Engine missing at cruise RPM - Pontiac GTO Forum
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post #1 of 15 (permalink) Old 12-19-2012, 04:30 PM Thread Starter
 
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Engine missing at cruise RPM

So I drove the car yesterday and I am getting a pronounced bucking/misfire at cruise RPM, tach isn't working currently but its from off idle to maybe 3000. It is most noticeable at steady RPM's... Was going to see if anyone had any suggestions, I will be going through the cap, rotor and wires to check for ignition issues. ALso my Holley 750 has not been rebuilt in at least 15 years so it could be the culprit. That said I am looking for a rebuilt quadrajet if anybody has one at a decent price. There are a few on ebay for 175-200 already rebuilt but they are for 1973's and I wanted to make sure they would work. Ideally I would like to procure the correct 400HO auto carb for 68 7028268.
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post #2 of 15 (permalink) Old 12-19-2012, 05:09 PM
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I had the same issue a couple of years back with my '65.....thought for sure it was carb related, it wasn't. It was too much advance at cruise due to a worn vacuum advance can bushing. Disconnect your vacuum advance and go drive the car. If the problem goes away, you need to cut the total advance down with a new bushing kit or an adjustable vac advance can. It'll take you 2 minutes to check, and then you can rule it out if the problem persists.
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post #3 of 15 (permalink) Old 12-19-2012, 05:55 PM Thread Starter
 
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Thanks geetee, I'll give that a try.
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post #4 of 15 (permalink) Old 12-19-2012, 06:06 PM
 
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Advance idea is a good one.

Also, make sure you're not getting cross-firing between #5 and #7 cylinders. Happened to me when I had a cracked spark plug. Can also happen if those two plug wires are running parallel and near to each other.

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post #5 of 15 (permalink) Old 12-19-2012, 08:11 PM Thread Starter
 
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Thanks, I'll fire it up in the dark this weekend and break out the spray bottle. I still want to find me a quadrajet
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post #6 of 15 (permalink) Old 12-30-2012, 11:06 AM Thread Starter
 
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Geetee, ran the car without vacuum advance connected and still had the stumble.
Didn't see any wires that looked like they were cross firing. I should have my new carb here in a week of two and I'll also grab new plugs and wires for grins. If that doesn't get it I'll dig deeper
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post #7 of 15 (permalink) Old 12-30-2012, 11:44 AM
 
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Engine missing at cruise RPM

Not sure if this could be contributing to your problem, but I had a similar stumble problem at high rpm and I found that my older plugs needed replacement.
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post #8 of 15 (permalink) Old 01-01-2013, 01:02 PM Thread Starter
 
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Thanks olde goat, I pulled all plugs and inspected yesterday, they look excellent. Inspected all wires and pulled cap and rotor (accel HEI) and after a test drive I think the problem may be gone. Maybe a wire was arcing or the seal between the coil and cap could have been on the wrong side of the carbon button. I don't know but I'll post when I figure out more.
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post #9 of 15 (permalink) Old 02-15-2013, 06:37 PM Thread Starter
 
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Re: Engine missing at cruise RPM

Problem is still there getting ready to rebuild my Quadrajet pretty soon so I may wait to do anything drastic. Problem is Intermittent and sure feels like an electrical issue but could easily be the old ass Holley. The problem is hard to describe and not serious, just enough to piss me off a lot! Might throw new wires and plugs on this weekend.
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post #10 of 15 (permalink) Old 02-15-2013, 09:10 PM
 
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Could be a flaky pick-up or ignition module.
Does the accel HEI have the moving advance plate with the two wires? A fraying or broken pickup wire can do this.

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