Converted to HEI, timing problems.
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Joined: Mar 2011
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From: Fairfield California
Converted to HEI, timing problems.
I just converted my 72 to HEI. With help from the timing sticky and other threads I have it in and it starts and runs. I'm trying to set the timing. Factory for my base 350 is supposed to be 8 degrees initial, which I imagine is a bit lower than necessary. I set it for about 10 and I hear no pinging or knocking, total comes out to about 31-32 degrees. I have yet to tweak with the adjustable vac advance. After I let it settle for a few minutes or take it for a spin, it begins to run a little rough, when I put the timing light back on I can see it's missing because the mark on the damper will occasionally jump to a well after TDC position for an instant. I've tried dialing it all the way to 8 degrees but still the same thing? What am I missing here? Again it's a stock 72 base 350 (far as I know anyways from PO) brand new HEI and wires. I'm using a dial back timing light for ease. It ran fine with the old points distributor, so It must be my error somehow. Any help is greatly appreciated!
Melting Slicks
Joined: Oct 2002
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From: St Louis MO
Spark plug gap won't change the timing accuracy or stability. You may have excessive resistance in the circuit feeding the HEI, or maybe the ignition module is failing.
Another cause for timing scatter is excessive vertical freeplay of the distributor shaft, but unless the distributor gear or camshaft drive gears are badly worn, I'd be surprised if it would scatter more than 5 degrees.
Another cause for timing scatter is excessive vertical freeplay of the distributor shaft, but unless the distributor gear or camshaft drive gears are badly worn, I'd be surprised if it would scatter more than 5 degrees.
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Joined: Mar 2011
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From: Fairfield California
I wouldn't think the distributor gear end play was bad since its brand new, I can't feel any slop if I try to move the rotor around. Can't speak for the cam gear but this wasn't a problem before the install so I'm assuming that's okay too. I ran another wire straight off the horn relay to see if my supply is bad, but it didn't change anything. To clarify what I'm seeing when I check the timing, the mark on the damper will stay steady on the zero mark then maybe every 10-15 seconds it will skip once very quickly to a spot quite a bit after where I've set my timing on the timing tab. Its so quick it's hard to catch, and there is no rhythm to it. The rpms bounce a bit and the engine stumbles just a tad at this point. It seems to rev fine and smooth, if I'm on the gas I don't notice the miss fire. Again thanks for any advice you can give!
Drifting
Joined: Jul 2002
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From: The only Corvettes in Highett Victoria
Timing mark???
'72,
It would pay you to rule out any slippage in the timing mark. Unless your harmonic balancer is new, it is possible that the rubber that bonds the inner and outer cast iron components has failed and allowed the outer ring, with its timing mark, to have rotated and causing an incorrect static timing position. This could also explain the "movement" you observe when the engine is running.
Regards from Down Under.
aussiejohn
It would pay you to rule out any slippage in the timing mark. Unless your harmonic balancer is new, it is possible that the rubber that bonds the inner and outer cast iron components has failed and allowed the outer ring, with its timing mark, to have rotated and causing an incorrect static timing position. This could also explain the "movement" you observe when the engine is running.
Regards from Down Under.
aussiejohn
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Joined: Mar 2011
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From: Fairfield California
So how would I go about checking this for slipping? I assume I would just take off the belts and see if it moves around or has play in it in any direction? Assuming that is the case does that mean where I think TDC is is way off? I figured if it was too far off the car wouldn't even start or run? It feels like I'm definitely in the ballpark as far as timing goes with a small stumble. If that is the case it seems to me I could just advance or retard the timing way out either way until it improves or gets worse and ignore what my timing light is telling me to some degree, which has not worked either. Or perhaps I'm thinking about it all wrong? 
Race Director
Joined: Mar 2006
Posts: 14,112
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From: Florida
I doubt your balancer ring is slipping off and then back on again. Usually they slip off and stay off.
I would suspect a cheezy module, unless it is a genuine GM and some new distributers do come with the gear clearanced to far.
I would suspect a cheezy module, unless it is a genuine GM and some new distributers do come with the gear clearanced to far.
Instructor
Joined: Jun 2011
Posts: 183
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From: Litchfield CT
Your symptom is that of either a defective ignition module, or the module pickup isn't properly fastened. The magnetic reluctor triggers a hall effect transistor. If the module moves, the point in time the transistor is triggered changes. When that changes, timing changes. Check the mounting screws on the module inside the distributor. If they are OK, I'd swap the module. They are cheap enough. If that fails. Put your old distributor back in and re-check your timing. I'll bet everything will be back to normal.
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From: Fairfield California
The ignition module is something that makes sense to me. I have checked and re-checked every thing else. Plugs, wires, timing adjustments, rotor etc. It is a brand new unit, but an off brand cheapo one so the module may very well be defective. I checked the mounting screws and the connections on the module and they appear to be good. I can't think of it being anything else if it all worked fine with the old distributor and now the new one throws off the timing. Ill look into getting a new module and if that doesn't work I'll just return the whole distributor. Thanks!
Le Mans Master
Joined: Sep 2004
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From: Livonia MI
Sound's like you've already got some pretty good advice here. Agree, while new, could be the HEI module, as not known to be the most reliable thing. Maybe a good time pulling the module and installing a MSD multi spark box. Did this on my car with a big cam, and idles much better. No complants at high rpm's either.
Melting Slicks
Joined: Oct 2002
Posts: 2,317
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From: St Louis MO
So how would I go about checking this for slipping? I assume I would just take off the belts and see if it moves around or has play in it in any direction? Assuming that is the case does that mean where I think TDC is is way off? I figured if it was too far off the car wouldn't even start or run? It feels like I'm definitely in the ballpark as far as timing goes with a small stumble. If that is the case it seems to me I could just advance or retard the timing way out either way until it improves or gets worse and ignore what my timing light is telling me to some degree, which has not worked either. Or perhaps I'm thinking about it all wrong?
Melting Slicks
Joined: Jun 2002
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From: Orrtanna Pa.
I'd try to seperate the plug wire you have the timing light connected to from all the other wires, and keep the pickup closer to the spark plug.
HEI distributors crank out major voltage and the other wires may be giving you a false read once in a while. (crossfire)
Or you can try watching under the hood at night (in the dark) to see if you have anything arcing.
HEI distributors crank out major voltage and the other wires may be giving you a false read once in a while. (crossfire)
Or you can try watching under the hood at night (in the dark) to see if you have anything arcing.
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Joined: Mar 2011
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From: Fairfield California
Waiting for some free time to look into it more. I wish it was just cross-talk on the wires but that wouldn't explain the car actually miss firing and idling a bit rough with the rpms bouncing a bit. Pretty sure the timing light is telling me the truth. One other thing that I don't think anyone has mentioned is a stretched timing chain? I have no idea how many miles this one has on it as I've only owned it for about 5 months.
Drifting
Joined: Dec 2004
Posts: 1,714
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From: Austin Texas
My car had the exact same symptoms as yours in regard to the bouncing timing mark. I recently performed a cam change and found that there was a large amount of play in the original timing chain. I replaced it with a new double roller model.
I no longer have this issue with my timing mark. All other factors, in my timing procedure are the same as before.
I no longer have this issue with my timing mark. All other factors, in my timing procedure are the same as before.
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Joined: Mar 2011
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From: Fairfield California
Been busy with work but have had some time to work on this timing problem recently. I went ahead and changed out my timing chain with a double roller. This did not fix the problem, but I do have a fresh chain and fixed an oil pan leak with a one peice gasket in the process. That being said I'm beginning to run out of ideas?