C3 backfire.

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Jan 1, 2018 | 06:52 PM

#41

Tamaren

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Cruising

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Quote:

Originally Posted by HeadsU.P.

Back to drawing board. Sharpen chaulk.

Maybe the valvelash is a little on the tight side on that one valve. Sometimes when setting the lash a lifter has bled off (slack). Then when you go to start it up, it builds up pressure in the lifter, slack or lash is removed. But now, does not allow the valve to close completly. Pop thru the EXH.

Once the car cools off, I will be sure to pull that valve cover and check! Also going to check plugs for idle mixture.

Jan 1, 2018 | 07:01 PM

#42

HeadsU.P.

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From: Cool Northern Michigan

Quote:

Originally Posted by Tamaren

Once the car cools off, I will be sure to pull that valve cover and check! Also going to check plugs for idle mixture.

Only happens "under load" is the clue here. But I don't know what. Normally one would say vaccuum leak. But you already went through this. Clue number two; passenger side.
So that isolates somewhat. Clue number three: only in the exhaust. H-m-m-m-m-

Jan 1, 2018 | 08:15 PM

#43

Rescue Rogers

Is my vette stock?? HAHA

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one cylinder is running lean. Bad plug maybe, mis-gapped. I would check all the plugs on that side for oil or gap. Maybe a pinched wire

Last edited by Rescue Rogers; Jan 1, 2018 at 08:17 PM.

Jan 1, 2018 | 08:56 PM

#44

71bbc

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From: Centreville, VA

another valve lash question...

So, I finished my build a few weeks ago. LS5 454 +0.030, all forged internals, 10.00:1 CR, alum heads, roller lifters, and rockers. Per instruction sheet for my lifters, I have to turn the nut somewhere between 1/2 to 3/4 turn after zero lash. I did so and motor was not happy. Erratic vacuum, backfires and asks for too much advance to stay running. So, pulled covers and readjust them while motor was warm and stopped, this time 1/4 turn after zero lash...
Big difference...Now at 18 deg BTDC it idles at 14" Hg constant, no erratic anymore. Fair idle considering my mild cam. Now, is something I need to worry about it? or just leave it like that. Thank you

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Jan 1, 2018 | 09:00 PM

#45

Tamaren

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Cruising

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Quote:

Originally Posted by Rescue Rogers

one cylinder is running lean. Bad plug maybe, mis-gapped. I would check all the plugs on that side for oil or gap. Maybe a pinched wire

I like this, will check. Will pull all and gap to .040.

Jan 1, 2018 | 09:04 PM

#46

Tamaren

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Quote:

Originally Posted by 71bbc

Holy cow, Nice build! If I get to that point that I will double check lash. Was just out there, all plugs were firing and all rockers look right where I left them. Mixture looked good to me, plugs just about right.

Jan 1, 2018 | 10:48 PM

#47

Tamaren

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*cough* So, I was out there checking valve lash (and needed a bit of adjustment, I'd say that was causing the vacuum issue)

And, uh, I may of found that, uh, cyl. 2 & 4 were switched at at the plugs....

Thanks everyone who recommended that and I told you it was fine... my bad.

Will drive In the morning, I'm pretty sure that's the issue.