Discussion Forum - Northstar Chevelle Club

jim larson · 2K+ Club

Any issues with running a 396 with the Vacuum Advance canister plugged?

Bobs_Place · Vice President

Jim
Sounds like to much advance at 1800.
There are a lot of different advance cans, the vacuum that they start and stop will very.
Also the amount that they advance when full in.
All in @ 2600 seems a bit aggressive, maybe ok if you don`t ping.
The advance can that you have and the mechanical advance curve put the timing to high at 1800 and light engine loads.
I run 16* with 20* start at about 1100, all in by 3200-3500 vac starts at 9-10in and full at 16in. My Idle vacuum is 6.5-7.5in.
What is the vacuum at idle, 1800 and cruse.

jim larson · 2K+ Club

Thanks Bob. So what might you suggest on a vacuum canister for me? I am kind of lost on how these VC work, when they come in and how much advance they add. Are you also suggesting I add heavier springs in the distributor so it's total advance comes in later?

Back in Black · Active Member

Did you try connecting the vacuum advance to ported vacuum?

dashboard · 2K+ Club

Read these, excellent explanation timing and vacuum advance.

www.chevelles.com/forums/showthread.php

www.chevelles.com/forums/showthread.php

Lost in the 60s · President

Back in Black wrote:
Did you try connecting the vacuum advance to ported vacuum?

Exactly...get it off the full vacuum at idle port (can't remember the technical name).

jim larson · 2K+ Club

I think the only option for a vacuum is on the throttle plate, below the throttle plates, thus manifold vacuum. I don't see another option on the original carb.

-- Edited by jim larson on Tuesday 29th of September 2015 07:22:58 PM

jim larson · 2K+ Club

Bobs_Place wrote:
Jim
Sounds like to much advance at 1800.
There are a lot of different advance cans, the vacuum that they start and stop will very.
Also the amount that they advance when full in.
All in @ 2600 seems a bit aggressive, maybe ok if you don`t ping.
The advance can that you have and the mechanical advance curve put the timing to high at 1800 and light engine loads.
I run 16* with 20* start at about 1100, all in by 3200-3500 vac starts at 9-10in and full at 16in. My Idle vacuum is 6.5-7.5in.
What is the vacuum at idle, 1800 and cruse.

Vacuum at idle is 13". at cruse it is 17" I haven't check it at 1800.

Right now with the VC plugged I have 14* at idle (650rpm), 28* at 1000 rpm, 34* at 2100 rpm, and 36* at 29 rpm. According to the guy do re-curved my distributor it is suppose to add 22* (I can't seem to find the exact sheet he gave me).

The VC when hooked up was adding 20* at idle, so at 1800 rpm I would have 34* of advance plus what the distributor was adding.

I am confused by all this. I think what you are suggesting is that I limit my VC advance to about 10* and that my distributor starts a little later with all in later by using heavier spring??

Where do you find all these different vac can with various totals and when they start and end advancing?

Jon H · 1K+ Club

I think you will find them in the second attachment Kevin sent you.

Bobs_Place · Vice President

Jim

As I recall, you have an OEM carb for your year or close to it? The OEM carbs don`t have a port that is ported. Ported vacuum simply put, no vacuum at idle, off idle and greater
both manifold vacuum and ported works the same.

The article that Kevin recommended is very good, well worth reading.

Do you have a vacuum gauge and can you get some Vacuum readings?
You will need to select a can that is all in or has not started to advance at idle. If vacuum or mechanical advance partially in you will chase the idle speed up and down.

If your cam is a long duration with a lot of overlap you can tolerate a lot of advance at idle, the engine likes it, therefor the vacuum advance at idle will help.
Lets say base timing is 10* plus vac adv16* totals 26*, that works, however if your engine idle at 10in vac and your vac can is full in at that idle vacuum you may have to much
timing at mid throttle, ie: 2800 rpm 36* and 16* vac adv is 52*, very good for cruise speed but at 1/2 or better throttle the vacuum advance has not retarded timing
back or close to 36* which will cause the engine to ping.

With 6-7in vac at idle, I installed a vac can that starts advancing at higher vac than idle and is all in at cruise vac, as soon as I tip the throttle in the timing start to retard.
Because the vac adv does not advance at idle my timing acts like it is hooked to a "ported vac" port. Would be nice to get more timing at idle but sometime we have to make trade offs.
When time permits I would like to advance the dist 2-4* to see how the engine acts, if it works well I would then have to shorted mech adv to keep WOT adv. down to 36*.

Read the article that Kevin recommended, or search "Timing 101".

Bobs_Place · Vice President

Jim

Your last post came as I was replying to the second to last that you posted, I type slow!

28* at 1000rpm seems very aggressive, with VC connected and light throttle you get a lot of timing very early.
My timing starts to advance at between 1000 to1100Rs.

Can you put in stiffer springs in to move the curve up, say 5-750Rs
Than try a B24 can P/N VC1804, gives you 20* vac adv@12-14in/with no adv@6.5-8.5in or
a B25 P/N VC1807=16*@13-15in/with no adv@5-7in.

I would suggest mid to upper 20s* at idle.
If you can, stretch the mech to 24* with base timing @12 use the B25 @ 16* would give 28*@ idle.

You stat that your timing is at 34@ idle with VC puded in.
But before you go out and buy a bunch of cans and rerecurve the dist, crank the timing down to say 28 or 25*at idle with CV in to see what the engine likes.
Drive the car at low levels, see were it`s best.
Than we can adjust mech and or vac adv. accordingly.
Hope that helps.

jim larson · 2K+ Club

Thanks Bob, yes I do have an OEM 65 dated holley carb. As I mentioned I have plugged my Vac Can, currently have base @ 14* and 22* in the distributor for a total of 36*

Currently my Vac Can is a VC 1765, which is suppose to add 0* @6" and 16*@12", but it adds 20* at idle My vacuum gauge says I have 12" of vacuum at idle. At crusing speed of 40, 45, and 50 mph on a flat stretch of highway I had 17" of vacuum when checked a few months ago.

Back to the VC advance of 20* at idle, either the specifications are wrong, my timing light is wrong, or the vac can was manufactured wrong?? There was no rubber bushing around the rod as there is suppose to be, as I was told. I have also heard that the timing lights that you can adjust up or down are not correct because of a lag in the computer in the light. Have you heard that?

I will try the stiffer springs. I am not sure how to tell if one spring is stiffer than the other except by feel, as I have 3 additional sets of spring.

I have three other VC sitting in the garage; but not sure of when and how much timing they add.

One is stamped 360 20, which is what the original 66 360HP engine had from the factory, the 360 is suppose to be the last 3 digits of the part # and the 20 is suppose to be the amount of advance timing added with all in at idle I am told.

Another is stamped 357 20, which was for the 66 325HP engine

Another is stamped 502 14, which I was told was for a Small Block

The third is stamped B1, it is suppose to be the same as a VC680 with 0*@8" and 16*@16"

If the stiffer springs work out, then I think I should try the B1 VC I have or the B25 you suggested. Any opinions?

Bobs_Place · Vice President

Jim
The VC 1765 is full advance at about 1 in vac less than the B24, I would not replace the VC, it`s not bad. The bushing will reduce the amount of advance. You can
also get a Vacuum Timing Limiter Plate P/N 99619-1 at Summit Racing for about $3.00 which limit in 2* each step.

You can check your timing light on your engine by setting the timing as hi on the timing tab ,say on the 16 or 18* mark than dial back the light to 0*.

The B1 can is what I am running, works very well for me. The timing starts above idle vac and is all in at cruise, when I tip the throttle in vac starts to drop/timing
backs down.

Not familiar with the 357 and 502, check them with a hand vac pump or just plug them in to the manifold.
As for the spring tension hook them together, stretch and measure them. Any stiffer will help. Also while your in the dist. check for a bushing that limits mechanical advance, you may need to pull it.

Have you driven the car car with the timing backed down, if not you need to, this will tell us were the engine runs best at idle, just off idle, start ability and low speed. Pull 6 to 10* out and try it. Don`t worry about losing your 36*total advance as we just want to find the best low level timing for your engine. Do this with the VC connected.

Seems to me that the VC is ok, but mech. adv needs to moved up, with vac advance at idle start mech advance around 800 to1000 with 8o12* an all in at 3000 to 3500 @36 total. If the eng likes 26 to 30 at idle than we can add that 18* vac adv to the advance that the mechanical is making at cruise.

Do you see where I`m going with this? The eng like lots of timing at cruise 48 to 52*, 36* WOT and,to get get good Idle quality timing must be stable.

jim larson · 2K+ Club

Thanks one again Bob. My cam is an old Cam Dynamics cam (stamped CD10306), which I am told is the same as the current Crane 284H12. The people of Team Chevelle seem to think this cam need 16* of base timing. That is why I had the base set to 14* because the distributor was recurved to add 22* of mech to reach the 36* total with VC line plugged.

I changed one spring to a stiffer spring and it seems to change a little when the mech from the distributor starts and is fully in. With VC plugged I had 12* @ 600 rpm, 12* @ 800 rpm, 20* @ 1000 rpm, 26* @ 1500 rpm, 34* @ 2700 rpm, and 34* @ 4000 rpm.

When I hooked up the VC, the engine went up to 750 rpm with 28* and at 850 RPM it still read 28* of advance, my conclusion being that the VC added 16* which is what it is suppose to add at idle with a manifold vac of 12-13".

The surging when driving at or below 1800 rpm was still noticeable; but reduced. I am thinking of changing one of the spring to a little more stiffer spring. Thanks for the method of checking which spring is more stiffer, I am assuming the shorter one.

Bobs_Place · Vice President

Good Job, heading in the right direction.

So you started with 14* at idle (650rpm), 28* at 1000 rpm, 34* at 2100 rpm, and 36* at 29 rpm
You are now at 12* @ 600 rpm, 12* @ 800 rpm, 20* @ 1000 rpm, 26* @ 1500 rpm, 34* @ 2700 rpm, and 34* @ 4000 rpm.
You reduced the timing 2* @ idle, 8* @ 1000, can`t tell when mech goes to max adv, should have gone up some.
Looks like you retarded base timing by 2* and the spring dropped 6*@1000.

What kind of weights do you have installed, are they after marked or original? The curve is very short.
Add more spring and or bring the timing down some, see how it runs.

We need to find out what the engine likes on the low end to help with setting the curve.

jim larson · 2K+ Club

Bobs_Place wrote:

What kind of weights do you have installed, are they after marked or original? The curve is very short.
Add more spring and or bring the timing down some, see how it runs.

I think the weights are original, at least they don't look like those in the kits Summit sells. I don't see a bushing like those in the re-curve kits; but it looks like there is some kind of ring around the pin that limits the travel of the weights. The guy ( don't remember his name) at the engine building shop in Apple Valley recurved it and said the distributor added 22*.

I added one stiffer spring today. With VC plugged base set at 12* with 600 rpm.

Also 12* @ 800 rpm

13* @ 900 rpm

14* @ 1000 rpm

19* @ 1100 rpm

and 34* @ 2700 rpm

When I hooked up the VC, the rpm increased from 600 to 800 rpm and advance went from 12* to 28* ( an increase of 16*). So it looks like all the advance went in @ idle. I don't understand how I can determine for sure if all the advance from the VC is in, other than running the engine at 2700 rpm and seeing if the advance reads 50* (an increase of 16* from the 34* to 50*).

I am going on a cruise with our local club tomorrow, so I will have to check the milage. It had been running at about 12.5 miles per gallon. It still kind of surges a little when under 17-1800 rpm. Which coverts to about under 10, 20, 30, 40 mph in 1st, 2nd, 3rd, and 4th gear respectively.

dashboard · 2K+ Club

Jim if you disconnect your vacuum advance and plug it at the carb do you still get the surging or bucking at The rpms listed above?

jim larson · 2K+ Club

dashboard wrote:
Jim if you disconnect your vacuum advance and plug it at the carb do you still get the surging or bucking at The rpms listed above?

No, it goes away for the most part. I should also mention that the surging is with a very light load on the car, almost coasting. When I get off the throttle and the car slows down by the engine turning less rpm once I get to around 1800 rpm, I get the surging. As soon as I give it a little gas, thus increasing the rpm and a little load on the engine it goes away. What people have been telling me and I gather from other comments that there is two much advance at a light load with the 1800 rpm.

Bobs_Place · Vice President

Jim

Put the timing light on, check timing at 1800 with the VC disconnected, I think you will find that your timing is quite high at this rpm.

Now retard the timing by 2 to 4* hook up the VC and drive the car. If the car still serges at 1800 take more timing out .

When you get the eng run clean at 1800 with VC connected, than check timing at high rpm and 1800 with VC disconnected.

IF our timing at 2700 is now 28* you will need to lose 6* @ 1800 Than you can adjust the base timing back up so you get the 34 or 36* @ high RPM.

I thinking they ground the original dist weights to get the curve were is`t at now.

Do you have another dist that you can get the weights from, if so you my want to try them, but before you swap weights, measure and adjust timing to eliminate

the surge to see were you need to be.

The vacuum advance is working good. 50* @ 2700 RPM will only happen on light throttle, as you add throttle, the vacuum will drop, timing will then back

down, 34*--36* if you stand on it.

jim larson · 2K+ Club

I think I follow you Bob; but for example, if I need to take an addition 8* of timing out at say 1800 rpm, how do I do that? Do I just keep adding stiffer springs?

Bobs_Place · Vice President

jim larson wrote:
I think I follow you Bob; but for example, if I need to take an addition 8* of timing out at say 1800 rpm, how do I do that? Do I just keep adding stiffer springs?

If you can find stiff enough springs, but that may just put the hole curve up higher, mech would`t start advancing untell 1200/1500 or so?

We need to stretch out the curve, That has to be done with the profile of the weights.

If you spread the weights on the dist, note the contact area the were the weight meats the dist shaft, I think they would have ground on that serface of the weights to ajust the curve. The original weights are straight along that serface.

Do you have another dist that you can get the weights from to compare against yours, you should see were they were ground.

The best way to fix it is to start with a new set of weights and grind them. I don`t Know if your weights can be ground again to slow down the curve at the 1800 mark.

BUT follow my last post first, need to find out were the timing is at 1800 and ware we need to be.

dashboard · 2K+ Club

Is your VA adjustable? Seems like your VA is adding to much across the board.

jim larson · 2K+ Club

dashboard wrote:
Is your VA adjustable? Seems like your VA is adding to much across the board.

No the VC is not adjustable and all is in at idle, 16*. I just read that article on todays gas you posted. It suggests 12* base, 24* mech. total 34* with all in by ? I have 12* base, 22* total and all in by 3000 rpm.

I drove the car over the weekend on a cruise, up and down hills and around corners, getting into it a few times. 175 miles and it averaged 13.7 miles/gallon. I think I will live with the little surging at 15-1800 rpm with low load on the engine. No heating issues, car ran fine, Maybe I will bump the base timing up to 14*. Thanks to everyone who chimed in.

-- Edited by jim larson on Monday 5th of October 2015 01:58:13 PM

Jon H · 1K+ Club

Jim I think your car will like the 14*, I ran my 396 and now the 454 at 14* and both were very happy.