Vaccum advance hose schematic from your '69 pls

[opelitis1]

Good afternoon John!!!  On another Board they were a talkin' about the above and you sent a copy that I had saved, but,  I done "dooed it again"..  It has gone from my favourite files.  Ain't gonna butter you up, but, it was a real nice clean concise way of hooking up...
Merci   JeanZ!!!

Ted

[JohnZ]

Photo below shows my "driving" setup. I pull the diverter valve signal hose off the tee in the choke pull-off diaphragm (which is a full manifold vacuum source), remove the stock vacuum advance hose from the vacuum advance unit to the pipe from the carb, cap off the end of the pipe from the carb, and add a length of rubber hose directly from the vacuum advance unit to the tee in the choke pull-off hose. Takes less than a minute to change it or swap it back to the stock configuration.

[opelitis1]

Nice neat functional setup...  If you were running a dble pumper instead of a vacc. carb, where would you get your vaccum from???  The intake manifold fitting for pwr. brakes looks awfully inviting that you have one side plugged off... Would this work or where else could I draw from???  T.

[JohnZ]

Nice neat functional setup...  If you were running a dble pumper instead of a vacc. carb, where would you get your vaccum from???  The intake manifold fitting for pwr. brakes looks awfully inviting that you have one side plugged off... Would this work or where else could I draw from???  T.

I have that vacuum fitting with the extra capped-off nipple as a convenience for connecting my vacuum gauge when adjusting the idle mixture screws; you could do the same thing and connect a hose directly from that nipple to the distributor (you'd still need to cap off the existing vacuum source you're using now). I'm not familiar with the vacuum sources on a double-pumper - I only deal with stock/OEM carburetors.

[sebastien]

John,I have a stock 68 327-210 2bbl carb how do you suggest I route my vacuun advance hose?I understand it is best to take the source from a full manifold vacuum source.

thanks

[68Zproject]

I did this on my Rabbit and connected a T to the hose comming off the intake manifold to the power brake can.

[opelitis1]

John, I never really thought about this until your statement re. "stock/OEM carbs" being of the vaccum secondary type only..  Guess Holley jumped into the mix, so to speak, with Dbl. pumpers for the racing public with their square bore 4150 series and lastly with their psuedo copies of the Q-Jets' small primaries/large secondaries baseplate alongwith with the dble. mech accel. pumps' features...  Funny how one gets into a mindset for years, and all of a sudden....  T.

[JohnZ]

John,I have a stock 68 327-210 2bbl carb how do you suggest I route my vacuun advance hose?I understand it is best to take the source from a full manifold vacuum source.

thanks

I'm not familiar with the configuration details of the 2-barrel carb, but if there's no port on it that shows full manifold vacuum at idle, you can use the vacuum fitting on the intake just behind the carb.

[jdv69z]

John,

I know you probably explained this before, but why do you go to full manifold vacuum from the stock configuration?

Jimmy V.

[JohnZ]

John,

I know you probably explained this before, but why do you go to full manifold vacuum from the stock configuration?

Jimmy V.

Use the link below to go to the text of my "Timing 101" article, which was published in the October/November, 2003 issue of "Corvette Enthusiast" magazine; it explains ignition timing and advance systems and how they operate in layman's terms.

http://www.lbfun.com/warehouse/tech_info/timing%20&%20vacuum%20advance/Timing101Article.pdf

[jdv69z]

John,

Thanks very much for the article. I'll make the switch. My initial timing is set a 12 BTDC. Should I adjust that? I know factory is 4 BTDC. It is 302. Will I be able to see a difference, like better idle, better throttle response off idle, and maybe lower water temp at idle?

As always, thanks for your invaluable assistance.

Between you and Jerry Mac's tips, I've really been able to improve my 302's performance. The little details do add up.

Jimmy V.

[rich69rs]

Check your total timing, first with the vacuum advance active at idle and then with the centrifugal advance active above 2500 rpm.  You can read total timing with a "setback" timing light.  That way you can tell whether or not your initial 12 degrees is ok.  I'm guessing that you'll be fine.

[JohnZ]

John,

Thanks very much for the article. I'll make the switch. My initial timing is set a 12 BTDC. Should I adjust that?  The little details do add up.

Jimmy V.

Your 12* initial (with VAC disconnected and plugged) should be fine, as long as your total timing (initial plus centrifugal, VAC disconnected) doesn't exceed 36*. Full manifold vacuum will bring your idle timing (initial plus vacuum advance) to about 27*. Your centrifugal curve should be set up tol be "all in" by about 2800 - that's a good baseline to work from for a normally-driven street engine.

[sebastien]

thank you John and Rich for all the info!
Rich you have a 2bbl/auto as well, what kind of setting do you have?

[rich69rs]

I've got mine set up on full manifold vacuum as John has recommended.  The 2Bbl Rochester doesn't have a full manifold vacuum source, but the connection at the back of the intake manifold where the power brake vacuum booster, RS headlight vacuum source, etc. all tie in provided a very convenient source for full manifold vacuum.

Been a while since I set it up, but as I recall, initial timing was about 10-12, degrees; total timing (vacuum advance + initial) was about 30 degrees at idle; centrifugal advance all in above 2600 rpm.