Intermittent Missfire???
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Melting Slicks
Joined: Jul 2010
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From: SUFFIELD CT USA 2023 C8 CORVETTE UN-MODIFIED FINALIST
2023 C8 of the Year Finalist - Unmodified
Intermittent Missfire???
My '77 has an intermittent missfire. It is basically a stock motor with a Quadrajet and has 103000 miles. The car starts and runs otherwise normally. Sometimes it will run normal for 10-30 seconds then it will have misses. It happens at all RPM's from idle up to cruise. It is very subtle and some people don't even notice it, one of my car buddies described it as not really a miss, another thought I had a leak in my exhaust! It may be more frequent when the car is cold. I have changed gas, looked for vacuum leaks and checked the PCV valve which seems to be operating normally. I added a can of Seafoam but have not driven enough to give it a chance to do its thing yet. This seems to be getting worse slowly, a couple of weeks ago I was not sure I had a problem, I am now! I did a total rebuild of the HEI w/ plugs and wires just before I put her to bed for the winter and the car ran great for April and the first half of May
Melting Slicks
Joined: Oct 2005
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From: Redruth Cornwall
If you rebuilt the hei, did you replace the module? Could be the module is overheating.
You could refit the original if you still have it and try.
Also did you use the correct heat paste?
I have read posts where after market modules don't seem to last as well as the original.
You could refit the original if you still have it and try.
Also did you use the correct heat paste?
I have read posts where after market modules don't seem to last as well as the original.
Melting Slicks
Joined: Aug 2012
Posts: 2,548
Likes: 379
From: Woodinville WA
sounds like a lean misfire - but honestly, it could be just about anything from bad gas to a burned valve (and everything in-between).
how did the pins look on the HEI when you rebuilt it?
how did the pins look on the HEI when you rebuilt it?
Thread Starter
Melting Slicks
Joined: Jul 2010
Posts: 2,762
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From: SUFFIELD CT USA 2023 C8 CORVETTE UN-MODIFIED FINALIST
2023 C8 of the Year Finalist - Unmodified
If you rebuilt the hei, did you replace the module? Could be the module is overheating.
You could refit the original if you still have it and try.
Also did you use the correct heat paste?
I have read posts where after market modules don't seem to last as well as the original.
You could refit the original if you still have it and try.
Also did you use the correct heat paste?
I have read posts where after market modules don't seem to last as well as the original.
Thread Starter
Melting Slicks
Joined: Jul 2010
Posts: 2,762
Likes: 820
From: SUFFIELD CT USA 2023 C8 CORVETTE UN-MODIFIED FINALIST
2023 C8 of the Year Finalist - Unmodified
The rotor and wires on the capacitor looked like toast all else looked good and everthing was new last December. Only about 500 miles since then!
Melting Slicks
Joined: Aug 2012
Posts: 2,548
Likes: 379
From: Woodinville WA
if those are worn, it will do much what you're saying because the timing advance won't be linear
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From: Orlando FL
How is the Dizzy Cap, if old, check the inside in between the contacts to see if you have any trails to another contact, I have had a problem once where it would jump or follow that trail to another contact and give a miss once in a while. I replaced and the problem went away.
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Melting Slicks
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From: SUFFIELD CT USA 2023 C8 CORVETTE UN-MODIFIED FINALIST
2023 C8 of the Year Finalist - Unmodified
Last edited by BPHORSEGUY; May 21, 2013 at 07:20 PM.
Thread Starter
Melting Slicks
Joined: Jul 2010
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From: SUFFIELD CT USA 2023 C8 CORVETTE UN-MODIFIED FINALIST
2023 C8 of the Year Finalist - Unmodified
How is the Dizzy Cap, if old, check the inside in between the contacts to see if you have any trails to another contact, I have had a problem once where it would jump or follow that trail to another contact and give a miss once in a while. I replaced and the problem went away.
Thread Starter
Melting Slicks
Joined: Jul 2010
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From: SUFFIELD CT USA 2023 C8 CORVETTE UN-MODIFIED FINALIST
2023 C8 of the Year Finalist - Unmodified
Melting Slicks
Joined: Mar 2002
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From: Beringen
Looks like not enough advance. Check the vacuum advance system if it's working properly/giving enough advance.
Verify that you have the plugs firing (cylinder balance test). You can do this by putting a piece of rubber piping between the dizzy tower and the plug wire and touching that with a grounded piece of electric wiring.
Verify that you have the plugs firing (cylinder balance test). You can do this by putting a piece of rubber piping between the dizzy tower and the plug wire and touching that with a grounded piece of electric wiring.
Thread Starter
Melting Slicks
Joined: Jul 2010
Posts: 2,762
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From: SUFFIELD CT USA 2023 C8 CORVETTE UN-MODIFIED FINALIST
2023 C8 of the Year Finalist - Unmodified
Thread Starter
Melting Slicks
Joined: Jul 2010
Posts: 2,762
Likes: 820
From: SUFFIELD CT USA 2023 C8 CORVETTE UN-MODIFIED FINALIST
2023 C8 of the Year Finalist - Unmodified
Looks like not enough advance. Check the vacuum advance system if it's working properly/giving enough advance.
Verify that you have the plugs firing (cylinder balance test). You can do this by putting a piece of rubber piping between the dizzy tower and the plug wire and touching that with a grounded piece of electric wiring.
Verify that you have the plugs firing (cylinder balance test). You can do this by putting a piece of rubber piping between the dizzy tower and the plug wire and touching that with a grounded piece of electric wiring.