BarryK

yes, if you have a full manifold source on your carb hook it up from there. if you don't you can pull it directly off the intake manifold itself by getting a connection from most of the parts suppliers or your local speedshop

77 zz4

I will try it after, but should I get a new vacuum can first? What will happen if I try it with the original can?

litevette

Man I love these kinds of threads. Speacially the links some have provided. I'm certainly learning a lot! Thanks all!!!

BarryK

ZZ

check and make sure your timing is set correctly and adjust if needed.
check the fuel/air mixture on your carb via a vacuum gauge - the proper setting is usually when you get the highest reading of vacuum at idle.
connect up your existing vacuum can and recheck your timing and make sure that when it's all in it's around 50*-52* but make sure it doesn't exceed 52*. If your total timing with vacuum can attached falls around 50*-52* than your existing can should be fine.

77 zz4

By 52* do you mean 16* initial plus 36* @ 3200 rpm?

I'm a little below that now. 12* and then 36* @ 3200. Mind you, that is w/o the vacuum can.

BarryK

ok, if you are at 12* initial and 36* total now, AND you are not getting any pinging or knocking, than hook up the vaccum advance. AFTER you hook up the vacuum advance recheck timing and make sure it doesn't exceed 52* (36* total PLUS 16* vacuum advance).
I'm guessing on the 16* vacuum advance since I don't know what can you have but it should be close to that.

As long as you are between 48*-52* with the vacuum advance hooked up you should be fine.

**EDIT: don't forget to make sure you are have the vacuum line hooked to a full manifold vacuum source, not a ported source.

77 zz4

Ok, I will try that later but, what happens if I'm not at 36 total?

BarryK

where is your total timing at if it's not at 36*?

anything below 34*-36* than it's too low, meaning it's retarded and you lose power, fuel economy, throttle response, and the motor runs hotter than normal.
If it's above 36* (some motors can get away with 38*) you are probably going to start hearing pinging and knocking from detonation. Detonation will kill your pistons quickly if you leave it at too advanced of a setting like that above 36*.

Fevre

This it good for older heads but today's vortec or kidney shaped comb chamber heads like less timing. The ZZ4 has the old style comb chamber so 36 should be pretty much what you need

Mike Stansbury

when mine is hooked up to full vaccum my idle stays way up and comes down very slow

zwede

This thread is from 2006. If you have a problem you want help with, start a new thread with more info.

Alex Tworkowski

Don't feel bad about your question. It's a good one. I didn't see anyone answer it, so I'll try. The motor pulls a hard vacuum when running with the throttle closed. That's because air can't get in in any great quantity. At that time your vacuum advance is in the full advance position. That is to say, it's at spec. as it was set w/ a timing light or whatever method you used. That's a correct setting for cruising but not accelerating. Accelerating can cause pre-ignition pinging where the fuel ignition comes before the spark plug tells it to, so it's necessary to retard the timing. The timing is retarded a bit when the throttle is opened because the vacuum drops as the throttle opens. Air is rushing in to fill that vacuum so the vacuum is low. The distributor has a vacuum sensor (just a diaphragm) connected with a vacuum line to the intake manifold or the carburetor. The top plate can rotate to adjust spark advance. As the rpm goes up, the vacuum goes up and the spark advance brings the ignition timing back to spec. Today's cars use computers to control most of the parameters involved in squeezing good performance out of your motor. Systems vary somewhat, but I think I gave you the basics. Idle is full advance as is cruising, throttle being closed or open moderately w/ higher rpm. Hard acceleration uses open throttle w/ lower vacuum and less advance. So it's called Vacuum Advance, which is what it is. I guess they didn't wanna call it "Less Vacuum Retard".

Alex Tworkowski

Clockwise distributor w/ ccw advance. Idle IS at full advance. Acceleration is retarded advance. You should see a clockwise rotation on acceleration under open throttle and gradual ccw as rpm goes up or throttle closes, or both.

Little Mouse

Pull the vacuum direct off the manifold it was done that way before emissions BS reared its head and they started ported vacuum off of the carb. If you do it off the carb you get no benefit of idle vacuum increase only cruise works then. But then "race cars dont have doors. toilets have doors ". Race cars dont have vacuum advance total waste of time on them.

Little Mouse

Pull the vacuum direct off the manifold it was done that way before emissions BS reared its head and they started ported vacuum off of the carb. If you do it off the carb you get no benefit of idle vacuum increase only cruise works then. But then "race cars dont have doors. toilets have doors ". Race cars dont have vacuum advance total waste of time on them. Worlds of street cars came from the factory with no vacuum advance.

SH-60B

[QUOTE=bondoboy;1555649722]I don't get where some of you say you can hook an advance to full vacuum. If you did this, you would have full advance at idle, and less advance when you speed up. When the advance is connected to a timed source, it advances when you reach a certain rpm. The only way a full vacuum advance would work is if the cannister were pulling from the other side of the dist. shaft. Chevy uses a clockwise distributor. When the vacuum advance starts working, it turns the point plate or pickup plate on hei counterclockwise. This makes the points open or the pickup to work earlier, which is spark advance. If you would have a full vacuum on a distributor like this you would be advancing fully at idle, and less as the rpms increase. This would actually hurt performance. I have installed rochesters and holleys both 2 and 4 barrel on various years of vehicles and they used a timed vacuum for advance. I asked people at work who have worked on old cars and they agreed. Unless the advance pulls clockwise or there is some other reason, full vacuum doesn't work.[/QUOT

Deleted, just saw its an old thread

Little Mouse

At idle your engine is at no load on it and at its highest vacuum pull its also at a good amount of vacuum when it's in at no real load when your just cruising down the hwy say on flat freeway. As others have mentioned at both times your car is running at its lowest amount of fuel to air with your right foot not on the gas. Now the age old argument about vacuum advance getting you better fuel milage that is a tiny argument not the reason you use vacuum advance to a benefit. So lets say its summer in Arizona or Las vegas its say 114 degrees your stuck in a traffic jam idling long periods of time your engine is gaining heat from all the idling and not as much air pulled through your radiator the extra heat, advancing the timing because you have no mechanical advance going in the dist because it takes a certain amount of rpm to make the advance plates in the dist move (called centrifugal advance ). To work. Bottom line it helps make the engine run cooler that's where the vacuum advance really shines on a street car and is a help. Also let's say your cooling system in this hot time of summer and that higher hp motor you built ( more heat generation from the engine ) is just barely capable of cooling the engine in the first place then the idle vacuum being raised above what's called just initial advance this is where the vacuum advance system for a street car really shines. At the risk of pissing people off the tiny fuel consumption thing is not much of an argument for one. Take full advantage of it and take vacuum directly from the intake manifold. Now if you want to pass the california air research test so you can drive your car there put it on the carburator then your engine will run slightly hotter at idle burn more exhaust hydro carbons. In conclusion to this long post take advantage of what the vacuum advance is really good for and dont ever hook it to your carb go directly to the manifold for the source all the time. As long as your cooling system in real good working order for you you can run vacuum advance or no vacuum advance your choice. But if your not going to take full advantage of something what's the point in doing it a hair better fuel milage not really. Anyone bought a vette for fuel milage screwed up from the get go.

Little Mouse

When the car companies dropped compression in engines for fuel reasons there was less heat build up in engines and the emissions folks were on there case and to stave off the catalytic converter as long as they could and its costs to them. They simply put the vacuum pull off the carb and at idle tests it burned a bit more exhaust hydro carbons to help them. The engines no longer building as much heat without compression so extra help at stop and go traffic high heat no longer as much of an issue they could get the CARB off there hiney in the state that sold the biggest car market sales for them. You want a better chance at cooling your car in bad conditions with that improved from 1970s nothing engines to your now more power. Put the vacuum advance back on the intake manifold like they did before CARB problems.

MelWff

Again, the thread is 14 years old

lars

Painful.