cold-excellent warm-stumbles at cruise? - Team Camaro Tech
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post #1 of 16 (permalink) Old Feb 17th, 06, 04:49 AM Thread Starter
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cold-excellent warm-stumbles at cruise?

Excellent working Quadrajet that I always ran went bad so I bought a new Holley 750 DP (QJ couldnt be rebuilt). It idles at a stop excellent. It revs at a stop excellent. When cold, it is excellent when cruising and under heavy load. Once I get around 195-205 in temp, it starts to stumble/surge/run poorly, only at cruise and partial throttle. It still idles and revs at a stop exceelent and runs excellent under heavy load. What gives? The fuel line is routed the same as with the QJ and insulated so I couldnt imagine hot fuel has anything to do with it unless Holleys are like that. I dont think it has anything to do with fuel pressure since it is the same (6-7) both warm and cold (although I havent checked it while driving). The vacuum from the manifold is the same both warm and cold. Any ideas.
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post #2 of 16 (permalink) Old Feb 17th, 06, 06:40 AM
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Re: cold-excellent warm-stumbles at cruise?

check your plug wires at night for leakage and or bad cap and rotor


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post #3 of 16 (permalink) Old Feb 17th, 06, 12:56 PM
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Re: cold-excellent warm-stumbles at cruise?

Check for proper choke operation also.
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post #4 of 16 (permalink) Old Feb 17th, 06, 01:05 PM Thread Starter
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Re: cold-excellent warm-stumbles at cruise?

Ive got the choke wired open and just play with it until warm. How would the choke being fully open when warm cause a problem?
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post #5 of 16 (permalink) Old Feb 17th, 06, 04:58 PM
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Re: cold-excellent warm-stumbles at cruise?

It's more likely that the carb is way rich for some reason (like blown power valve). And when it warms up its way rich. What do you think? Check a few plugs after your next hot cruise. You don't want to read the plugs from a cold idle.

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post #6 of 16 (permalink) Old Feb 18th, 06, 07:30 PM
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Re: cold-excellent warm-stumbles at cruise?

If you have the choke wired open, then that means no choke at all during cold start. Yet you say it runs great when cold, which means running very rich. Could be power valve (as stated above) though new Holleys have protection against blowing them, unlike older Holleys which would spit and blow the PV immediately.

Vacuum leak would also be a big suspect, IMO.

Did you set float levels?

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post #7 of 16 (permalink) Old Feb 19th, 06, 07:29 AM Thread Starter
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Re: cold-excellent warm-stumbles at cruise?

I set the floats but I havent done anything with the mixture screws and thought the carb was slightly too large for the engine. Instead of getting rid of the carb, would it be of any benefit to lean out the mixture a bit with the mixture screws and change to smaller jets?
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post #8 of 16 (permalink) Old Feb 19th, 06, 07:56 AM
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Re: cold-excellent warm-stumbles at cruise?

I agree on checking your plugs and plug wires. The next check would be for vac leaks.
If you are running to rich you should be able TO smell it and possibly see the black smoke comming from you pipes when you rev warm.
You shouldn't need to touch a new Holley they are all preset From the factory.
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post #9 of 16 (permalink) Old Feb 19th, 06, 08:42 AM
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Re: cold-excellent warm-stumbles at cruise?

Mixture screws only affect the idle and slightly off-idle. You'll need to adjust those for your engine. I usually set both sides equally, then (one at a time) turn in til engine starts to stumble a little, then back out til runs good.

Jet size won't correct your problem. I remain convinced you got something else going on there. There's just no way the thing should run good when it's cold with no choke, IMO. To me, that means it's rich as a bit*h. Do you smell rich mixture or see black smoke out tail pipes?

Have you checked the basic tune? i.e.; plugs, wires, cap, rotor, timing, etc?

Also, look at any vacuum lines very carefully and replace if questionable. I have also seen guys use incorrect carb base gasket, which caused vacuum leak.

Did you check the power valves?

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post #10 of 16 (permalink) Old Feb 19th, 06, 08:43 AM
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Re: cold-excellent warm-stumbles at cruise?

Excellent working Quadrajet that I always ran went bad so I bought a new Holley 750 DP (QJ couldnt be rebuilt).

What failed in the Q-jet that it couldn't be rebuilt?
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post #11 of 16 (permalink) Old Feb 19th, 06, 11:38 AM Thread Starter
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Re: cold-excellent warm-stumbles at cruise?

Cant remember exactly why QJet couldnt be rebuilt but the shop that I had used in the past said it couldnt be. I havent been able to find any vacuum leaks. Guess, I will run a line to view the vacuum at cruise to see if I have any unusual issues. Will one of those air/fuel gauges tell me if I am waaaaay rich when I heat up or are they worthless?
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post #12 of 16 (permalink) Old Feb 19th, 06, 01:48 PM
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Re: cold-excellent warm-stumbles at cruise?

Quote:
Originally Posted by wheezer
.........Will one of those air/fuel gauges tell me if I am waaaaay rich when I heat up or are they worthless?
They are very limited. If you know someone with a dyno or an inspection station, maybe have them stick a wide-band O2 sensor up the tail pipe.

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post #13 of 16 (permalink) Old Feb 19th, 06, 03:54 PM Thread Starter
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Re: cold-excellent warm-stumbles at cruise?

Tough to work on the car and get it warm when its 10 degrees so I'll have to wait a few days however I figured I would see approx where the idle mixtures screws were at just for the hell of it and they were 1/2 turn from bottoming out on the lean side (thats clockwise right)? What gives? I thought they came fat from Holley and I certainly wouldnt consider 1/2 turn fat, although I have little to no experience with Holleys.
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post #14 of 16 (permalink) Old Feb 25th, 06, 07:40 AM
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Re: cold-excellent warm-stumbles at cruise?

Quote:
Originally Posted by wheezer
Cant remember exactly why QJet couldnt be rebuilt but the shop that I had used in the past said it couldnt be.
Sounds like someone at the shop needed a Q-Jet. Usually the bases get warped from overtightening and some guy's don't think they can be fixed. Most likely what they saw. Naturally, new base takes care of that. I have noticed a trend though that most shops don't want to "re-build" if they can "re-bolt" a part. LOL

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post #15 of 16 (permalink) Old Feb 26th, 06, 08:43 PM
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Re: cold-excellent warm-stumbles at cruise?

What intake are you running? If it's the cast iron Q-Jet version, circa 1968/1969 it will route hot exhaust directly to carb. Q-Jets used the stainless mounting "gasket" to keep the fuel in the carb from percolating.

On your intake where the carb mounts,..is there a trough in front of the primaries? If so, this could be your problem with poor performance at temp.
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