GT 500 - need your help with ideas on what could be wrong

[Peter L.]

  I had the same issue with a Boss 302. It turned out to be a bad factory tach. They run in series with the ignition system. Easy to check. Just unplug the tach and bypass it all together. Easy to check and eliminate as a possible cause.

[shelbydoug]

Compression tested ok if 150 -180 is good. checked play in rotor again and noticed I could spin all the way around with a little force. Rotor is good and not cracked. Decided to remove points plate to look underneath. Things seemed to look good there. weights still in place. So is it possible the pin on gear on Dist. sheared?

Yes. This is actually a fairly common failure on 351c's.

The reason being, that car uses rubber"umbrellas" instead of positive sealing valve stem seals.

With age, they dry out, become fragile and crumble into the oil which gets returned to the oil pan.

Eventually the oil pump sucks up enough of them to freeze  or bind up the drive gear while the engine is running.

The weakest link fails. Sometimes referred to as a fuseable link. That being the roll pins that hold the drive gear to the distributor shaft.


I've seen them just bent 90 degrees and not sheared. So probably the pump gear just bound up momentarily and cleared themselves enough. This will start a complaint about the engine "jumping timing".

The original oil pump drive shaft is a piece of hardened hex rod. I've seen  thm twisted like a licorish stick so in that case it worked as a fuseable link and twisted before the pins sheared.

People who race will tell you that they have seen the oil pressure gauge readings bounce around under high engine load. That can be attributed to the hex shaft twisting and bouncing back.

Once you go to a machined "molly" drive shaft it will no longer flex and you had better verify the integrity of the drive pins.



The other thing which COULD have significance is that the outer ring ON THE HARMONIC BALANCER, IF NOT BONDED, with age has a nasty habit of rotating 90 degrees on the hub. That's just little enough to keep the engine running, but "just a little funny".


Here's the question that will point you in the right direction on the balancer. Have YOU had the balancer rebuilt? 

[csheff]

trotrof1, I had no pop back thru the carbs, seemed like it wanted to start but just wouldn't.
Doug, THE HARMONIC BALANCER looks good from looking at it. But will know more when I get new pin though gear get it back in and try to fire up. And no on having the balancer rebuilt.

In the mean time my dist. has a C3 so must not be original one. Would original one be C5AF-12127-E? I would be interested in finding a correct one for the car with correct date if possible. Car was built Jan, 29th 67 So any date Nov, Dec, Jan?

Also when I get the correct Dist., if I have it re-curved what info do i need to set it too?

[csheff]

Thanks Bob that is the plan

[csheff]

Thanks everyone for your inputs.
Was a sheared pin on Dist gear. After a few brain farts it's running again. Just have to ck and adjust timing.

[Bob Gaines]

Glad to hear that you solved the problem and reported back. Hopefully this thread will help others in the future who are searching for answers.

[trotrof1]

No substitute for hands on mechanical repair. Way better than just handing it off to a shop.

[Greg]

Great job, a lot of satisfaction in diagnosing and fixing the problem.

[NC TRACKRAT]

Hopefully, you found both pieces of the sheared pin?

[JWH]

Glad to hear that you solved the problem and reported back. Hopefully this thread will help others in the future who are searching for answers.

+1

Great to hear you are back on the road and nice work figuring it out.
Jeff