I have a thread going in the brakes section and kind of added another vacuum question to that thread and I think it would better fit here. I have followed John Z's tune up for a 69 Z 28 as close as I can. My problem is that my vac. advance can applies when it gets enough vacuum but it won't release on it's own, I have to pull the vacuum line off to get it to release. The vac. can is a vc-1810 and it is hooked up to manifold vacuum. When I hook it up to ported vacuum it seems to work fine. I have 4" of vacuum at idle and 14" at about 3,000 rpm, which is max vacuum until I let off on the throttle.
10 degrees initial advance, I have always added a few degrees for altitude
28 degrees mechanical all in by about 3,000 rpm
vc-1810 advance can
70 primary jets-John Z recommends 72-I use 70's for the altitude
74 secondary jets-again different but for altitude
AC45S plugs
Thanks in advance, Greg

Does the vacuum advance can 'release' when you shut off the engine?

I tried that today and yes the can does release when I shut off the engine.
Thanks, Greg

And when you open the carb butterflies, the can doesn't move?

alan

You pulling vacuum from the carb venturi's ?

When I rev the engine the advance can applies fully. Then when I release the throttle the engine idles at 1300-1400 rpm instead of 900-950 rpm. When I unplug the vacuum hose from the advance can the engine returns to normal idle. I then plug the vac. hose into the advance can and it idles at 900-950 rpm until I rev the engine or drive it normally. I have the vac. hose hooked up to manifold vacuum. The advance can seems to work normally, applies and fully releases, when plugged into ported vacuum.
Thanks, Greg

Greg, 4" hg vacuum at idle is very low. You should be able to get 9 to 10" The VC1810 cannister does not fully deploy until 8". It might not even start to deploy until 5". Your initial should be more like 16"-18" so your mechanical should be 20"-18". This would help get the vacyuum up a bit. We are talking about a 302 with the 30-30 repro cam right?

I don't know what type and brand of distributor you're running but I'm running a Mallory "Unilite" with an adjustable vacuum advance canister.....I had this same problem you're experiencing now (engine won't return to idle).....I mean, it would eventually, but it took a few seconds.....What I had to do was decrease the amount of advance down to like around 6-8 degrees and that helped alot.....Hopefully you can fix your problem but with 4"Hg of vacuum @ idle, I don't know!.....I was going to also suggest to increase your initial timing to around 16 degrees.

Gary, yes it is a 302 with the Crane Blue Print series cam. Just so I'm understanding you correctly, I should adjust my initial timing up to 16-18 degrees with an additional 18-20 degrees mechanical advance? Will the vc1810 advance cannister fully release if the vacuum doesn't drop below the 3-5" that the can needs to start deploying? As it is when I rev the engine the vac. can applies and makes the engine idle at 1200-1300 rpm until I remove the vac. hose from the advance can, it then drops back to 900 rpm. Do I adjust my idle at this point from 1300 rpm down to 900 rpm?
Thanks, Greg

Ron, I am running all stock components. Is there an adjustable advance can that bolts onto stock distributors?
Thanks, Greg

have you checked what vacuum you have when the engine is at the high idle? i think what is happenning is when you rev the engine and create higher vacuum, the can deploys more, and when you let go of the throttle the can keeps it self deployed because of the higher vacuum it is creating.

Sounds like the cannister is not deploying at all until you rev the engine. If the vacuum drops below 8" then the timing will dither and the engine will probably die, if adjusted to 900 RPM with the canister deployed. You need to get to 9" before you can even use the VC1810.

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I pulled my hair out for two years trying to make my 302 run good and idle good for around town driving.
Like GaryL I had a Comp Nastalgia plus cam. Unlike him I could only muster up 7" of vacuum. I didnt have an original style carb so I installed a holley 700 dbl pumper for more throttle response.
The first important thing I did was to drill A hole in each of the two primary throttle plates as suggested by Holley Tech. (3/16 I think) This gave more air without disturbing the idle circuit to much.
I installed a different bushing in my original dist. to limit my mech. advance to 15. My initial is around 20. I think I made it myself because MDS said thier kit wouldnt work. The mech timing come in at 1500 and is all in at 2800. I left my Vacuum can on the dist. alone and didnt use the echlin because was idleing too erratic (the idle vaccume was about the same as the vacuum nessesary to pull the advance in so it would idle fast or too slow depending on its mood at that time! I also think this helped to prevent pinging.
One last thing I would like to mention is that I installed a pertronix electronic kit inside my original dist. It helped idle and overall performance
I have no ping w/93 octane. 11-1 compression(weisco piston kit for 302)
Gary...

Ron, I am running all stock components. Is there an adjustable advance can that bolts onto stock distributors?
Thanks, Greg

Yes, there sure is!:yes:.....Accel PN 31034 (Points Dist) Accel PN 31035 (HEI Dist)....Crane Cams also has one, PN 99601-1 (Points Dist) and PN 99600-1 (HEI Dist).....These will also take care of that annoying ping, under a slight load, by decreasing the amount of vacuum advance down to around 6-8 degrees!.....These vacuum advance canisters are adjustable, with an allen wrench, inside the vacuum advance hose nipple.

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I pulled my hair out for two years trying to make my 302 run good and idle good for around town driving.
Like GaryL I had a Comp Nastalgia plus cam. Unlike him I could only muster up 7" of vacuum. I didnt have an original style carb so I installed a holley 700 dbl pumper for more throttle response.
The first important thing I did was to drill A hole in each of the two primary throttle plates as suggested by Holley Tech. (3/16 I think) This gave more air without disturbing the idle circuit to much.
I installed a different bushing in my original dist. to limit my mech. advance to 15. My initial is around 20. I think I made it myself because MDS said thier kit wouldnt work. The mech timing come in at 1500 and is all in at 2800. I left my Vacuum can on the dist. alone and didnt use the echlin because was idleing too erratic (the idle vaccume was about the same as the vacuum nessesary to pull the advance in so it would idle fast or too slow depending on its mood at that time! I also think this helped to prevent pinging.
One last thing I would like to mention is that I installed a pertronix electronic kit inside my original dist. It helped idle and overall performance
I have no ping w/93 octane. 11-1 compression(weisco piston kit for 302)
Gary...
I'm just curious but how did it idle when you pulled the vacuum hose off?
Not at all or what?
I'm having an idle problem and have the adjustable can that came with the distributor and I make less than 7" of vac at best, 15" around 1,200rpm which isn't even idle speed.

When I un-hooked the hose from manifold vac. I think it idled faster but not necessarily smoother if it was idleing slow at the time because the engine got more air. (vac. can not engaged) If I reved the engine up to get the vac. can to engage, and unhooked it It would drop the idle and stall.
The bottom line was I needed tons of mech. advance and the idle screws turned in too much. As if I had a bad vacuum leak.
Hope I answered your question.

I have been busy but did get to work on my car last weekend. I set the initial timing to 17 degrees and the vacuum rose to 5" at idle and 15" max at about 2000 rpm. I then readjusted the lifters to .030 and the vacuum rose to 6 1/2" at idle 16" or 17"max. With the altitude I don't know that I will ever reach the same readings you guys get that are closer to sealevel. The idle is much smoother and the throttle response is way better. Just curious Gary, what jets are you running in your carb?
Thanks, Greg

There are several guys with Z's in the Denver area. Perhaps they can chime in with their vacuum readings so as to get a baseline of what is typical at that elevation?

One of the guys has a Z that was getting lousy readings. He lives in New Mexico. After much trial and error it turned out his timing chain was off by one tooth. He reset the chain and the car runs like gangbusters now.

alan

I have been busy but did get to work on my car last weekend. I set the initial timing to 17 degrees and the vacuum rose to 5" at idle and 15" max at about 2000 rpm. I then readjusted the lifters to .030 and the vacuum rose to 6 1/2" at idle 16" or 17"max. With the altitude I don't know that I will ever reach the same readings you guys get that are closer to sealevel. The idle is much smoother and the throttle response is way better. Just curious Gary, what jets are you running in your carb?
Thanks, Greg

#72 front, stock back.

thanks guys great info.
I have a 69 z28 with a comp cams copy of the orginal cam lobes only
the lash is 12 and 14 different ramps. So they have a little more duration than stock. I get 6-7 inches vacuum at idle.

I was runing 12 degrees basic timming at idle, 22 full mechancial for a max at 34 in by 3000 and 9 on the vac can.

The problem I had was using manifold vac rather than ported.

the engine is right at the point where you rev it and the vac can kick in.
With the additional vac advance the rpm goes up and keeps the vac up so it stays at 1800 -2100 rpm at idle until slowly it comes back down.

When I went to ported signal all was ok though it seemed to run warmer at idle.

Looking for any suggestions on the best setup for performance

thanks guys great info.
I have a 69 z28 with a comp cams copy of the orginal cam lobes only
the lash is 12 and 14 different ramps. So they have a little more duration than stock. I get 6-7 inches vacuum at idle.

I was runing 12 degrees basic timming at idle, 22 full mechancial for a max at 34 in by 3000 and 9 on the vac can.

The problem I had was using manifold vac rather than ported.

the engine is right at the point where you rev it and the vac can kick in.
With the additional vac advance the rpm goes up and keeps the vac up so it stays at 1800 -2100 rpm at idle until slowly it comes back down.

When I went to ported signal all was ok though it seemed to run warmer at idle.

Looking for any suggestions on the best setup for performance
You would want to use manifold vac, ported was mostly for emissions, but I did find that by putting the vac line on ported is a band aid fix for low idle vac and the wrong vac. advance canister along with the carb in the wrong phase (off of idle circuits)

thanks guys great info.
I have a 69 z28 with a comp cams copy of the orginal cam lobes only
the lash is 12 and 14 different ramps. So they have a little more duration than stock. I get 6-7 inches vacuum at idle.

I was runing 12 degrees basic timming at idle, 22 full mechancial for a max at 34 in by 3000 and 9 on the vac can.

The problem I had was using manifold vac rather than ported.

the engine is right at the point where you rev it and the vac can kick in.
With the additional vac advance the rpm goes up and keeps the vac up so it stays at 1800 -2100 rpm at idle until slowly it comes back down.

When I went to ported signal all was ok though it seemed to run warmer at idle.

Looking for any suggestions on the best setup for performance

If you use the VC1810 can the idle will stay high until the vacuum slowly drops since it deploys at 8". The ported vacuum needs the original can that does not deploy until 15" or so.

I have a Crane Blueprint 30-30, 18 deg initial, 18 deg mech by 2800 RPM, 15 deg full manifold vacuum advance, #72 primary jets, Thorley headers and it runs fine on 92 octane w/o pinging. It runs as good as it can run for a stocker.
No hesitation at all. It does suck the gas, though.

The problem I had was using manifold vac rather than ported.

the engine is right at the point where you rev it and the vac can kick in.
With the additional vac advance the rpm goes up and keeps the vac up so it stays at 1800 -2100 rpm at idle until slowly it comes back down.

When I went to ported signal all was ok though it seemed to run warmer at idle.

Yes, it is possible to need to use ported vacuum because you can't keep the canister fully deployed at idle. Sometimes, with a crazy cam (race motors mostly) it could even be best to lock out the distributor at 34-36, and run the vacuum advance on ported for a lil better cruise.

Still, that's a situation to avoid if you can. GaryL worked his pretty hard a couple years ago, and got it right where it needs to be.

Yes, you need LOTS of advance at idle. 25 to 30 degrees is about right, and you pretty much need vacuum advance on manifold to get it (or locked out timing).

Those adjustable canisters adjust the amount of vacuum that triggers the advance, not the amount of advance. The screw adjusts the spring thst the cansiter pushes against. I've heard they get a lil "screwy" if you loosen it up too much. Could still be worth a try tho.

Those adjustable canisters adjust the amount of vacuum that triggers the advance, not the amount of advance. The screw adjusts the spring thst the cansiter pushes against. I've heard they get a lil "screwy" if you loosen it up too much. Could still be worth a try tho.

As u screw the allen screw in, most also reduce the amount of advance
eg if u have one screwed out, it starts about 7" all in say 14 " and may have say 9 degs
As u screw in it also reduces the amount of movement of the spring as it loads it
till u end up with starting about 11" all in at 14" and only have 6 degs

BUT..there is another type, and if u dont know u have it it will screw your head up if u acidently change the postion inside.

If u push the allen key hard in, the rear mount inside that holds the spring turns and will sit in 1 of 6 cam positions....and then u can screw the allen key in each of these for fine adjustment

For using manifold vac...set the cent to the rpms and amount of advance you want
say as Jim says about 34 to 36 degs
what ever u have at idle is 'it'
Find what the idle vac is, then set the VA to be all in around this piont, then adjust the VA as to how much advance u want on top of the total cent for cruise total about 38 to 48 degs
The VA when connected must be all in at idle. so you idle advance will be what is in the cent plus the VA
To high an intial cent causes hard starting and exessive current draw thru the armiture soldered joints that can cause hot start issues later.
I have moded my cent to have 12 deg intial total of 32 degs ..20 in the cent and the VA with 6 degs
which gives me total idle of 18 degs, and total at high rpm cruise 38 degs
My idle is 650 rpms no vac Ad 6 to 7 " vac with VA connected idle vac is 15/16" at 650 rpms