Vacuum Cans

[John Walker]

This is for Duke/ Clem or any of you engine "guru's" Some time back (2 years ago) I put a GM crate motor(454HO) in my wife's 1973 coupe with a modified 700
R4. I also installed an H.E.I. with mechanical tach drive and a 750 Demon carb. This is my problem, I have now gone through 4 vacuum cans (blown diaphrams) 3 in the last 6 months 2000 miles. The engine will pop back through the carb from time to time, not hard enough to kill the motor or do any damage( I thought)but I can't keep cans alive. I am running 12 degrees initial advance and the napa/echlin can that Duke recomends and when the can is new the engine runs very well, 19 mpg on the highway, pulls very strong, no detonation etc. The engine pulls about 13/15 inches of vacuum at idle and the distributor is run on non-ported vacuum at the carb base. Can I get heavy duty vacuum cans or use another source or any other ideas. Thanks in advance. John

[Duke W.]

Re: Vacuum Cans

I think the best approach is to go after the root cause of the problem, which is the backfiring. It could be carb or ignition related. The HEI can cause backfires, but it's usually a two-second warning that it's about ready to quit - been there a couple of times. Just the same, it would be a good idea to throughly check the HEI primary wiring for problems. Look inside the distributor and make sure the pickup coil wires are not chaffing or possibly causing an intermittent short or open. The baseplate/pickup coil moves in response to the vacuum can, so the two leads to the module will eventually fail, and they must have plenty of room to move and not rub against anything.

I've experienced both an open pickup coil and failed module on my Cosworth Vega. Both resulted in backfires just before the system quit. Once, at Riverside, the car let out such a violent pop that the guy riding with me thought the engine had blown. I knew it hadn't because it didn't feel like I ran over a land mine (been there too). A quick check after coasting into the backdoor of the pits showed the pickup coil was open. I now carry a spare HEI in the spare tire drop center, but I digress...

If you provide a detailed explanation of the conditions under which you experience the backfire, I'm sure Clem can give you some hints on how to adjust or modify the carb to prevent further occurances. Chances are a jet change or pump shot adjustment/modification will eliminate the transient lean condition that is probably causing the backfire.

I don't know of any "heavy duty" vacuum cans that could better survive a backfire, but then I don't ever recall hearing of a backfire breaking the diaphragm. Once way to bandaid this would be of place a restrictor somewhere in the signal line. This would attenuate the pressure wave, but could cause the vacuum can response to be lazy and cause transient detonation when you open the throttle because the vacuum advance does not back off quick enough.

In any event, this would be just a bandaid, and the best way to solve the problem is to eliminate the backfiring.

Duke