1969, 350, 300hp, Auto
I just got a new vacuum advance and noticed that the stamp on the old one is different from the one on the new one. Is there a certain one I should put on the car. the old one is stamped with MS 357 20, The new one is stamped with B10. I am thinking of replacing it because the old one is very very tight, It is very hard to move by hand, and when I remove the vacuum hose that is connected to the carb there is no effect.

Thanks for any help.

 

Please, Anybody got any advice on this, Please

 

I don't know about stock vacuum cans, but most folks around here have advanced their mechanical ignition and will recommend you get an adjustable vacuum can so you can dial down the advance. You need this because stock vacuum cans will advance the ignition too much when you advance the mechanical. There have been mentions of a vacuum can from Napa that Lars and others have used and I've also seen one from Crane mentioned in tuning threads.

 

The only way to tell is to run a timing light while running the engine. Or better yet, to hook up a vacuum pump to the advance can and check it with known vacuum applied to it.

Your vacuum advance is not really an important factor in performance unless it's not releasing. Your mechanical advance setting is for power. The vacuum advance pulls in more advance when cruising under light throttle conditions to improve fuel mileage. As soon as you whack the throttle open, the vacuum drops and the vacuum advance goes away.

Vacuum advance can be tuned so that you can pull the best mileage and still have it roll off enough to avoid detonation under acceleration. If your engine wants, for example, 35 degrees of mechanical advance for best performance, but 50 degrees for best fuel mileage, so be it. But if that extra 15 degrees of advance doesn't go away when you open the throttle up, you'll have a rattle can for an engine in short order. Most stock cans pull about 15 degrees regardless of what engine they are on, so having this or that can probably won't matter. And most stock engines pull similar vacuum numbers at similar throttle openings.

I'm not sure if this really answers your question, but I'd be happy to make a second attempt if I missed the mark. Hopefully I didn't just rehash the basics that you already knew.

Tom

 

Oh, and another thing. You mentioned no effect when pulling the vacuum hose off the carb. The hose is supposed to be connected to a ported fitting, thus, no vacuum at idle. So if you're testing this with the car idling, there SHOULDN'T be any effect.

 

Thank you, any info is good info, I'm new at this so I can use all the advice you guys want to give. Thanks again.

 

Here is an article written by Lars on timing.
https://www.corvetteforum.com/techti...=109&TopicID=3

Here is another with some advice on vacuum advance. Both good papers. Print them out and work with them. It will make the car run much better.
http://forums.corvetteforum.com/showthread.php?t=995446

 

I just saw a post from Lars in another thread regarding the vacuum can. The one he recommends is NAPA part # VC1838.

 

Yep, I just saw that one also, Thanks

 

Send a note to Lars asking him for his Vacuum Advance papers. They explain the whole process in great deal -- so much so that now even I understand how it works and what I'm trying to achieve.

 

 

I'v seen him on here before but since I'm new I don't know how to contact anybody, Is it as simple as an email or is there something else I do to contact. Thanks again for everybodies help.