Engine Timing off by 20 Degrees?

There is a chance that the distributor was not tightened down enough and it rotated on you. mark it with a wax crayon after timing it correctly and see if it moves.

was it surging? if so, it cold be a vaccume leak.

Tyler

Timing chain stuff won’t change your spark timing. Either the distributor rotated (too loose), or your balancer is failing and allowing the outer ring to rotate such that it appears that your timing moved. Unlikely but worth mentioning is that perhaps your distributor gear is wearing… once they start going, they can go quick.

While you're marking stuff, mark the balancer too.

gsxrken said:

Timing chain stuff won’t change your spark timing.

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how come not? if the chain jumps and turns the cam a tooth off from the crank it effects the timing. The distributor will stay the same relative to the cam, but not the timing marks on the crank.

correct?

It effects the valve timing via the cam, not ignition timing. The small gear for the chain and the harmonic are held in place by the woodruff key on the crank. The cam will move in relation to the crank, but the crank won't move.

The chain can jump and effect valve timing via the cam, but the crank and the dizzy will not move when the chain jumps.

I hope you don't take offense Max Power, but experience and logic tell me you're incorrect. Since the cam drives the distributor, not the crank, if the chain is very loose, both cam timing and spark timing will be erratic relative to the crank as seen by a timing light. But what is described does sound most likely that the distributor simply moved. If you're using a 40 year old harmonic damper, I would replace it anyway, as the elastomer has definitely seen better days. If the outer ring has moved 20 degrees relative to the hub, then when you set the timing mark to zero degrees, the piston will be noticeably down in the bore. If the piston and timing mark agree, then the rotor will be pointing clockwise relative to the #1 terminal on the cap. Move the distributor back and time it with your light again. Tighten everything back down and drive off into the sunset. Since there was a noticable change in idle, it suggests something other than the damper ring moving, but more than one thing is often wrong with a 40 year old engine. Has the timing chain been replaced recently? If it is so worn it jumped a tooth, you would have more than just a change in idle quallity. It would have poor throttle response, a bad off idle stumble and possibly backfire especially when you mashed the throttle. Good luck and let us know how it works out.

You are correct 69gmachine. Even though the distributor housing and harmonic are bolted in a fixed location, the cam jumping a tooth would alter the shaft within that fixed distributor housing, altering a timing reading. Should have thought that further through.

If the balancer slips, it screws up the engine's balance too. I don't know how much can be felt by a 20 degree slip, but I had a friend with a 94 GT stang come a few months back that had the balancer slip 90 degrees (1/4 crank turn) Shook it's ass off. As for old balancers,slipping, that's just as likely as a newer one. The only one I've seen slip was that 94 balancer. I've got a 70 balancer on my 331 that's as good as it was thirty years ago.

I'm sure there are lots of good useable old parts out there, but I'm just not willing to take a chance anymore.

I went through 2 OEM balancers on my old 351 before I found a good one. The first two were the thin 69 351W balancers, and weren't stable enough for the machine shop to even balance. The third one was from a wrecking yard. It had a D2 part number and was a lot thicker and heavier than the 69 351 balancers but it had 3 bolt holes and it balanced OK. For the next engine I bought a new one. Just not worth the time hunting used parts.

Who has to hunt for em? I got a half dozen piled up on a shelf I think the one I used on the 331 was also a D2 351W balancer. Big S.O.B. too.