My 1972 Centurion Convertible with 47,500 miles is completely stock with the 455/Rochester combo. Since I inherited it from my dad (been in the family since new when my grandpa bought it) it has had a hesitation off idle. I had carb rebuilt but the problem persists. When I last drove it a couple weekends ago it was doing the same thing. While parked with the car running, for curiosity sake I removed the gas cap. At first there appeared to be no difference, but I was applying the throttle gently. When I got to our office street, I applied heavy throttle, and much to my surprise, no hesitation and the rear wheels actually spun. This continued for several more minutes before once again having the hesitation return. And with additional attempts at more aggressive throttle application an engine stall would be induced. Any ideas on the issue? Could it potentially be a fuel tank venting issue? Anyone experience the same and been able to successfully resolve?
It can easily be timing related. Make sure your vacuum advance on the distributor is functional. Just because it is hooked up doesn't mean it works. In 1972, Buick engines used an emissions system called Transmission Controlled Spark (TCS). It eliminated vacuum advance in 1st and 2nd gears, and allowed it it in 3rd. The first thing I eliminated on my 72 was the TCS. Ran a vacuum hose directly from the carburetor to the distributor so it had full time vacuum advance. Also, if the engine (and this is a BIG IF) has the stock distributor, AND you have it initially timed at 4* BTDC, you are leaving alot of power on the table. The stock 1972 455 distributor is part number 1112110.
The first thing I would take a look at is the secondary vacuum break (also known as choke pull-off). If it was replaced when the carb was rebuilt it should still be checked out to eliminate it from possible cause. If it is defective (as in not holding vacuum), then when you hit the throttle to the point of opening the secondary throttle plates the secondary air valve will flop open causing the situtuation you described. It can be checked with a hand held vacuum pump or simply disconnect the small hose and push the valve closed and hold your finger over the vacuum hose port. If it won't stay retracted its bad, but if it stays retracted that portion of the test is good. After you release vacuum it should extend smooth and easy. If it snaps out its bad. Thats just one system to test and eliminate. That secondary air valve also has a wound spring that is adjustable. With the engine off push the air valve open and make sure it closes by itself. Adjusting that spring is a trial and error method but is very easy to do and should not take long to do. Also a worn accelerator pump well can also cause this type of problem. Make sure the primary discharge nozzles are squirting fuel from the slightest touch of the throttle, and throughout its calibrated travel. Good Luck, Larry.
One more thing to mention. If in fact your hesitation is accelerator pump related it wouldn't hurt to inspect the check ball and its seat for crud and or pitting. Larry
all the above info is very accurate and on point, it would make things alot easier if you shot a short phone video of the car acting up and putting it on you tube for these expeirenced tuners to see first hand whats happening, that said, assuming your tuning exp is minor, after checking everything posted above, look over in the carb w engine dead, open the throttle and see if the accelerator pump shot looks strong squirting down in the front of carb, if so, loosen the allen head set screw that secures your secondary air flap adjustment screw, while holding a screw driver in the slot so you dont loose all your base line adjustment, now tighten that screw at least a 1 round, and retest if the bog goes away great, if it gets better tighten up a little more, short of a video showing us what its doing, youve prettty much been advised to the max, good luck!
Thanks for the input gentlemen, I am going to have a chance to get under the hood on Sunday. I know it can definitely run better as I have a 71 GS Stage 1 that runs very well with no hesitation at all. I will see about shooting a video of the condition.
