Unknown Engine Stumble/Hesitation

Hi
So at idle: no problem. At load: hesitation/stumble. What about when crusing at highway speeds?

If it happens at highway cruising:
I think the jets are too lean. At idle and low speed the idle screws seem to be OK. But before changing your jets, what does the plugs look like after a few miles of highway cruising?

If it only happens at load, but immediately:
It seems to be an ignition issue. When not at load the power required to fire the plug is less than when at load. Any number of components in the ignition system could be marginal, the coil, the wires, the plugs (gap too large) etc. [Personal experience: Had installed a Mallory Distributor that did something like this. I don't know if it got too hot, or the ground was bad or it was too close to other wires or what, tried to fix all of that, without success. Finally put on the old (restored) points distributor and all problems were gone]

If it only happens at load, after a 5-10 seconds:
Could be that it's running out of fuel? Clogged filter, bad fuel pump?

This was some suggestions. Unless one of them makes you say "that must be it", I'd suggest you do the following: Make it happen, stop the engine immediately, pull a plug. Is it wet with fuel? Then you have an ignition problem. Is it dry? Then you have a fuel delivery problem.

Allow me to recommend http://www.tuningmadeeasy.com/.../spark-plug-reading/ for reading plugs.
And for a full tuneup that I recommend you do (cheaper than replacing all components until it works) http://www.tuningmadeeasy.com/tuning/

Hope this helps. If it's not clear you're welcome to contact me directly

When I first got my car it did this too. I changed cap, rotor, wires, nothing helped until I replaced and regapped the plugs. I run a MSD and a gap at .050...the gap was too close and I opened it up.

" It stumbles and hesitate at times under load"

What carberator ? Is it a Holley .. is the stumble on acceleration ?

What distributor ? Is it a hesitation upon acceleration ?

Does it stumble and hesitate then catchup ?

Ron

Hey Folks:

Cheers for info so far. So to answer some questions.

It is a Holley 750 DP with a HP body (no choke tower, etc.)
The stumble is on acceleration/load, and seems to go in waves. When it gets hotter, it does it more.
The dizzy is stock, but with a brand new Ignitor 3... (however it was doing this prior to the new ignition unit..)
The hesitation/stumble, is like it misses a beat, one or more cylinders don't fire, the car does not jerk around, but you can definately feel that it slows. It last for a fraction of a second, then goes back to normal. Then it might do it again a few minutes after... quite a lot actually.
I just put a brand new fuel filter in, holley part. So all the ignition is new, coil, ignitor 3, cap, rotor, MSD leads and new fuel filter.
The only thing I have not changed is plugs. I did pull one, and it looked a good brown color, no oil, not burnt out white. But the plugs are a hotter number.
I will pull them and replace with stockers, and gap. Any suggestion on best gap setting?
As for fuel mixture, I I believe the jets are 70 and 80.. I could go richer.
Thoughts?
Cheers JC71

Here's what I would do:
1. get new plugs, 0.045"
if problem still there
2. go through a full carb adjustment as on the link I wrote earlier
if problem still there
3. take off all your fancy ignition stuff, put back on the old stuff or borrow. If that helps, then install the new components one at a time

I know, it's a lot of trial and error. But that's the best I can do, I can't pinpoint it from your description

Just my opinion:
Check your fuel float levels first.
Then the accelerator pump adjusment, and or the accel. pump diaphram.
The Holley if out of adjustment can be a bear to troubleshoot if not familier with them.

Jeff