Classic/Antique Car Repair: 68 ply valiant 273 motor, vacuum leaks, vacuum leak

Question
QUESTION: I just rebuilt the motor for my 68 Ply Valiant Signet. I bought a rebuilt Carter 2bbl carburetor to replace the Holly that was on it (it was the wrong one for the car). I have a new distributor with after (pertronix) electronic ignition. When I started the motor it ran ok but when I let it idle after breaking in the new cam it would surge. My vacuum guage showed a change from 14-15 inches of vacuum to 20-21 as the engine would surge. When I increased the idle speed from about 750 to 1500 rpm the guage was rock steady at 20 inches. I suspect the carb is no good, but what do you think? My next question is about ign timing. The book says 2.5 deg ATDC. If I set the timing there the motor won't run. I'm almost positive the cam timing is correct. Any ideas here?
Thanks
Arnie

ANSWER: 1968 cars were whole new breed of performance problems for the mechanic. It was the first year of more stringent emissions controls including some real tricky vacuum gargets that advanced and retarded timing in relation to engine temperature and throttle position. It sounds as though some of these items are malfunctioning and causing some timing changes. Hook up your timing light and watch the marks when the engine is playing games. Vacuum leaks can be found easily by taking your handi-dani Enzymatic torch and removing the burner and metering valve. Add a piece of flexible fuel line several feet long now to the pipe where the torch used to be. With the engine running search all the place that might have a vacuum leak with the propane. Any vacuum leak will cause a change in the way the engine runs. To check the carburetor, stick the hose into the carburetor air inlet. Slowly turn on the propane. If the engine picks up speed and smoothes out, the carburetor is defective or out of adjustment. If the engine speed falls off as soon as the propane is turned on the carburetor is running too rich and needs adjustment. Good luck.
Brad

---------- FOLLOW-UP ----------

QUESTION: Hi all, I'm back. I've been able to determine the rebuilt carb was no good. I took it back and got a replacement and the car runs good. Now for another question. My book says the ign timing is to be 2.5 deg ATDC. If i set it there a couple of things happen. 1) the exhaust manifolds get REALLY HOT and 2)the car runs poorly if at all. My vacuum guage shows 15-17 inch of vacuum (late timing on the guage scale)I set the timing to about 10 deg BTDC, my vac guage shows 20 inch of vacuum very steady, the engine starts very easily, and runs great. I'm afraid to leave the timing advanced so far so as to burn intake valves, but it runs so well there! Any ideas on this one?
Thanks
Arnie

Answer
Yep, it tells me that the car was originally equipped with a solenoid switching device the routed vacuum to and away from the distributor at certain engine temperatures and speed. The system was supposed to make the engine burn cleaner but for the most part it made the engines run crapy. Most of these systems were removed and bypassed very early on in the cars life. 10 degrees of ignition advance without emissions controls sound in the range of acceptability. Just make sire that there is no spark knock when the engine is put under load. If there is back the timing down just a tad until the spark knock goes away.
Brad