Hesitation off idle

[Oldsguy]

On my '77 cutlass, 350 with a quadrajet. Tuned up this weekend, inspected carburetor but didn't attempt to adjust it yet. The accelerator pump works, I think the idle may be low, I didn't check it but should have but it sounds like about 800 rpm in park. Could that be it? I know the solution would be to just re-build the carburetor which I will in the future but, for now, any adjustment or repair ideas in the meantime? Idle circuit works well, power circuit works well, secondaries come in smooth. For those who aren't familiar, 39,000 mile car, rarely driven, probably has some long storage issues.
edit: I checked the numbers on the carb, it is accurate for a 1976 Oldsmobile so I suspect it is original. Not electronic.

[joe_padavano]

Dan,

Hesitation off idle could be a symptom of crud inside the carb or a mis-adjusted float causing a slightly lean condition. It could also be the sign of a mis-adjusted or worn accelerator pump. And, yes, it can also be the sign of something completely unrelated to the carb. Without getting into the carb, you can at least band-aid the problem by tweaking the accelerator pump linkage or lever to increase the stroke of the pump. You need to be careful and do this a little at a time, since raising the pump cup too high up into the well can expose the bottom of the fill slot. I usually bend the pump link to shorten it, thus increasing stroke, but you can also adjust it at the end where it presses on the pump plunger.

Again, this is only a band-aid until you rebuild the carb and adjust it to spec.

[Oldsguy]

Yes, I was thinking about that fix but wasn't sure. A friend thought about checking for good vacuum advance too since it is supposed to provide that first shot of advance until the centrifugal weights kick in. I couldn't believe that the tune-up specs call for initial of 20 degrees! I am just not used to the tune up specs on the later seventies I guess. Seemed a little too steep to me. There were 14 degrees of initial advance so I didn't change it, at least not yet. I also haven't checked how much vacuum it is holding at idle, which may lead me to a vacuum leak on one of the myriad of hoses and sensors on top of the manifold. The car has been well maintained but I realize at that age parts can degrade, especially rubber ones, regardless of mileage on the car. I am really resisting the urge to strip down all that stuff to the bare minimum but my goal is to keep this car original, as long as I can fix this and keep it running smoothly and well I am not going to do that.

[BILL DEMMER]

intake air pre-heater system working?

[Oldsguy]

Yes, that valve is working well.

[Olds64]

Make sure the idle speed and idle mixture screws are set properly.

[88 coupe]

Originally Posted by Oldsguy

........ checking for good vacuum advanceb ........

If it's a small hesitation, it would be a good call. Lack of advance, and a vacuum leak at the same time

Originally Posted by Oldsguy

........ it is supposed to provide that first shot of advance until the centrifugal weights kick in ........

Not even close.

Vacuum, and centrifugal, advance mechanisms are totally different operations, and are not related in any way. Mechanical is tied (tightly) to engine RPM, while vacuum works primarily with the carb. When coming "off idle" there is a hesitation while the "pump shot" travels to the the combustion chamber, and one of the lesser functions of the vacuum system is to add advance, to the lean mixture, in order to smooth the transition.

Originally Posted by Oldsguy

........ the tune-up specs call for initial of 20 degrees ........

Low compression/lean mixtures need more advance.

Originally Posted by Oldsguy

........ 14 degrees of initial advance so I didn't change it ........

Try it at 20° and see if the hesitation goes away.

But don't bet on it, because I don't know as much as I think I do.

Norm

[Oldsguy]

Thanks Norm for helping me out on the difference between the vacuum and mechanical advance. I suspect I may need to take a closer look at the vacuum advance then. I suppose setting initial to 20 degrees certainly wouldn't hurt anything, since that is where Oldsmobile says it should be . The hesitation is a small, slight one, more noticeable with a very slight and slow accelerator pedal depression say from a stop sign, old man style, from 0 miles per hour to 3 miles per hour. Any other pedal action and the acceleration is smooth without any hesitation. I can overcome the hesitation by "goosing" it. As Joe eluded to, I really should rebuild the carb at some point.
Everyone, thanks for your comments and suggestions, in retrospect I really didn't complete the tune up and should have. I didn't set the timing per Oldsmobile, I didn't check advance, I didn't check for vacuum leaks, etc. I suppose I am getting lazy, and old...........both.

[BILL DEMMER]

infrequent use is saying to me, your accelerator pump cup is being allowed to "dry out" and shrink-causing a weak/delayed initial pump shot.

correct timing is a good thing, too


bill

[Abob89]

Hmm I just drove my '76 Cutty Supreme home from Seattle, WA to Vancouver, WA today and had the exact same issue. I give 'er some gas and there's this half a second-ish delay before it goes. But then again the car hasn't been turned on since November of last year.

Perhaps the same issue...

[Oldsguy]

I just spent a few more minutes with the car in the driveway. I adjusted the timing up to about 18 (not sure as the timing tab doesn't actually go that high, interesting...) and clicked the idle speed up about 150 rpms. Viola! no more hesitation. If I had not checked the tune up specs I would have never dreamed that initial timing that high would be applicable but as Norm said, low compression/lean mixtures need it. By the way, the fresh air intake hose was a horrid looking shiny aluminum one used on clothes driers. I picked up 18 inches of 4 1/8 inch black poly vinyl/ wire reinforced hose with cam lock clamps on each on at the local NAPA store, to the tune of $15.00. About 85 cents/foot is worth it, that aluminum was glaringly hideous.

[Olds64]

EXCELLENT! I bet you gained at least 25 horsepower by putting on that black poly vinyl reinforced hose. It is better at resisting heat than the aluminium stuff and will allow you to get a cold air induction effect. Definitely worth $.85/foot.