I have a 72 fastback with a 351c. The vacuum advance on my distributor seems wrong the cap is supposed to lock in place on it but it doesn't.Could that affect the timing or the performance of the engine if someone changed the vacuum and put the wrong one in. I can't seem to tune the engine right. The idle on park is at 800 rpm's and when i put it in drive it goes to 400-500.I've tried to adjust the carburetor but can't seem to get it to run well.Any help would be appreciated.

 

can you re-explain that one?

"The vacuum advance on my distributor seems wrong the cap is supposed to lock in place on it but it doesn't"

is that two sentences like:
"The vacuum advance on my distributor seems wrong" and "further the distributor cap is supposed to lock in place on it but it doesn't"
or are you talking about a vacuum advance cap I never heard off?

and yes, if the distributor cap doesn't lock in place you'll never have accurate timing. or will loose it soon.