QuestionI have a '66 Mustang with a rebuilt long block 289 and C4 auto transmission. The engine has a two-barrel (Autolite 2100) carburetor with a nagging off-idle stumble or hesitation, which happens whether hot or cold. The engine has solid compression in the 180 psi range in all eight cylinders, also has steady 21" Hg vacuum at idle. The base timing has been advanced from the recommended 6 BTDC to around 10 BTDC - with no different results.
The engine has the following items replaced, rebuilt, or repaired to-date either in response to, or because of the driveability problem. The carburetor has been rebuilt - twice by experts at Pxxx Carburetors of New Mexico, new vacuum advance, new stainless steel vacuum advance line from carb to distributor, new Pertronix Ignitor (electronic) ignition module replacing the points and condensor, new spark plugs and wires, new ignition coil, rebuilt distributor (stock Autolite), new distributor cap and rotor.
AnswerThe cause of the off-idle hesitation is the incorrect setting (or failure) of the accelerator pump inside the carburetor, in about 99% of these cases. I know you've had the carburetor "rebuilt" repeatedly, but your experience with "rebuilt" carburetors is exactly the same as my experience with them - most of the rebuilders are not really able to fix the problems, either because the problems are beyond fixing, or because they don't have the equipment to flow test the carburetor after it is put back together.
I'm sorry you have spent so much money on this car, but it does seem that your engine is in good condition (in fact that 180 PSI compression figure is a bit TOO high!) I don't agree with the Pertronix electronic ignition modification, but I doubt it is responsible for your problem, so don't fiddle with that now.
If you want to prove to yourself that the carburetor is the culprit (which I very strongly believe), you can spend a few more dollars and buy a new (not rebuilt!!) carburetor for it - I believe NAPA still stocks the right carburetor for your car for around $300, and I think your misery will be over.
If you want to do a bit of checking yourself, you can look at the discharge stream from the accelerator pump nozzles when you take the air cleaner off and have a helper mash the gas pedal to the floor. You should see two healthy squirts of liquid fuel coming from the nozzles in the carburetor. The streams should be around 1/8 inch in diameter, and should contain close to a teaspoon of fuel, each. You will have to hold the choke flap all the way open to see down there.
If you want to give the carburetor guys one more chance, take it back and tell them I said the accelerator pump is set way too lean - maybe they can fix it for you. In fact, you can drive the car to their shop and show them the hesitation - they should immediately jump on the problem and fix it.
There is one other possibility, and that is that the vacuum advance lines are not connected to the right port on the base of the carburetor - make sure the line to the distributor is connected to the "ported vacuum" fitting on the carburetor - this is the one that is somewhat elevated above the base plate. This line is not supposed to be connected to the intake manifold vacuum port, which is on the base of the carburetor.
Dick
