# Engine missing at cruise RPM



## FlambeauHO (Nov 24, 2011)

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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## geeteeohguy (Feb 2, 2008)

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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## FlambeauHO (Nov 24, 2011)

Thanks geetee, I'll give that a try.


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## jmt455 (Mar 26, 2012)

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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## FlambeauHO (Nov 24, 2011)

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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## FlambeauHO (Nov 24, 2011)

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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## olde-goat (Feb 1, 2009)

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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## FlambeauHO (Nov 24, 2011)

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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## FlambeauHO (Nov 24, 2011)

Problem is still there :banghead: 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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## jmt455 (Mar 26, 2012)

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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## FlambeauHO (Nov 24, 2011)

The Accel unit only has the plate that contacts the carbon button and 2 wires, no moving parts. I have thought about throwing a new distributor in but I'm gonna wait until the carb is rebuilt.


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## geeteeohguy (Feb 2, 2008)

What is the symptom? A chuggle? A lurch? Hiccup? a surge?? Fuel and ignition can be tough to sort out....it's one or the other. WHEN does it happen? In the cool of the morning or the heat of the afternoon? What does the car do? Die out completely? Just buck and surge? What about Wide Open Throttle...does it keep pulling, or does it buck and surge? I would install the original points dizzy if you have it and take it for a spin. If it doesn't buck or cough under full power, the fuel lines/pump/filter are ok. It COULD be an internal circuit in the carb. Carb problems are usually ignition problems, though. An other thing, on a dark night, open the hood....(not in a closed garage)...you can see cross-fire and short out of ignition wires very easily with the engine running.


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## FlambeauHO (Nov 24, 2011)

Just took it out, running perfect for first ten minutes and then a very slight surging at cruise. Also feels like it loses just a touch of power. Also a very small surge at most throttle settings. Does not seem temperature dependent. WOT feels fine with no surge but maybe a tiny bit of power loss.. like I am towing a small trailer. Don't have original distributor, may get new wires and plugs today.


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## geeteeohguy (Feb 2, 2008)

My weak a$$ 4runner does that when the AC compressor cycles! Do you have an insulator or thick gasket under your carb? It almost sounds like partial fuel evaporation....Does it ever do it when the engine is cold-ish, or only when fully warmed up?


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## FlambeauHO (Nov 24, 2011)

I do have a thick gasket, maybe a little less than a 1/4". I think it usually does it at temp but I'm not positive. Today I changed the oil with Rotella T 15w40 and some ZDDP additive. Purchased new plugs, wires, cap and rotor today and ordered a cheap HEI distributor with coil to eliminate that. Noticed today that during the missing/chugging shifting is late like the modulator isn't getting enough vacuum so we will see tomorrow if any of that stuff helps. Remember the Holley hasn't been rebuilt in 15 years... ordering qjet parts Tuesday.


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