Engine Hesistation/Stumble 1964 327/300

[Bruce Boatner]

This problem really has me stumped. My 1964 327/300 with a Carter AFB and PG hesitates/stumbles noticeably while accelerating from a dead stop. I have to "feather" the pedal to keep from the engine from stalling. Once I'm rolling I can put the hammer down with no problems--only at a dead stop do I have to tease the throttle. The problem is less severe after the engine warms up, but not appreciably different. Needless to say, not being able to take off strong from the hole is a big buzz kill.

I have eliminated all of the usual suspects having replaced the plugs, wires, points, rotor, distributor cap, etc. I completely rebuilt the carburetor making sure that the float settings were accurate. I also made sure the pump lever was adjusted properly (1/2" from the air horn). The problem was much less severe after I rebuilt the carb, probably because the pump diaphram was dried out and the float settings were wrong. I see no smoke after the engine "catches" so I suspect my problem is not due to an overich situation.

I'm thinking the problem is related to the carburetor, but I do not know what to try next. I would really appreciate any advice on where to go from here.

[Bob R.]

Re: Engine Hesistation/Stumble 1964 327/300

You might want to check the vacuum line from the transmission and the line from the vacuum advance. It sounds like it could be a vacuum leak.

[Wayne K.]

Re: Engine Hesistation/Stumble 1964 327/300

Bruce,

You may also try adjusting the pump lever so as to get a better shot out of it and see if that helps.

Wayne

[Bill Jackson]

Re: Engine Hesistation/Stumble 1964 327/300

Maybe one of the AFB experts could tell you if a larger accelerator pump discharge nozzle is available. Sounds like inadequate pump shot off idle.

[Bruce Boatner]

Re: Engine Hesistation/Stumble 1964 327/300

Wayne,

Thanks for your advice. I did try adjusting the pump lever "downward" because I was unclear if the 1/2" adjustment was from the "base" of the air horn or the nodules on the air horn next to the pump rod. Adjusting downward seemed to create slightly more hesisitation. I guess I could try an upward adjustment and see what happens, although this would probably produce a leaner shot. Easy enough to try though.

Have you ever heard about changing the size of the nozzle on the pump?

Bruce

[Randy Adams]

Re: Engine Hesistation/Stumble 1964 327/300

Had the same trouble with my '64. Check your vacuum advance weights. Make sure they haven't got a little rust on the bottom of them, they need to be able to slide.

[Wayne K.]

Re: Engine Hesistation/Stumble 1964 327/300

Bruce,

I'm not very familiar with AFB's but I had the same issue with my Holley and the increased pump shot took care of it.

Wayne

[Duke W.]

Re: Engine Hesistation/Stumble 1964 327/300

A few questions:

Does it idle smooth both in drive and neutral? What RPM reading at both conditions? (I believe the GM spec is 450 in Drive, but I'll let you check that.)

How far from the seat are the idle mixture screws?

Does it have the proper vacuum advance and does it function properly?

How does the engine respond to light on/off throttle transitions that are part of normal drving in traffic or maintaining a constant speed with slight upgrades and downgrades?

Duke

[Gene M.]

Re: Engine Hesistation/Stumble 1964 327/300

It shouldn't be the accelerator pump if you rebuilt it unless the rubber on the piston is no good. Verify that the pump is putting out a good shot. Gotta be a vacuum leak some place or late timing. The stock set-up for AFB calibrations as it was designed should not need modification. This is assuming the engine is as the General made it. Look for intake manifold bolts that need to be torqued to spec. This can be a source of vacuum leak. Another is the ignition advance. Check the weights and springs as some suggested. Verify that the vacuum canister is working. Power brakes and vacuum advance hose ok? Lastly verify the total advance is comming in on que. My 65 did this 4 or 5 years ago and it was advance.

[Bruce Boatner]

Re: Engine Hesistation/Stumble 1964 327/300

Duke,

The car idles reasonably smooth between 500 to 600 rpm while in drive (moves up to around 800 rpm in while park--smooth idle there). Idle gets a little rough below 500 rpm. While adjusting the timing, I lowered the idle slightly below 500 while the car was in nuetral/park. It idled OK, but not as smooth as I would be "comfortable with"

If memory serves, the idle screws are approx. 1 turn from fully seated. I adjusted the screws to obtain maximum vacuum using my trusty gauge. I adjusted both screws equally (i.e., same number of turns between left and right.

I checked the vacuum advance and it is functioning properly. Definite advance in timing with the can connected at idle.

The engine reponds normally at all times other than a standing start. Reponds well to light taps as well as mashes. Hard to comment regarding performance on upgrades/downgrades because I live in Houston. Seriously, my office parking garage has fairly steep ramps and I not encounter hestiation while driving up or down the ramps.

One thing I did notice that seemed unusual based on my limited knowledge. My idle speed actually INCREASES at low idle when I disconnect a small vacuum line. I always thought it was normal for idle to decrease when introducing a vacuum leak.

As always, thanks so much for your help!

Bruce

[Duke W.]

Re: Engine Hesistation/Stumble 1964 327/300

Nothing jumps out, except that last part about disconnecting the vacuum line and seeing increasing idle speed which would indicate a rich condition, but at 1.0 turns out your idle screws are lean relative to the nominal 1.5 turns out setting. Conficting data! Usually if you cause a vacuum leak by disconnecting a vacuum line the engine will lean out, lose revs, and run rough.

The engine should idle nearly butter smooth at 500 in neutral and 450 in Drive, so something is probably amiss, but it's subtle.

If you haven't already fiddled with the accelerator pump linkage try richer idle screw settings say 1.5 and even two turns out, and see if there's any effect.

Duke

[Bruce Boatner]

Re: Engine Hesistation/Stumble 1964 327/300

Gene,

Pump seems to be putting out a good shot, but I guess I don't have enough experience to know how good is good enough.

I re-torqued the intake manifold bolts and verified that the vacuum cannister is working. I checked the advance weights and springs and they appear to be fine. I have not mapped the advance curve yet, but did check that the timing advanced smoothly as the rpm's went up. At one point, I had the wrong vacuum can on the car creating too much advance at low rpm and noticed no difference in the amount of hesitation.

I've checked all the vacuum hoses I can get to and could not find a leak, although there may somewhere I did not know to look. If all else fails, I guess I could find a garage with a smoke machine to look for a leak.

Bruce

[Wayne C.]

Re: Engine Hesistation/Stumble 1964 327/300

I've heard that a good way to locate vacuum leaks is to use a propane torch... turn it on, don't light it, and move it slowly around hoses and intake manifold edges, etc aiming it from just a few inches away... if the engine noticeably picks up speed suddenly, then a vacuum leak is pulling the gas into the intake and burning it.

[Mike M.]

Re: Engine Hesistation/Stumble 1964 327/300

My experience tells me that off-idle hesitation is usually caused by a "lean" condition. So what could be the cause? All of the responses have been good in that they are fixes for a lean condition (richened carb power circuit, vacuum leaks or ignition advance problems). But my question is: did the car at some point earlier run without hesitation, i.e., what has changed? Did this problem gradually creep in or did it occur all at once? Did you do any major engine work, or had the car been sitting for a period of time?
One trick I have used to determine if the problem is carb or timing related is to advance the static timeing by 4 to 6 degrees and see if that has any major effect on the hesitation. If it helps, then more than likely your problem is in the distributor, if not, then the carb.
Lastly, I have seen this problem caused by a sloppy timing chain (twice, both on SBC's). This retards the camshaft. If the problem seems timing related and the distributor checks out fine I would check the timing chain, unless you have replaced it and know it is good.

MIke

[Frank H.]

Re: Engine acc. pump/ Hesistation

Bruce
Its not how big a shot its putting out ,its when,with engine off,look down the carb and note when accelerator pump begins to squirt,I think you'll see your hesitation is the pumps slight delay, to correct you need to remove the top of the carb,remove accelerator pump and adjust by
pulling its shaft up while holding the plunger end,while it looks like the rod is threaded,its really ribs that lock it into the plunger base.
go two ribs and reassemble and test it,
They make or used to make oversize (dia.) pumps ,your bore is wore just a little.

Frank