I was checking the timing on my '56 292. I'm running a recently rebuilt t-pot carb with a modification to run a single vacuum line to the vacuum advance. I have a distributor from a '62 Y-block and a single vacuum advance. When I adjust the timing with the vacuum line disconnected to the I think "3 deg" mark on the damper and then reconnect it after it's sets it runs poorly. It advances quit a bit with the line reconnected at idle. When I set it to the spec'd degrees with the line connected it runs much better, like it should. How much should the vacuum advance advance the timing at idle? It seems too much which is causing a poor running condition as specified in the shop manual.


~DJ~ AKA "Bleach"
1956 Ford Fairlane Town Sedan 30K original miles

Ordinarily there won't be any additional advance when checking the ignition timing with that setup you describe, but that is dependent on the engine RPM. Once the throttle butterflies open up even a little there will be some vacuum applied to the vacuum advance diaphragm. Factory idle spec was something like 450 - 550 RPM. Anything much above that will start to show significant ignition advance above the base mechanical setting. Make sure the crankshaft damper ring isn't defective, these are very often slipped off their "clock" and make setting the initial timing very confusing.

The additional ignition timing advance provided at a higher idle RPM is probably why most people like their idle RPM increased over factory, it smooths out audibly and runs noticeably cooler. In this situation 20° BTDC or even more at idle wouldn't be unusual. Remember there is no engine load at idle and this won't hurt anything. It will not pass any sort of smog testing however.

If the distributor vacuum advance diaphragm is defective then it will introduce a vacuum leak and run poorly, be sure to check for this. A lot of people like to run 10° to 12° initial or base timing regardless.