I have a question as to particular cause(s) for an off-idle stumble. I have a 289-2V, w/ auto C4 trans in my '66 Mustang. Here are the following tests, repairs, and conditions; carburetor (stock 2100) rebuilt - twice - by experts at Pxxx Carburetors. They noted no such stumble when they tested on their 302 mule engine. I've even timed and re-timed the engine at idle following their recommended procedure by not using a timing light, but using the old 'ping' and advance method.
Installed Pertronix ignition module, new; spark plugs, wires, fuel filter, distributor advance. The engine will idle OK, but has an off-idle stumble as soon as the throttle is cracked.
The engine compression is strong and balanced in all eight cylinders, and the manifold vacuum is a strong and steady 20-21 inches Hg with no gage needle bounce or movement. The distributor vacuum advance diaphragm tests OK - hold vacuum when applied by my Mity-Vac.
Any other suggestions?

 

That 'stumble' is also called a 'flat spot' or 'hesitation' by others. I've got it on one car, I know others that have it -it's a carburetor trait. Sometimes it can be the accelerator pump putting too much raw gas into an open venturi, sometimes it's not enough. That's an adjustment.
My recomendation - learn to drive around it, don't 'punch it' when you start from a stand still in other words. Something else you might try is to raise your idle speed a bit, say 50 or 100 rpm to about 750 or 800 rpm.
Consider a Pertronix ignition too.
Jim

 

If the 2100's have any way of tuning you could try the accelerator pump circuit as well, bigger/smaller squirters, changing the amount of fuel delivery etc.

 

I stored my 65 every winter for 10 years, and I always filled the tank with gas before storing it and put in some stabil. Well, after several years of this, the old gas just got some water in it from condensation. The 65 would run good for a few seconds, then stumble. It was the water in the gas. So, I drained out the old gas and put in a fresh tank. Ran like a champion after that.
How old is your gas?

 

The 289-2V ignition already has a Pertronix module (first edition) in the distributor. The carb accelerator pump has been replaced with new, and adjusted by both the 'experts' at Pxxx Carburetors and myself. The twice-rebuilt carb was tested using the same choke, throttle plate, accelerator pump, and idle mixture settings on their test engine, and then re-installed on my car - with vastly different results. The folks at Pxxx Carburetors say there was no such stumble or hestation on their 302 test engine after they rebuilt the carb - on two seperate occaisions. By the way, the carb was originally shipped to these experts for rebuilding and repair specifically because of the nagging flat spot, or hesitation, or stumble - however the term is used.

66GTKFB - there is no way to 'drive around' this hesitation, or stumble - it occurs as soon as the throttle is cracked - from the idle position. It happens regardless of the throttle opening speed, i.e. whether you feather or mash the pedal. Also, the stumble is severe enough that raising the idle speed as suggested won't affect it - the idle speed now is at 700 RPM.

67mustang302 - the fuel in the tank is maybe two weeks old - no stale gas in this case.

Given that the same carb gave different results ( no hesitation vs. hesitation) on two different engines - I'm wondering what arethose differences that I need tocheck? I've already ruled-out / repaired the most common causes; weak ignition, faulty vacuum advance, vacuum leak, cylinder compression / imbalance, poor fuel, etc.
Thanks for the suggestions! Keep the ideas coming 'cause I know I'll figure this one out!

 

You didn't answer me. How old is your gas?

 

Note the previous reply in regard....'the fuel in the tank is maybe two weeks old - no stale gas in this case.'
Besides, the age of the gas would not explain why the stumble exists only at one portion of the RPM range - if the fuel was bad this would affect all driving speeds and RPMs - not just at idle.

 

Vacuum leak? Maybe you aren't getting a good seal between the carb and intake manifold (where the rebuild shop had a "near perfect" seal). Almost all of my carb problems have been related to a vaccuum leak, but I usually blamed the carb first.

Just a thought.

-Edit-
I just re-read that you testedfor leaks.Maybe check again?

 

cruchyskippy - thanks for the suggestion...please note the previous post, in regard to the vacuum leak testing.....'the manifold vacuum is a strong and steady 20-21 inches Hg with no gage needle bounce or movement. The distributor vacuum advance diaphragm tests OK - hold vacuum when applied by my Mity-Vac'.
This is the current condition; strong and steady vacuum @ 20-21 inches Hg with no vacuum gage needle bounce. I took themanifold vacuum reading direct from the intake manifold port for the auto trans modulator hose. The engine idles smooth, with little to no shake at a steady 700 RPM's. I had previously tested for a carb and intake gasket leak (despite the normal manifold vacuum gage reading) using carb cleaner sprayed around all the sealing surfaces - with no change in idle speeds or vacuum conditions. This tells me that the vacuum seal (gasket integrity) is OK in the area of the intake and carburetor.I can just about rule out any vacuum leakage as the root cause.

 

I'd try adjusting the timing with only eliminating the stumble in mind, if it runs like dooky at rpm worry about that later. At least if you can cure the stumble this way you will have determined that it is timing/vacuum related.