Stevejr

Hello, our car occasionally misfires during acceleration on the highway, but not from a stop. One of my friends rebuilt the Quadrajet, thinking that was the problem. The '71 carberator is on a '72 engine. Maybe that's the problem. Could it be the fuel pump? There is excess cranking and stepping on the gas pedal before it starts up, but the car does sit for at least 2 weeks before its started. The engine has only 55,000 miles, and the plugs, wires, coil, and cap were all replaced. Any help would be appreciated. Thank you.

Vintage Chief

If what you're suggesting truly feels/sounds like a misfire, then no - the issue would most likely not reside w/ the carburetor itself. Generally speaking, a misfire would suggest an electrical issue associated w/ the dwell and/or timing of the engine (perhaps the distributor, rotor and/or points).

I suggest you perform a thorough engine tune-up to specifications as outlined in your CSM and/or the label on your fan shroud. You should begin w/ relatively new (or brand new) correct spark plugs with the appropriate gap, a relatively new (or brand new) distributor rotor, relatively new (or brand new) distributor contact points (if you are using contact points). Your coil should be relatively new (or brand new).

This must be performed in the following order from (1) to (3):

(1) Set dwell to specs
(2) Set timing to specs
(3) Adjust A/F mixture screws to highest achievable vacuum

oldcutlass

Hard starting is normal for these cars when they sit for weeks. What your describing is normal. The missing at highway speeds may be timing related. To far advanced timing with the added advance from the vacuum advance unit. I'd start by looking at my tune first.

Fun71

I agree with what the others have said. To address your questions, the 1971 and 1972 engines were nearly identical, so the perceived carb mismatch is not an issue. Also, the fuel pump is likely not an issue either.

When you experience these misfires, is it during steady cruising?

Stevejr

It doesn't happen during steady cruising. Only occasionally, when I accelerate during the cruising.

Vintage Chief

Question for you. Do you think it's a misfire or does it appear to "bog down" when you accelerate? Sort of like - a hesitation in acceleration at first, then it catches up quickly and is OK after that i.e cruises after an initial hesitation or bog down? It might be the case your accelerator pump needs changing. A very simple process, but you did say your friend recently rebuilt your carburetor. Most Quadrajet rebuild kits come w/ a new accelerator pump - but not the linkage itself.

Stevejr

I believe he did change it, since it came with one. I do get a hesitation, which I thought is a misfire. It sounds like a snapping sound, a single snap, then it rides normally. When it happened one particular time, the car almost stalled.

Vintage Chief

Well, it does appear to be leading more towards fuel flow than an electrical misfire. If the car almost stalled, it suggests fuel starvation, but that isn't to say (1) vacuum advance isn't sufficient and/or (2) engine/system vacuum might be suspect (even the float in the carburetor possibly).
These annoyances are difficult to address on someone else vehicle w/o first-hand knowledge of the response of the vehicle.

I'd still begin w/ the basic tune-up to ensure dwell, timing & A/F mixture is set to highest achievable vacuum then move on from there.

Is this something you are going to address on your own (turn your own wrenches) or something you'd pass along to a mechanic?

Stevejr

Another friend is one of my mechanics, not the one who rebuilt the carb. I'll have him check all the basics first, like you said, but not until the Spring. Then we will see what that leads to. Thank you for all you help and expertise. Have a Healthy & Happy New Year!

Vintage Chief

Very good. Have a Safe and Happy New Year.

bccan

Obviously check the tune first.

Check and/or replace the rubber fuel lines between the gas tank & steel lines, they may be soft & sucking closed under high fuel demand ie accelerating on the highway or up a hill. It’s a half hour task that is probably due whether it’s actually you’re problem or not.

Hard starting after sitting for weeks is likely leaking well plugs in the carb. Worth fixing if you’re in the carb but quite optional as a stand alone project. As Eric mentioned this could be considered normal & IMO aggravated by fuel evaporation with oxygenated/alcohol mix fuel formulations.

droldsmorland

Please don't forget to come back here and close the thread with the fix. Important to close the loop for future searches.

69CSHC

Absolutely ! one of the issues I had with the 69, my situation was most exposed at the track. Would literally have several popping hesitations 2/3s of the the way down the track. Very inconsistent, one pass would barely do it or be noticeable and the next would be a disaster. 1 minute the car was running like a Bull, John Force burnout in the pits... Then nail the gas at the line and pop stalls. Pass is shot because the timer started. 2 hours to get to track, 2 hours to make a pass at times, and a complete waste of time and money if you only have one shot etc... Tinker, tinker, tinker, still hit or miss as to whether the car would make a decent pass. Bothered the hell out of me. Obviously I went after this issue with a vengeance.

Those rubber hoses needed replacing, and the air fuel mixture screws on the new carb needed plenty of adjustment. 3 MPH gain in performance at full weight full options and a slower track.

Ah yes, I wish I could like this comment 1000 times. So important and seemingly so seldom done. Cant tell you how many times over the last 15 years I have come across a question and tons of suggestions about what may be the problem. Only for the OP to never say what if anything worked. Not just on this site, across all sites.

Stevejr

Thanks to everyone for all their help. We will find out in the Spring what the actual problem was.

Vintage Chief

It is indeed spring or later. Did you figure it out?

Stevejr

Everything was put on hold because of the pandemic. My mechanic/friend seems to think it's the carburetor. He is going to rebuild it this summer. Hopefully, that's the problem.

Stevejr

Well, my mechanic / friend finally rebuilt the Quadrajet today. The car has never ran so well! The tires screech out of the hole, and there is no hesitation or misfire upon
acceleration. He seems to think the accelerator pump was bad. She runs like a new car! Thanks to everyone for their input and concern regarding my Cutlass.

Vintage Chief

Good deal. Happy motoring!