I am having a hard time figuring out my timing. I rebuilt my motor with a bigger cam and am having troubles with the vacum advance. When i hook my vacuum advance up to either manifold or ported vacuum on the carb, my timing does not advance. Before, i would set at zero and then plug in vacuum advance and it would move to 8. i cant seem to get the timing right. Mt vacuum guage reads 9 to 10 on the manifold. My question is, what is the required vacuum to advance the distributor at idle. Should the vaccum advance when plugging the vacuum advance to the carb. If i attach a mitty vac to the distributor, my weights and springs move. Do I just not understand the timing and vacuum advance or am i just missing everything. The motor is a mild built 350, cr 9.6:1. holley 6210 carb and a 4 speed. Any advice would be greatly appreciated. Sorry for long post.

 

If you apply vacuum to your vacuum advance canister and it does not move after, say about 16", your canister is broken. Personally, I have only seen broken canisters that would not hold vacuum (and they would not advance).


I read your post wrong. I thought you said applying vacuum with MityVac had no effect.

 

The vacuum can installed now is calibrated to open at much higher vacuum, probably at the stock values of 13-15 in/hg..

If you only have 9-10 in/hg. vacuum at idle, you need a can that activates at about 7-8.

 

I agree, you need the "B28" advance control. It is made for bigger cams. The NAPA/Echlin part number is VC-1810 and is all-in at 8"Hg. This is the non-HEI part. See Lars' paper on "Distributor Vacuum Advance Control units".

 

Your cam sounds strong ,, set total advance and forget the vac advance..

 

so if i do not plug the vacuum advance up, do i just need to figure out my springs to get the timing all in at 36 like i read. It looks like the way its set up is that my total timing is only 24 or 26 degrees. my inital timing is 12 now.

 

Yes your mechanical needs to be set at 3000 all in 36 degrees.. What it is at idle will not be a concern.

 

You can if you choose...but I would rather... if it was in my shop...I would be installing the vacuum advance that earlier posts gave you part number for and install it and see what happens from there. And it would be connected to ported vacuum.

Even though GM did go to connecting the vacuum advance to manifold vacuum in later years...this manifold vacuum hose did not go directly from the carb to the vacuum advance pod directly....or at least none that I have ever seen. GM installed a ported vacuum switch in the coolant system...and when the engine warmed up enough ...the coolant temp would switch this ported vacuum switch to allow vacuum to go to the vacuum advance pod and keep the engine running. Having the vacuum advance pod being connected to manifold vacuum...without this switch...can cause it to be really temperamental when the engine is being cranked when it is cold.

I also use a vacuum gauge to set timing and use the timing light as an information source.....but to each his/her own.

DUB

 

Once you get that can then you'll need to set your initial timing to more like 16* to get a decent idle. This combined with 20* of mechanical advance will give you 36* all in at WOT. If you use manifold vacuum it will advance your idle timing more. I have found that this helps with a larger cam that produces low cylinder pressure at idle speeds as long as manifold vac is steady.

 

you can also buy a after market adjustable vacuum canister .what carb do you have ?read Lars paper ,or a good carb book to set the carb correctly .maybe you can gain a little more vacuum at idle .

 

You say in your post that the engine is a "mild built 350". The vacuum level you refer to is usually associated with pretty gnarly cams. Tell us what cam you are running, so we can get a better idea of what you're working with. It may be a matter of tuning. Any cam that can be referred to as "mild" should generate at least 15" of vacuum at idle.

Scott

 

Is this a drag or road racing car that is at wide open throttle the majority of the time? If so, you might consider not hooking up your vacuum advance. If this is a street car, even one making 600hp, you should have the vacuum advance set up and working on full manifold vacuum. At idle, the engine will run cooler and with better combustion efficiency. At cruise, the engine will runner cooler, have better throttle response and get better mileage.

Not sure what you mean by "if I do not plug the vacuum advance up". Just to be safe, I'm stating that the vacuum advance should be disconnected and the vacuum source plugged when you are setting the timing. With your cam, you need plenty of initial, like 16 or so, but make sure you have 36 in by 3000 to match what seems like an aggressive cam from your 10" idle vacuum. That's what I have with 92 degrees of overlap.

 

Is it possible that you have ported/manifold vacuum confused in your post?

 

That's nonsense. Your method sets the timing for a load/speed point where the engine spends 1% of its life, and ignores the low load/RPM points where it spends 99% of its life.

 

Read . learn , and try to relax .. I am not ignoring anything your just working off a flawed idea ... Lets not get into a huge debate , post after you look around and search how to set the timing ..

 

No...I do not think so....only because I have to repair many in a year due to incorrect vacuum supply going to the vacuum advance pod.

As I have always been told and experienced.

"Ported vacuum" is a port on the carb that when at idle has no vacuum coming out of it ( depending on how high the idle is set). When the throttle is increased...the vacuum beings to "come on" and supplies this vacuum to the vacuum advance pod or whatever you are controlling for this purpose.

"Manifold vacuum" is vacuum that the engine is creating at idle and is constantly there at idle. This value of vacuum will change during different throttle angles...but it still there and when idle is returned...it is there.

Why did you ask??? Something I wrote not make sense....which I know can occur.

DUB

 

Not again.............

 

Ported vacuum was an emissions concept, the lack of vacuum advance, and thus less timing at idle, made for high exhaust gas temperatures, so when you pump air into the exhaust using the A.I.R. system there was additional burning of combustion byproducts. This is strictly an emissions, not a performance, concept. If you want the engine to have some performance, run cooler and idle cleaner, run full manifold vacuum. A big cam needs it more than a small one and it needs more initial timing also, due to increased exhaust gas dilution. The extension of this is that a single plane may need more initial timing than a dual-plane due to the dilution differences.

You don't want high exhaust gas temperature, that energy is better used in the combustion chamber as cylinder pressure to apply to the face of the piston making some torque. Read Lars's excellent paper and the associated paper on vacuum advance by Duke Williams.

Also, having the timing all in by 3000, but not paying attention to what your initial is, is leaving some performance on the table. Of two engines both with 36 degrees all in, which one will operate better, the one that has 4 degrees initial or the one at 15 degrees?

Yes, reading is good.

 

Thank you.

 

My understanding is mechanical advance adds 20 from initial . So if you set it at 4 degrees you will never hit more then 24 degrees total timing. I could be wrong but I think I am right on this. So if you set say 34 total as I have on my fast burn heads my initial is around 14.

Now if you have a stock motor running low quality gas the all in setting of 32 to 36 degrees may not work for you, and you may want to have initial timing at 4 degrees..