Hesitation on acceleration: what to check and in what order?

[willcarter]

Hi all,

Hoping for some insights. I have a ‘57 with a Holley 1273 carburetor. The carb has recently been rebuilt and restored by Holley. It was reinstalled on the car by a local shop. They did not, however, do any fine-tuning/adjustments at all; they just bolted it back on and hooked everything back up. (Their one “carb guy” is out for an extended period of time with no firm date of return).

I am still having a problem that I was having before the carb rebuild: significant hesitation and/or stalling upon acceleration. The problem is worse from a dead stop than while in motion; it’s more likely to stall completely if I give it any kind of sharp acceleration from a dead stop. Even while in motion, though, it will sometimes stall if I apply sharp acceleration (say, a quick pump of the gas pedal about 1/3 of the way down); but while in motion, it is more likely that I’ll get significant hesitation/stumble than a complete stall. The condition is the worst from a dead stop on an incline.

While in motion, I can usually prevent the hesitation from becoming a stall by (1) sloooowly accelerating rather than giving it a hard shot of gas/a firm push of the pedal as you normally would from a complete stop and/or (2) when I feel it starting to hesitate and stumble, “feathering”/playing the gas pedal until it smooths out again. (“Feathering” probably isn’t the right word, but hopefully you know what I mean. It reminds me a bit of when I first learned to drive on a stick shift and had to play the gas pedal to keep from stalling out while working the clutch.) But it’ll still occasionally stall out even under smooth and very modest acceleration while in motion.

(I should add here that in addition to the carb being recently rebuilt by Holley, I also recently had the fuel pump replaced with a brand-new one and that I have an auxiliary electric fuel pump. I have not systematically tested whether leaving the latter on all the time makes a difference. I don’t think it does - I have left it on for a couple of 10-15 minute drives - but I haven’t tested it systematically or for long periods because everything I’ve read suggests not leaving it on all the time.)

I’d like to try my hand at figuring out what’s going on. That’s in part from wanting to learn how to do some things myself and in part out of necessity; there are very (very) few shops near me that have anyone with any real experience tuning a carb and the few shops that do have such folks have months-long backlogs (I’m not *necessarily* assuming the issue has to do with the carb needing to be fine-tuned; see below).

I’ve done a lot of reading up on this, and it seems the most likely culprits (in no particular order) would be:

1. The carb’s accelerator pump needs to be adjusted;
2. I have a vacuum leak somewhere;
3. The choke may not be opening properly or at the right time;
4. The float level needs to be adjusted/the gas inside the carb chambers needs to be leveled on both sides (not exactly sure what this one means; I got it from here: viewtopic.php?p=117855&hilit=Carburetor ... b3#p117855)

I think I understand each of these in theory; whether I can properly deal with them myself in practice remains to be seen. My question for the moment is: of these possibilities (and any others I may have missed), which would it make sense for me to try to diagnose and adjust first as a matter of logic and of their relationship to each other? Does that make sense? In other words, does it make more sense to start with one of the above in particular and then, if changing it makes no difference, move on to the next logical one?

My completely uninformed instinct would be to check for vacuum leaks first simply because checking for and fixing a vacuum leak wouldn’t involve changing anything that could be detrimental (whereas, by contrast, I could easily mess up adjusting the accelerator pump, the choke, or the float levels/the levels of gas in the carb chambers, since I’ve never done any of that before). So, it would seem that that would be the place to start; and if I find and fix a vacuum leak and that fixes the issue, there’d be no need to mess around with, and possibly mess up, the carb-related matters.

(If vacuum leak is the logical place to start, then my follow up question would be about the best ways to identify and fix it. I’ve read enough to know that the main DIY methods of looking for a vacuum leak involve spraying flammable chemicals (carb cleaner, etc.) or soapy water. Only the latter is a realistic option for me; I’m far too nervous and inexperienced to be spraying flammable chemicals in what for me will be random places to see if it causes the RPMs to rise and hoping they don’t catch fire. But I don’t know whether the “spraying soapy water” method is likely to be useful.)

Thanks for any advice!

[paul2748]

Regarding a vacuum leak. Obviously check all the vacuum hoses to make sure they are in good condition. Don''t forget the ones under the dash going to the heater and wipers.

Next step is to make sure the applicable bolts are tight - the intake manifold and carb. The intake should be checked with a torque wrench to factory specs.

Sometimes the hesitation problem can be solved with a little more initial advance. 10 to 12 degrees is not out of the question. Factory is around 6, these engines seem to run better with more than that.

[willcarter]

Thanks, Paul. Will check the hoses and bolts. (And I don’t know anything at all about “advance.” I mean, I’ve heard the term but literally know nothing else about it.)

For now, I’m mainly looking for insights into the “order of operations” in which to proceed. I’ll then reach out for more help once I know what I’m attacking first!

Regarding a vacuum leak. Obviously check all the vacuum hoses to make sure they are in good condition. Don''t forget the ones under the dash going to the heater and wipers.

Next step is to make sure the applicable bolts are tight - the intake manifold and carb. The intake should be checked with a torque wrench to factory specs.

Sometimes the hesitation problem can be solved with a little more initial advance. 10 to 12 degrees is not out of the question. Factory is around 6, these engines seem to run better with more than that.

[ward 57]

No need to be afraid of carb cleaner. it will not blow up in your face. Did you keep your old fuel pump? New replacement pumps are notorious for failing. Better to have your original rebuilt. I would point to your accelerator pump. A fast and easy check is to remove the air cleaner and when not running hold the choke butterfly open and work the throttle by hand, you should see a good shot of fuel into the carb venturi. If not, it needs to be adjusted. A simple screw on the pump lever is all it takes to increase the throw. If you have not yet purchased a shop manual by all means get one. It is full of trouble shooting info.

[Jimntempe]

As another poster mentioned, you can look down the carb throat past the choke plate and see if you are getting a good squirt of fuel when you move the throttle. If you are lucky all of your problem is that the accelerator pump is not working and that would fit the problem you are having. It would be the very first thing I would check.

Assuming your car is stock and the carb was rebuilt to stock it should not require any adjustments other then to adjust the idle speed and idle mixture screws and only minor adjustment of that is likely to be needed.. but.. a poor adjustment of them would not cause it to have bad hesitation and stalling out once underway, they mainly affect idle quality.

Since you had the same problem before you had the carb rebuilt odds are it's not the carb OR there is a serious flaw in the carb, crack in the body, badly warped body. If not then you have to look for something else wrong with the fuel/air system on the car.. a big vacuum leak perhaps, or the ignition timing is way off.

One common way to look for vacuum leaks is to spray carb cleaner or propane around the various places air might be leaking in, the base of the carb, the various vacuum hoses and fittings, the manifold where it mates to the heads. If there's a leak it will suck in the spray and the engine will speed up from the extra shot of fuel.

It might also be that the diaphragm on the vacuum advance on the distributor is leaking which potentially could cause both a vacuum leak and a failure of the advance to move under acceleration. You can check that by sucking on the end of a hose attached to the vacuum pot and see if it holds vacuum.

You would need a timing light to check the spark advance setting and I'm guessing you don't have one so leave that for the shop to do if you wind up taking it back to them.

[CSPIDY]

If you have a vacuum leak the engine idle will be high
I would put a vacuum gauge on the engine
This will help diagnose the problem

The float adjustment on these Hollys is very simple
There is a large screw on the side of the bowl, take it out and the level should be at the bottom of the hole
Much like checking the oil in the rear end or a manual transmission

[willcarter]

I do still have the original fuel pump. (Pictures attached). Other than having “made in USA” stamped on it, I can’t tell what make or model it might be. If anyone knows, that would be great.

I went with a new fuel pump rather than having this one rebuilt because the shop was unable to locate a rebuilder and said that this one was non-original, so they advised getting a née fuel pump.

No need to be afraid of carb cleaner. it will not blow up in your face. Did you keep your old fuel pump? New replacement pumps are notorious for failing. Better to have your original rebuilt. I would point to your accelerator pump. A fast and easy check is to remove the air cleaner and when not running hold the choke butterfly open and work the throttle by hand, you should see a good shot of fuel into the carb venturi. If not, it needs to be adjusted. A simple screw on the pump lever is all it takes to increase the throw. If you have not yet purchased a shop manual by all means get one. It is full of trouble shooting info.

[Jimntempe]

No need to be afraid of carb cleaner. it will not blow up in your face. Did you keep your old fuel pump? New replacement pumps are notorious for failing. Better to have your original rebuilt. I would point to your accelerator pump. A fast and easy check is to remove the air cleaner and when not running hold the choke butterfly open and work the throttle by hand, you should see a good shot of fuel into the carb venturi. If not, it needs to be adjusted. A simple screw on the pump lever is all it takes to increase the throw. If you have not yet purchased a shop manual by all means get one. It is full of trouble shooting info.

As to carb cleaner.. I don't think I would disagree when it comes to the holley but in general I don't like using carb cleaner on an installed and assembled carb after an experience I had with a quadrajet. I sprayed a bunch of cleaner in it and some time later when I finally took it apart I discovered that the carb cleaner had cleaned off deposits in one part of the carb but in areas with very small clearances, which was around the power piston, the deposits had concentrated and redeposited and glued the piston to the walls.

[paul2748]

The make of the old pump is not relevant as long as it says made in USA.

Here are a couple of rebuilders who have a good reputation

Arthur Gold Rebuilders 508-210-0891 (Mass.)
Terrill Machine 254-893-2610 (Texas)

I have used Arthur Gould for two pumps. One is in use right now and no problems so far. The other is a spare and has not been used. No experience with the other rebuilder

I do still have the original fuel pump. (Pictures attached). Other than having “made in USA” stamped on it, I can’t tell what make or model it might be. If anyone knows, that would be great.

I went with a new fuel pump rather than having this one rebuilt because the shop was unable to locate a rebuilder and said that this one was non-original, so they advised getting a née fuel pump.

[swatson999]

These are the exact symptoms I had when I purchased my 56 last year. Even the "feathering" of the accelerator pedal matches. Turned out to be the accelerator pump (whoever rebuilt the carb sometime in the late 90s when it was restored *forgot to put in the needle valve* for the pump, so the engine was being flooded whenever the pump was activated. Local carb rebuilder fixed 'er all up for me, and demo'd the gush of gas that was being pumped out without the needle. (I had him overhaul the whole carb, just to make sure nothing else was missed and it has proper gaskets and seals and all for today's gas...working like a champ now!).

I'd say start there... :)

[willcarter]

Thanks! I’ll check to see if the needle valve is there. There’s a useful video about that here:

https://m.youtube.com/watch?v=_xHI8S-lymQ

If I understand the terminology right, the “needle valve” is in the “squirter,” not part of the accelerator pump itself. Does that sound right?

Anyway: I will check that. I’d be surprised if Holley forgot to put the needle valve back in when doing my carb rebuild, for two reasons: (1) I’d think they’d do it correctly and (2) the hesitation on mine is the same before and after the rebuild. But perhaps servicing that portion of the carb isn’t part of the rebuild service?

These are the exact symptoms I had when I purchased my 56 last year. Even the "feathering" of the accelerator pedal matches. Turned out to be the accelerator pump (whoever rebuilt the carb sometime in the late 90s when it was restored *forgot to put in the needle valve* for the pump, so the engine was being flooded whenever the pump was activated. Local carb rebuilder fixed 'er all up for me, and demo'd the gush of gas that was being pumped out without the needle. (I had him overhaul the whole carb, just to make sure nothing else was missed and it has proper gaskets and seals and all for today's gas...working like a champ now!).

I'd say start there... :)

[Rusty57]

Here are 4 simple steps you can perform and post the results. Then we might be able to better help you diagnose your problem. Without results from checks and test we are all just guessing and speculating. My experience is that this approach will not solve your problem.

What carburetor (List number) is on your engine? I remember that in another thread you posted a picture of a 1273-2. I just wonder if you really got your original carb back. And with the original float bowls.

With the engine not running, what do you see when you look down the primary throttle bores and open the throttle. Do you see 2 nice small streams of gas, no gas, or a big flood of gas?

When the engine is running and at operating temperature, what is the position of the choke plate? Once again, a picture will really help!

Using a 2' piece of 3/8" hose, can you hear any vacuum leaks when you hold one end to your ear and move the other around the carburetor and manifold when the engine is running? If you want to get more professional go to Harbor Freight and buy a mechanic's stethoscope. Thy are on sale for $4 right now. Otherwise a whole $5.

By the way, I use WD-40 to check for vacuum leaks. I agree with Jimntempe. Spraying carb cleaner on an assembled carb can create more problems than it solves. Plus WD-40 does nor attack paint like carb cleaner.

[willcarter]

Thanks, all! I really do appreciate your continued advice.

Got the car back: here are my initial findings:

1. It is indeed the same carb that I sent in to Holley (picture attached). I have no way to determine whether it still has the original float bowls.

2. The choke appears to be fully open once the car’s at operating temperature. (Second picture)

3. With the engine off, I see what appear to be two good streams of gas pumped when I operate the throttle. I say “appear to be” just because I have no baseline against which to judge it. But it neither appears to be a “flood” nor a nominal amount; looks like two strong squirts with each pull of the throttle.

Next up will be to check whether the needle valve is in fact in the squirter. If it is, I’ll then check the setting of the accelerator pump.

[willcarter]

To follow up:

I removed the screw over where the needle valve is supposed to be. There’s no needle valve in there as far as I can tell (see attached picture).

(Also: I dropped the plastic gasket when removing the screw. I can see it: just gotta figure out how to fish that sucker out! It’s plastic, so a magnetized tool isn’t going to help…)

But before I say “a ha, missing needle valve; that’s it!”, a question. I have the Holley 4150 service manual. As you’ll see from the attached pictures of the relevant pages, it makes no reference to a needle valve. #31 and #32 are a “pump check ball weight” and a “pump check ball.” I recall reading somewhere during my research that a Holley carb can either use a needle valve or a check ball & weight. So now I’m stumped: I can’t see clearly enough into the hole to tell whether there’s a ball and weight there or not. As you can hopefully see in one of the attached pictures, there is a silver reflective bit at the bottom of the hole, but I can’t tell whether that’s the top of the weight (#31 in the manual) or just the bottom of the hole (meaning I’m missing the needle valve or the check ball & weight). And I’ve never seen another carb for comparison, so I don’t know what this assembly should look like.

Any help is appreciated!

[Rusty57]

Thanks for those details. That is good information to consider.

Those appear to be the original float bowls. Sometimes the float bowls will be replaced with newer style ones that have externally adjustable float levels.

I do not think your problem is the accelerator pump discharge. If the check ball was not in place the gas would just flow out of the nozzles with no control. It would actually siphon out of the accelerator pump well.

There is

You are correct that this vintage Holley used the weight and ball.

Here are 3 more simple checks.
Have you checked the float bowl fuel levels when the car is idling?
When you do the accelerator pump discharge check, what is the relationship between when the nozzles start to discharge and how far you have opened the throttle? Does the discharge start early in the throttle travel or later?
Is the lockout link between the primary and secondary throttle shafts in place and operating correctly? If this link is not operating correctly the secondary throttle plate could be trying to open too soon. That will bog down the engine because there is a lot of extra air with no fuel to help keep things going There is no accelerator pump on the secondary circuit.