| Engine misses on stone cold start-up [message #355260] |
Sun, 31 May 2020 07:06  |
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RF_Burns
Messages: 2277
Registered: June 2008
Location: S. Ontario, Canada
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Senior Member |
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So I've been trying to figure out an engine miss from stone cold start-up for a long time now... it's an on and off project. Once the engine is warmed a bit and rev'ed to clear it out, its good for the rest of the day, even after sitting for hours it will then start and run fine.
So I start the engine, it starts right up but has a definite miss. When put into gear and rev'ed to move it, you can definitely feel it missing. However give it a minute, then rev it up to 1500rpm, it will run rough.. blow a bit of black smoke then smooth out and run fine for the rest of the day.
I have a cold intake manifold (cross-over blocked) so I thought it was fuel condensing on the runners. Also the pre-heat cowling from the exhaust manifold is long gone so it doesn't get pre-heated air. When I rev'ed it, the increased air flow picks up the condensed fuel and makes a rich mixture for a few seconds then its cleared out and all is good. I have Throttle body EFI (originally Howell now EBL with Rochester TB) so adjusting the startup mixture is easy. Maybe fuel-drop-out is normal with a cold manifold. When I start it and pull it out of the shop, there is a heavy gas smell, so I can likely lean the choke setting even more.
What drives me nuts is the miss on one cylinder (I believe its only one). Over the years I have changed plugs and wires (DickP's wires and his recommendation of two different spark plugs). New coil and pickup of the correct polarity from DickP. New ignition module. BTW it did this running on OEM HEI and also now on Spark Control from the EBL.
I've tried starting it and let it run for 30 seconds or so, then pulled the plugs to see if I could troubleshoot which plug the miss was on. I could not see much difference between them.
I tried wrapping little copper wire out of each of the sparkplug wires at the distributor. Using them I could short the spark for each cylinder and see if I could determine which one was missing. I could invoke a miss on every cylinder, even while it was missing. So this lead me to believe it must be a random miss and not one particular cylinder.
It never backfires. However, even warm when rev'ed up to 1500-2000 rpm it runs a bit rough... not smooth like I would expect. It was rebuilt by DickP back in 2009 and I've asked him about it a couple of times... he though it was fine. I didn't specifically ask to have it balanced during the rebuild... I didn't know of such a thing at the time.
Anyway, does anyone have any ideas on my "missing" problem? I've lived with it for years but it just bugs me. I was thinking of putting a pickup coil on the HV output of the coil so I could look at it on my oscilloscope, but with HEI, there is no way to get to that. Maybe scoping the Tach output would give me some indication if an Ignition system problem...?
What say ye?
Thanks
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.
1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
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