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Old 09-28-2021, 08:22 AM   #1
s10mk
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HEI ignition problem help.

Hi everyone,
My car has developed a high rpm/ high load miss, can someone help me work through it. It's in my 67 Biscayne with a Chevy 350 and an HEI distributor. I have a good 12v to the HEI, and I did all of the basic resistance checks on the coil and pick-up, and those checked out ok. Im also having some trouble with my tach being very erratic, aftermarket Bosch. Sometimes the tach needle will swing up and down a thousand rpm while the engine is cruising steady. And sometimes the tach will bounce up and down while the engine is misfiring. I don't know if the two issues are related, but it gives me something to think about.
Could my tach be shorting out, causing a misfire? Or is the tach just reading whats happening with a dying coil/ module?
Thank you
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Old 09-28-2021, 04:31 PM   #2
Wrenchbender Ret
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Re: HEI ignition problem help.

I would look at the spark plugs & check the plug wires, dist cap. & rotor. It sounds like a high tension breakdown. Also could be a carb problem.
George
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Old 09-29-2021, 09:31 AM   #3
VetteVet
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Re: HEI ignition problem help.

How are you feeding the HEI distributor BAT terminal. If you are using the old coil resistance wire you will have to replace it with a new wire from an ignition on power wire.

I would just disconnect the tach wire at the distributor and see if anything changes with the misfire. Like wrench says, it could be a carb or plug problem.
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Old 09-29-2021, 03:47 PM   #4
Dead Parrot
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Re: HEI ignition problem help.

2nd on pulling the tach wire. If that doesn't work, try a new set of plugs. I chased a similar issue on a 79 Camaro. The carb and plugs were both new. After a session of getting the carb setup, it developed a high speed miss. After chasing a bunch of other things, tried a new set of plugs. Problem solved. Best guess is I developed "High Speed Glazing", described in one of the lower panels in the How to Read Your Plugs photo array. Basic idea is the plugs have a light coat of black carbon from running too rich then are subjected to high heat from running too lean. The carbon turns into a thin coat of conductive material that looks like a good plug color. Something that might happen while adjusting a new carb. I guess by then I had the carb adjusted good enough as the problem didn't happen again.
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Old 09-30-2021, 06:11 PM   #5
bob arrington
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Re: HEI ignition problem help.

carbon tracking
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Old 10-04-2021, 06:01 AM   #6
s10mk
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Re: HEI ignition problem help.

Quote:
Originally Posted by VetteVet View Post
How are you feeding the HEI distributor BAT terminal. If you are using the old coil resistance wire you will have to replace it with a new wire from an ignition on power wire.

I would just disconnect the tach wire at the distributor and see if anything changes with the misfire. Like wrench says, it could be a carb or plug problem.
I replaced the old resistance wire to the bulkhead connector with a new 12 gauge wire, my next step will be to disconnect the tach signal wire like you said.
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Old 10-04-2021, 02:06 PM   #7
s10mk
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Re: HEI ignition problem help.

So just a quick update. I replaced my plugs and wires. And I still had a mis in conjunction with a wonky tach. I disconnected the tach and the mis was still present. Then I replaced my coil with one I had laying around. The misfire went away and my tach is acting normal again. So I don't know what to make of that. Even with my old coil, my plugs made a good spark and the old coil passed the standard resistance checks. But the car appears to be running better...sooooo, I don't know what to make of that
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Old 10-04-2021, 07:48 PM   #8
dmjlambert
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Re: HEI ignition problem help.

Sounds like you found the problem, that's what I make of it. Good deal. Resistance checks may not tell you the whole story.
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