but for anybody who runs across this problem down the road... If the motor stalls and then seems to be running faster than the tach reads (and the advance is still advancing) try swapping out the ICM.

Peace,
Jim

 

Great to read. Makes sense that erroneous tach info would point to ICM. Thanks for posting your solution.

Seek and ye shall find.