QuestionQUESTION: Have an Autolite 2100 carb that has been rebuilt by Pxxx Carburetors twice - still has same off-idle stumble and hesitation. The carb is installed on a mostly stock, rebuilt 289 engine with strong compression in all eight cylinders, and strong idle vacuum (20-21") with steady guage needle. The engine readily starts and idles smooth without much vibration.
The carb has a new accelerator pump; diaphragm and spring. The engine has the following new parts; vacuum advance, spark plugs, ignition wires, distibutor cap, distributor vacuum solid line (stainless steel) to the carb, and Pertronix ignitor module (1st generation). I've timed the engine using the Pxxx method recommended seat-of-the pants; with proper advance beyond the factory 6 degs. BTDC just until the engine starts to stumble and exhibit hard starting - then back (retard) a little - lock down and test drive. The off-idle stumble still remains.
Any ideas or suggestions at this point? I've contacted; e-mail and telephone the folks at Pxxx Carburetors and they have not yet responded nor offered any suggestions to cure this problem.
ANSWER: Hello Jim,
This is a puzzler as everything you've done seems to make sense. I have only 2 suggestions at this point and you'll probably not like either:
A) Until the problem is fixed, back the timing down to stock specs. Although I don't think this is the cause, sometimes excessive advanced or retarded timing can lead to problems like this. The approach I like to use is to get the car back to completely stock, get it running right and then make modifications. (the Petronix is fine: fire doesn't seem to be your problem)
B) I still suspect the carb. despite the rebuilds. Get into your engine bay where you can see down the throat of the carb, manually open the throttle (car is turned off, of course)and look at the stream of fuel being dispensed into the venturis. You should see a strong, uninterrupted stream. If you see a sputter in the stream or no stream off throttle, you've still got a pump problem. You should see a strong stream of fuel beginning immediately after opening the throttle. If this is not so, check to make sure the diaphragm is not leaking, the linkages aren't bent and make sure even connection holes are not galled out or binding. If external looks OK, check your internals, particularly float levels and even the floats themselves for porosity if plastic or leaks if metal. Finally, if all of the above seems OK, and the problem still exists, check the metal in the accelerator pump circuit. This is very difficult to diagnose but I've seen carbs with blockages or internal cracks in this area more than once--after all, these are 50 year old carbs! If this is the problem, you can rebuild until the cows come home but a defective casting will bugger you up every time. If this is the case, recommend either getting a new Holley 2BBL from Summit or the like or check e-bay of all places. I've bought good OEM cores there for reasonable prices!
Sorry for the long reply but it sounds like you're past the easy fixes. Without a strong stream and/or volume of fuel, your Mustang turns into a bucking Bronco (sorry for the pun)
Good luck!
Bob
---------- FOLLOW-UP ----------
QUESTION: Bob, thanks for the reply and suggestions. In regard to these you have offered, item A) I should have mentioned that the original stock timing setting @ 6 degs. BTDC did not change the off-idle stumble one bit. Still present at the initial stock setting, as well as during the incremental advance setting(s) as per the carb rebuilders suggestions.
In terms of item B) The carb has been looked at and examined, as well as hot tested at the carb rebuilders shop. They are nationally recognized and have stated the carb has been gone through for any flaws - at least on two (2) different occasions. I'll take your advice and go through with an eye for more of these details. One other bit of information, a few years ago, I did swap the carb with another - same type 2100 model - from a friends restored Mustang. Without any other changes - just basically bolt -on and go - the engine ran great; no hesitation / stumble at all!
AnswerHello Jim,
Sounds to me like you answered your own question. The fact you substituted another 2100 and the problem went away points to your existing carb. as being the problem despite apparent careful rebuilding and bench testing. I am not a carob. expert myself but the information I passed on was the result of working with an old-time carburetor guy. It's becoming a lost art!!
Specifically, I work as a project manager for an old car dealership and thus, have had the opportunity to see a variety of fuel systems. The carbs. I mention went through a similar, rebuilding process as yours but failed to perform on the car. Both were Carter but I think that was coincidental as one was a BBD series (similar application to 2100) and an AFB 4 bbl. In both cases, we went so far as to even suspect the quality of the rebuild kits!! And in both cases, a new (old) core for one and replacement of the main bowl casting on the other, problem solved!
Hope this helps. You sound like me--objectively measure the problem and methodically fix it. But a car is complex enough that this approach sometimes fails and you need to rely on experience and replacement of components you can't conclusively prove are bad but are consistently causing the same problem.
Bob
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