Question for all those on manifold vacuum

[jcd9730]

I've tried to run the advance off manifold vacuum, and it idles like crap.

From what I understand, the vacuum can is set to start advancing at a certain level of vacuum by a limiter, ie at 10Hg and then advances until full deployed.

My question is, if I have it hooked up to manifold vacuum, that means at idle the vacuum can will be seeing full vacuum (17 -18Hg in my case) right at idle. This would then fully advance the timing, right at idle and cause it to idle like crap since there is way too much timing advance at idle.

Right?

I just don't see how it would work with manifold. I'm not disputing that manifold is better, I just don't see how this works

[geezer#99]

THat's how it works, but the question is what's your initial timing set at?
And what's your total timing at idle? Initial plus vacuum advance. It might be too much!
Some vac pots can add as much as 26 degrees.
If you know your total at idle then reduce the initial to compensate.

[jcd9730]

Initial is at 8, total is like high 20s i think. I couldnt get it to not ping even with heaviest weights with the total more that 30.

So I should be setting intial timing WITH vacuum hooked up? I've always set mech. advance and then hooked up vacuum. But that was with ported source

[geezer#99]

You still set initial with vac advance plugged off.
It likely is pinging because of the higher vac advance and the vac pot is slow to move to reduce the timing. Some pots are adjustable. Use an 1/8 inch allen wrench in the vac pot nipple. THey have a spring in them. THe vacuum pulls on the diaphram inside and compresses the spring. When you step on the gas pedal, the manifold vac drops and the spring pushes the diaphram back and your timing drops. THe spring in yours might be set up to be slightly weaker so it's slower to reduce your timing.
You can change pots or make a limiter to reduce how far they move.
There's a number stamped on the arm of the pot to tell you the number of degrees it adds. Unfortuneately it's underneath on the bracket where the pot bolts to the dizzy.

[jcd9730]

It adds 13Hg, and its adjustable. When I say pinging I meant at WOT with vacuum advance plugged.

I though that the screw is for adjusting how far it advances total? And that still wont affect it idling like crap?

[geezer#99]

OK!!
With only 8 degrees initial have you verified your timing marks are accurate?
IIRC the screw controls how fast or slow it works depending on your vacuum.
I suppose if you where to adjust the spring for maximum tension it could limit the total.
Do you have a dial back to zero timing light?

[jcd9730]

Yeah, I checked TDC when I did the head gasket. Its at least real close. And yeah, I have a dial back

If the screw adjust how fast or slow the vacuum advances, idle would cause the vacuum to fully advance, right?

Sorry, Im just trying to understand this. I've been puzzling over this awhile

[geezer#99]

[QUOTE

If the screw adjust how fast or slow the vacuum advances, idle would cause the vacuum to fully advance, right?

[/QUOTE]

Maybe! Depends on how much vacuum you have at idle. IIRC the spring has two functions.
1. Set the point at which the the advance starts with a set amount of vacuum.
2. Control how fast the advance goes to zero.
If your pot is set up with your spring being the weakest possible then it likely advances your timing to the max with very little vacuum. And drops back to no advance slowly.
I've tuned with them but found it was so much easier to set them up as weak springed as possible and built a limiter to control the total advance.
Are we getting anywhere or still going around in circles?

[jcd9730]

No I get what you're saying. I'm going to try it with the can adjusted all the way out, and play with the limiter that I have in there. I just thought the spring adjusted total vacuum advance but Im probably wrong. Though in hindsight, I think thats the function of the limiter...

Thanks!

[geezer#99]

If you haven't seen the internals there's a pic of one in this link. They're basically all the same design. Some are adjustable. THe pic one isn't. The adjustment screw in yours adjusts the tension on the spring using a small plate on the end of the spring and a screw which pushes on the plate. Compress the spring, it gets stronger, the diaphram moves slower and needs more vacuum to overcome the spring tension.When the vacuum is taken away the stronger spring pushes the diaphram back quicker and the advance from the pot goes to zero.
If yours is set up weak it takes less vacuum to overcome the spring and with no vacuum the diaphram is slower going back to zero.
http://www.thesamba.com/vw/forum/vie....php?p=5920512

[jcd9730]

Ohhh I see.

So basically I have to figure out where to adjust the can/limiter to where it doesn't advance at vacuum below idle vacuum (~18hg)?

[geezer#99]

Bingo!! THe light bulb is on!! LOL!!
However that might not be possible with your pot. Your 18" of vacuum might be more than the spring can hold back even set at max tension.
You'll need to experiment.
You might need to make a limiter.
I'll find a link for a limiter!

[jcd9730]

Hahaha! I can see the light!

The can I got came with a handy little limiter already

[geezer#99]

And here's a link with some pics of how I did mine.
http://www.chevelles.com/forums/showthread.php?t=343241

[jcd9730]

Wow thats a lot of good info! And good info about the weights too! That may explain why I was having trouble with the total timing too.

Thanks a lot, this has been quite helpful!

[geezer#99]

One last question!
What are you doing awake at 3 am. Working nightshift maybe!
My excuse is old age. Worn out by 9, off to bed and sleep till 3. Ah!! The golden years!!

[jcd9730]

Haha yeah, I work nights and school during the days.

Not much work to do at 3am, so I always end up thinking about whatever task is at hand with my truck.

Hey, nothing wrong with going to bed early!

[geezer#99]

I hear you on the nightshifts. I did my time!
Worked a lot of nights over 35 years. Mostly 12 hours. FRom 8pm to 8 am. Of course had dayshifts in there too. Just the opposite. 8am to 8pm. Worked 14 total shifts in 21 days then a week off. Needed it.
Good troubleshooting with you this mornin'!!
Keeps the old brain going!

[jcd9730]

Yeah, night shifts mess with your head...

Thanks, I believe you have set me on the right track to get this cranky beast running smooth again!

[jcd9730]

Boom, nailed it. Running smooth with manifold vac. Feels smoother already!
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[geezer#99]

AWesome!!
What did you have to do?

[jcd9730]

Well, I guess when I installed the can, I totally misunderstood the limiter (this piece- http://www.summitracing.com/parts/crn-99619-1) so it was advancing the timing right off the bat. Once I threw that out, I just had to adjust the can so it wasn't advancing at idle. Easy to do when you have a better understanding of the function....

Fully screwed in, it was adding 22 degrees at idle, so I just unscrewed til it was perfect! Easy as that. I don't even think I need to add a limiter like in the thread you linked.

[LONGHAIR]

You really should have the limiter. It is intended to stop the advance at a specific point.....That is it's job, not the vacuum screw's.
The vacuum canister should be pulling all of it's advance at idle, up to the point where the limiter stops it. The adjustment screw is there to change the way it releases that advance.
Vacuum advance works opposite of how most people think. It is all in at idle and "goes away" as you open the throttle, which is when the mechanical advance is coming in.
You should have no vacuum advance at all when you are WOT.

[jcd9730]

Yeah, but vacuum is higher at part throttle than at cruise. So the vacuum advance shouldn't advance until the vacuum is higher than vacuum at idle. Right?

And the limiter I had increases timing at idle. It limits the total vacuum advance but also increases intial advance when on manifold

[msgdsrf]

Typically, vacuum advance at idle is a good thing. Keeps things burning clean and lowers operating temp. There's no load on engine so pretty hard to have too much if your total isn't too much to begin with. As you give it throttle the load increases and vacuum decreases the timing as mechanical advance comes in. Longhair is 100%.

The Summit can adjust for limit and this in a very small way may effect initial curve but not much.