vac. advance canister ?

[blazer2007]

My recent tune up found vacuum canister was not working . Is a stock 350 4 barrel 9-1 comp ,very very mild cam . Old can says AV 20 but was wondering if I would benefit with a adjustable vacuum advance with such bad gas these days or just go to the #20 canister . I did read that the buick AR 12 was a good canister to use on a HEI 350. Not a race truck just a daily driver . Is in a 80 truck

[PGSigns]

I would put in the one it calls for and see how it runs. You can go with the adjustable if you have a good dial back timing light to track the changes. While you have it apart check to see if the springs and weights under the rotor look in good shape. I have seen may a one that were flat worn out and not working like they should.
Jimmy

[Dead Parrot]

A lot of the adjustable units only adjust how much advance you get, not when it comes in. And some of them come in pretty fast once you hit the magic vacuum. Almost a binary on/off switch.

Not sure how many of the OEM #s are still available.

Might want to check when your mechanical starts to come in and when it is all in. You may want to adjust while you have the dizzy open.

[blazer2007]

Thanks , just trying to get my head wrapped around tuning . I just thought 15 BTDC + 20 vacuum was alot . More reading needs to be done.

[cadillac_al]

I wonder if it came with a 20 or if it got replaced at some time. It does sound like a lot for a truck.

[crazy longhorn]

Blazer, each engine combo "tunes" a little differently That said, I ran a 9 to 1 , 355.....214/224 on cam duration. If you run low gears, or a loose converter(mine had 4.10 gears)....they tend to tolerate more advance, as they are not lugging the engine? That old 355, liked around a 32-34 total (initial + mechanical advance). But she liked another 12-14 on the vacumn advance(crankshaft, & hooked to a full vac port). You will need to tune & play, to find the right spot.....what I tell, is general info on the SB chev. best of luck...tuning is 1 step at a time. Have fun, any ?, ask! Longhorn

[blazer2007]

I know is an art form and trial and error not like installing a water pump where is pretty straight forward. Doing lots of reading and u tube stuff. Just hate the dist. at the back. MY ribs hurt LOL

[blazer2007]

If I have 12 HG of vac. at idle would it be safe to assume that a AS-20 vac. can. wouldn,t even be working ?

[geezer#99]

No!
IIRC the stamped number on the pot arm is the number of degrees of timing it adds.
You should get a mityvac pump and test the vac pot for how much timing it adds with how much vacuum you apply to it.

[cadillac_al]

Originally Posted by blazer2007

If I have 12 HG of vac. at idle would it be safe to assume that a AS-20 vac. can. wouldn,t even be working ?

Maybe if the vacuum can was working the vacuum would come up. That's low vacuum for a 9:1 small cammed engine. I would get another 20 vacuum can and see how it works. If you have pinging issues you can just buy another one with less advance.

There used to be a book at the parts stores that told how much advance each can had and what vacuum it came in. There were literally hundreds of different combinations. Computerized engine management sure killed that business.

[geezer#99]

Maybe he should just buy or build a limiter like in this link.
http://www.crankshaftcoalition.com/w...EI_distributor

[blazer2007]

Thanks caddilac ,There is a good write up called tuning 101 I believe and it lists all the cannisters.
Geezer my 12 HG vacuum is with the dist disconnected , don,t know if that means anything as the old cannister has failed and is disconnected now.

[geezer#99]

What makes you think the vac pot is dead?
Did you draw a vacuum on it?
And if so, how?

[blazer2007]

Originally Posted by geezer#99

What makes you think the vac pot is dead?
Did you draw a vacuum on it?
And if so, how?

Idle stays the same when disconnected.

[geezer#99]

That don’t prove anything.
Put a long vac hose on it and use your lungs to pull a vacuum on it. If the idle increases it’s good.

[blazer2007]

Originally Posted by geezer#99

That don’t prove anything.
Put a long vac hose on it and use your lungs to pull a vacuum on it. If the idle increases it’s good.

Will do, I don,t have a mighty vac but a friend does but he,s not around

[Captainfab]

If your vacuum advance is connected to ported vacuum, it isn't going to change the idle at all. Now if you connect it to manifold vacuum, then it will advance the timing and increase the idle.

[blazer2007]

It,s on Manifold vacuum

[crazy longhorn]

Short horn here... (thats for you geezer)! I totally agree, but will ask....what is the initial timing/total, & what is the duration on the camshaft? Also when is the mechanical curve "all in"? longhorn

[geezer#99]

Mr. Short Horn!
Good to see you’re still kickin’!
Anywho I found some info on vac cans I figured I’d post.

https://www.crankshaftcoalition.com/...ance_Specs.pdf

[rpmerf]

Originally Posted by crazy longhorn

That old 355, liked around a 32-34 total (initial + mechanical advance). But she liked another 12-14 on the vacumn advance(crankshaft, & hooked to a full vac port).

I have mine set up similarly. 350, 9.5:1, 184/194 cam. 14-15 base + 20 mechanical = 34-35 all in, and ~12-14 vacuum. When I had more vacuum advance, it would ping while the choke was on. When the choke is on, the engine runs richer, so the air fuel mixture in the cylinder burns faster.

[crazy longhorn]

Originally Posted by geezer#99

Mr. Short Horn!
Good to see you’re still kickin’!
Anywho I found some info on vac cans I figured I’d post.

https://www.crankshaftcoalition.com/...ance_Specs.pdf

Still kickin, been playing with grand kids & a new pup! Good info on the linkey.....as usual, right on Shorthorn

[blazer2007]

Ha d to fix the Jeep for winter. Anyway got a chance to do all this reading but was wondering what they mean by tip in. Is this the first # on the the specs. Ex. AR-12 starts 7-9 - 7@10-12. AR-15 3-5 -7.5@9-11.

[geezer#99]

Exactly where do they mention tip in.

[blazer2007]

Too much info in my head, has to do with a limiter plate but isn,t what I wanted to know. Are the first #,s HG starts at 7-9 . Is this when the vacuum starts to work at idle. If so why do some start at 7-9 and others at 3-5 . Is this due to the camshaft profile on different motors ?