Mechanical and Vacuum Advance

[John Lokay]

There have been a number of questions posted on the "Board" concerning this topic and here is another one. It is my understanding that Vacuum Advance kicks in when intake manifold vacuum is high which happens to take place at idle and subsequently diminishes unpon acceleration. My questions are as follows: 1) The original vacuum advance unit on my 1966 425hp coupe puts out 12 degrees of maximum advance. If I set initial advance at 12 degrees, and do not plug the vacuum advance line, does the engine see 24 degrees of total advance? (12 degrees initial +12 degrees vacuum; 2) At what point will there be no vacuum advance; will this happen at full open throttle say around 3000 RPM; I assume vacuum advance diminishes as the throttle opens; 3) A friend told me that to prevent total advance from exceeding say 38 degrees I should plug and run the car without vacuum advance. I am reluctant to do this as I believe the GM engineers had a good reason for incorporating the vacuum advance unit on the distributor; any thoughts?

On an unrelated topic, yesterday I posted a question concerning an ignition switch problem and someone responded that I should have a key "cut from knock out numbers". What exactly does this mean?

Thanks for your responses!

[Gene M.]

Re: Mechanical and Vacuum Advance

On the original keys the knock area that a key ring goes thru, well that piece you "knock out" has a number on it. A GM dealer can cut you a new key if you supply him this number.

[Gene M.]

Re: Mechanical and Vacuum Advance

On the original keys the knock area that a key ring goes thru, well that piece you "knock out" has a number on it. A GM dealer can cut you a new key if you supply him this number.

[Chuck G.]

Re: Knock Out Numbers

To cut a key from the knock-out numbers....here's what it means. If you pull your lock, you will see a series of numbers stamped on the case....4 digits if I remember. Using a Briggs and Stratton key book, this number translates to the tumbler numbers used in your lock. A locksmith can use this code and cut you a "correct" key just using this code. However, if the tumblers in your lock have ever been replaced, then the code on the outside doesn't match the tumblers on the inside, and the key will be incorrect. A locksmith can look at your tumblers, or put any series of tumblers inside, and cut you a fresh key using a hand held "punch type" device. Hope this helps. Chuck

[Chuck G.]

Re: Knock Out Numbers

To cut a key from the knock-out numbers....here's what it means. If you pull your lock, you will see a series of numbers stamped on the case....4 digits if I remember. Using a Briggs and Stratton key book, this number translates to the tumbler numbers used in your lock. A locksmith can use this code and cut you a "correct" key just using this code. However, if the tumblers in your lock have ever been replaced, then the code on the outside doesn't match the tumblers on the inside, and the key will be incorrect. A locksmith can look at your tumblers, or put any series of tumblers inside, and cut you a fresh key using a hand held "punch type" device. Hope this helps. Chuck

[Jack H.]

Re: Mechanical and Vacuum Advance

Most guys don't have the original key punch outs because they're 'bye bye'. Also, you don't need to see the dealer as most good locksmiths have a look up table for the codes (AND key cuts + tumbler config and how to decode the code IS given in your Corvette Shop Manual/CSM). Last, the factory original ignition lock barrel has the code embossed on it (other locks like door/GB do NOT).

Now, you have to ask yourself a question -- can you trust any codes you find? If car has been through multiple owner hands, what's been replaced and what hasn't???? But, a locksmith can pull the ignition switch, do disassembly read the tumbler wafers (or you pull it and walk it in), inspect for missing/worn parts and refurbish to like new condition. -----------------------------------------------------------------------

On other issue of advance. The 38 degree TOTAL advance (static + dynamic) advice given you is a good guideline. If you've too much advance, you risk burning/punching holes in your pistions. BUT, engines that have been 'tricked out' for full race may exceed these guidelines.....

You have 'static' advance (what you set by wiggling distributor) that comes from idle RPM with vac line disconnected & plugged. Then, there's two forms of dynamic advance. One is done with centrifical force based on throw out weights in the distributor, the other is done with engine vacuum. Together static + dynamic is what your friend was talking about.

Centrifical advance can be changed with different advance 'curves' and springs. Vac advance can be changed by swapping vac advance assy's. Each distributor (DR P/N) has unique, engine matching, advance silhouette. Often this is 'quoted' at key check points in the CSM (rear spec pages).

When you start to see centrifical advance and how much by RPM will vary with engine/distributor. Same goes for vac advance. So, it's NOT possible to tell you when/how much you'll get on your car. We can only quote you the DR/factory spec for your system (who knows who's done what to what you're working with?) which you can look up yourself....

[Jack H.]

Re: Mechanical and Vacuum Advance

Most guys don't have the original key punch outs because they're 'bye bye'. Also, you don't need to see the dealer as most good locksmiths have a look up table for the codes (AND key cuts + tumbler config and how to decode the code IS given in your Corvette Shop Manual/CSM). Last, the factory original ignition lock barrel has the code embossed on it (other locks like door/GB do NOT).

Now, you have to ask yourself a question -- can you trust any codes you find? If car has been through multiple owner hands, what's been replaced and what hasn't???? But, a locksmith can pull the ignition switch, do disassembly read the tumbler wafers (or you pull it and walk it in), inspect for missing/worn parts and refurbish to like new condition. -----------------------------------------------------------------------

On other issue of advance. The 38 degree TOTAL advance (static + dynamic) advice given you is a good guideline. If you've too much advance, you risk burning/punching holes in your pistions. BUT, engines that have been 'tricked out' for full race may exceed these guidelines.....

You have 'static' advance (what you set by wiggling distributor) that comes from idle RPM with vac line disconnected & plugged. Then, there's two forms of dynamic advance. One is done with centrifical force based on throw out weights in the distributor, the other is done with engine vacuum. Together static + dynamic is what your friend was talking about.

Centrifical advance can be changed with different advance 'curves' and springs. Vac advance can be changed by swapping vac advance assy's. Each distributor (DR P/N) has unique, engine matching, advance silhouette. Often this is 'quoted' at key check points in the CSM (rear spec pages).

When you start to see centrifical advance and how much by RPM will vary with engine/distributor. Same goes for vac advance. So, it's NOT possible to tell you when/how much you'll get on your car. We can only quote you the DR/factory spec for your system (who knows who's done what to what you're working with?) which you can look up yourself....

[Gary Schisler]

Ignition switch

Another alternative, if you don't have the knock out with the four digit number, is to pull your ignition switch, using the procedure in the GM service manual, and take that to a good locksmith. My little town of 10,000 had a old locksmith who made me a key from the ignition switch itself and then rekeyed one door lock to match that. As I recall, this was all for around $20-30.

Pulling the ignition switch takes about 5 minutes. Very easy. Gary 21316 66 327/350

[Gary Schisler]

Ignition switch

Another alternative, if you don't have the knock out with the four digit number, is to pull your ignition switch, using the procedure in the GM service manual, and take that to a good locksmith. My little town of 10,000 had a old locksmith who made me a key from the ignition switch itself and then rekeyed one door lock to match that. As I recall, this was all for around $20-30.

Pulling the ignition switch takes about 5 minutes. Very easy. Gary 21316 66 327/350

[Gene M.]

Re: Mechanical and Vacuum Advance

Jack,

Thats cool, it makes it even easier. Learn something everyday. Thanks for the tip.

[Gene M.]

Re: Mechanical and Vacuum Advance

Jack,

Thats cool, it makes it even easier. Learn something everyday. Thanks for the tip.

[John H.]

Re: Mechanical and Vacuum Advance

John -

Yes, with the vacuum line connected, your engine will see the 24 degrees advance (at idle, or when you take your foot off the gas on the road, either of which results in highest possible manifold vacuum condition). You also have centrifugal advance in the distributor in addition to vacuum and initial - this varies by distributor application, but generically is about 24 degrees, all of which is "in" by 2500 rpm or so. Plugging your vacuum advance will have no effect at all on limiting total advance, as max advance is achieved at high rpm under load, and there is essentially no vacuum under this condition. Your total advance will always be your initial plus your centrifugal. You can dope out how much centrifugal advance you have with a dial-back timing light - rev the engine to about 3500-4000 rpm, pull the trigger, and turn the dial-back knob until the "0" marks line up on the balancer and timing tab - the dial will tell you what your total advance was, and subtracting your initial advance will give you the amount of centrifugal advance in your distributor. You can do the same thing with a conventional (non-dial-back) timing light if you put a timing tape on your balancer and read it direct.

[John H.]

Re: Mechanical and Vacuum Advance

John -

Yes, with the vacuum line connected, your engine will see the 24 degrees advance (at idle, or when you take your foot off the gas on the road, either of which results in highest possible manifold vacuum condition). You also have centrifugal advance in the distributor in addition to vacuum and initial - this varies by distributor application, but generically is about 24 degrees, all of which is "in" by 2500 rpm or so. Plugging your vacuum advance will have no effect at all on limiting total advance, as max advance is achieved at high rpm under load, and there is essentially no vacuum under this condition. Your total advance will always be your initial plus your centrifugal. You can dope out how much centrifugal advance you have with a dial-back timing light - rev the engine to about 3500-4000 rpm, pull the trigger, and turn the dial-back knob until the "0" marks line up on the balancer and timing tab - the dial will tell you what your total advance was, and subtracting your initial advance will give you the amount of centrifugal advance in your distributor. You can do the same thing with a conventional (non-dial-back) timing light if you put a timing tape on your balancer and read it direct.

[Joe L.]

Re: Quadrajet Carb. Help!

Ron-----

Leakage through the float bowl well plugs is just about the only way that a Q-Jet carb can "leak down" on sitting. There are four well plugs that are possible culprits; the two larger ones that are oriented straight down and the two smaller ones that are angled in. All it takes is ONE of these to leak and you'll have the problem that you're experiencing. Usually, it is very difficult to tell if these plugs are leaking by looking at them. Even if they leak at a VERY slow rate, the float bowl will empty overnight.

I recommend sealing these by cleaning, roughening and applying a liberal amount of an epoxy cement like JB Weld.

[Ron Anderson]

Re: Quadrajet Carb. Help!

Joe. The J-B Weld is curing as we speak!I also picked up a new float and a needle valve today.

Thank You, Joe, for your help,

Ron