Expert: Brad Sears - 7/6/2010

Question
I have a 1968 Camaro, 327 4 brl. It is running bad, breaks up on low acceleration. I installed 2 NEW Edelbrock carb. only 40 miles on 2nd carb. Backfires through exhaust when decelerating. Installed new fuel pump, fuel filter, 40 miles on fuel pump, fuel filter, MSD cap and rotor I have MSD distributor. Have vacuum advance on port side of carburetor which is what instructions say. Timing is set at 6 deg BTDC. When I hook up vacuum hose on distributor timing stays at 6 deg, doesn't go up like it should, if I hook dist. vacuum hose to other vacuum port of carb (which is the 'wrong' port#, then timing goes up when I hook up vacuum hose to dist. Car still runs bad on low acceleration. It seems like it is loading up when running poorly. 600 miles on spark plugs, 7,000 miles on spark plug wires. Runs excellent on full acceleration taking tach to 3000 or more. Never breaks up on full acceleration. When 'cold' engine runs fine, when accelerating when the car is 'sitting still' runs fine>without a load on it. No smoke coming from exhaust. I have had Edelbrock Carb #600 cfm# on car for 15 years. Never had this problem.
Thank you in advance for you help.

Answer
First of all the distributor should not advance at idle hen the vacuum line is connected to the ported outlet on the carburetor. It will only advance off idle at speeds of 1000 rpm and higher and at only light to moderate throttle. There are several items that could cause you problem. The first is a vacuum leak in the intake system that is leaning pout the mixture during periods of normalcy high intake manifold vacuum which would be light acceleration. The backfiring through the exhaust is also an indication of a vacuum leak. The other is a worn breaker plate in the distributor that is changing the dwell and timing when vacuum is applied to the distributor, again under light acceleration. When the engine is cold it is on the choke and would compensate for a vacuum leak. If this is not the problem let me know and we will go to the next step.
Brad