[cobalt327]

Originally Posted by Custom10

I am quite sure that changing the springs nominal position or position without any vacuum will effect the point at which it starts to move due to a vacuum force applied to the diaphragm.

You need to - like I did- test this w/a vacuum pump equipped w/a gage. I stand by my previous statements.

[cobalt327]

Originally Posted by kyle86

Have you found that the preload on the spring doesn't cause it to drop out faster if limited from the back (Crane) verses the front?

Say on the AR16, around 4" the arm starts to move and it's total advance is in around 15". If it's limited to 10 degrees the way I did mine, wouldn't the advance start to move around 4", but hit the limiter around say 8" vacuum instead of the total 15" needed to move the whole arm then entire distance?

Then if it's limited to 10 degrees the crane way from the back side of the arm, you would think that it would have to make it all the way to that 15" before all the advance was in regardless of where it started since the arm has to move all the way to it's factory stop right? I don't have access to a vacuum pump right now so I don't know.

What I found was what I posted previously, Kyle. The amount that the arm is moved by a "front" OR "rear" limiter does not change in any way the amount of vacuum it takes to move the arm initially or when the arm retracts- it it the same as an un-limiter-equipped can.

Do what I did and test this for yourself. You don't even need a gage-equipped vac. pump- use your mouth and see what you "see". It will not be as accurate as my gage, but I'm sure you will be able to tell something.

BTW, the can I tested is an OEM-type can, w/o internal adjustment.

[kyle86]

I'll put a t off my vacuum gauge later today and try it for myself.

Earlier today though I decided to up the jets. It was really hot out and I was getting some spark knock during acceleration. Also it was dieseling when shutting off. I decided to shut it off while cuising and look at the plugs. When I pulled the new plugs I put in 3 days ago, they were pale white still, the old ones I had in also had sides that were wiped clean. I moved from 72 jets (factory) to 74s and most symptoms went away except for the preigniton tiping in from cruise.

It seems like 74 is slighly rich, and 72 is too lean. I might eventually buy some 73s and see how they work. I'm going to play with this vac advance can one more time and see if I can get it straightened out. Otherwise I'm buying an adjustable one.

[Custom10]

Originally Posted by cobalt327

You need to - like I did- test this w/a vacuum pump equipped w/a gage. I stand by my previous statements.

I had a chance to test an OEM(non adjustable)type vacuum can last night on the bench. The starting position of the connecting rod does effect the start point of the movement. Pic one shows the rod against the vice grip jaws at about 4"hg this is just below the start point,the rod has not moved yet. Pic 2 shows the rod starting to pull away or move at approx 4.5" hg. I then clamped the vice grip 1/8" closer to the can simulating what the crane plate would do if installed on the back side of the rod as I suggested. Pic three shows the rod now starts to move at 7.5"hg so that is 3" hg difference. I then tested my HEI with an adjustable can same thing. In both cases the plate does effect the starting point in relation to the vacuum applied.

[kyle86]

Originally Posted by Custom10

I had a chance to test an OEM(non adjustable)type vacuum can last night on the bench. The starting position of the connecting rod does effect the start point of the movement. Pic one shows the rod against the vice grip jaws at about 4"hg this is just below the start point,the rod has not moved yet. Pic 2 shows the rod starting to pull away or move at approx 4.5" hg. I then clamped the vice grip 1/8" closer to the can simulating what the crane plate would do if installed on the back side of the rod as I suggested. Pic three shows the rod now starts to move at 7.5"hg so that is 3" hg difference. I then tested my HEI with an adjustable can same thing. In both cases the plate does effect the starting point in relation to the vacuum applied.

Great experiment!!!

I will definitely be swapping out my limter plates.

As for my problem, the tip in ping went away after I increased the jet size another step. Never would have thought it wanted this much fuel.

The only time it pings during cruise now is when I punch it. If I tip in or roll in, it accelerates fine but if I stomp on it, it will chatter. I'm guessing this is an accelerator pump problem and may be able to get tuned out with some cam changes.

[F-BIRD'88]

You can also limit the other end to limit max possible vac advance under high applyed manifold vacuum (deceleration , shifting etc) some cans can flash over 20deg.
The cans origionally from lw compression cars with egr need tweeking for cars that don;t have egr and or high compression ratio.

good post. good pics.

Post some stuff on the adjustable can.

[cobalt327]

Great job- glad that it is settled.

[cobalt327]

I had a spare bit of time away from the work in front of my monitor, so I took up a buddy's offer to raid his Auto Zone's store to measure all the vacuum advance cans I wanted. I had almost decided to not bother (too much like work comment above), but in the interest of making sure, I went ahead anyway.

Needless to say, what Custom posted earlier has been shown to my satisfaction to hold true throughout the stock I measured.

While this comes as no surprise to anyone, w/the possible exception of myself, I wanted to be as sure as possible, given the two cans I own (that I will sacrifice at least one of to see what's up w/it) that seem to go against the trend. They are used, and although they appear to work normally otherwise, there's no telling what is going on w/them, and I doubt I'd ever use either on an engine.

Something I did notice, is if there's some amount of "preload", the amount of vacuum required to move the arm doesn't always increase in a linear fashion. Another thing was my Mityvac's gage was too small to give a good amount of sweep, so seeing a small deviation wasn't possible. So, I plumbed a BIG vacuum gage to it temporarily to test with.

Now, in the case of Kyle's problem, though- I do not see the difference between front or rear limiting having an affect one way or another on the pinging he's getting at throttle tip in. The reason I believe this, is the advance is at the max while at a light throttle cruise. It wouldn't matter if the can came in at 2 in/Hg or 5 in/Hg or 10 in/Hg- it's all in at the throttle setting where he's getting pinging at tip in. That is a function of TOTAL timing- either by the vacuum can, or by the total mechanical timing and the tip in point of the vacuum advance can wouldn't change that.

So all that said, hopefully my earlier dismissal of Custom's testing being wrong will be forgiven. Fact is, it is right on.

[ericnova72]

i missed this thread when it started, but agree with F-Bird, it is a very good informative thread. Anyone dealing with timing issues with performance engines on the street should be directed to this thread, it encompasses pretty much everything you need to know, parts to use, and trends to these parts.

I've been using the Crane limiter plate and an adjustable can since I discovered it 15 or so years ago, but still have trouble convincing people that it makes a big difference when done right. Most don't want to put in the effort and just put there heads in the sand and dump the vacuum advance.

[Custom10]

Now, in the case of Kyle's problem, though- I do not see the difference between front or rear limiting having an affect one way or another on the pinging he's getting at throttle tip in. The reason I believe this, is the advance is at the max while at a light throttle cruise. It wouldn't matter if the can came in at 2 in/Hg or 5 in/Hg or 10 in/Hg- it's all in at the throttle setting where he's getting pinging at tip in. That is a function of TOTAL timing- either by the vacuum can, or by the total mechanical timing and the tip in point of the vacuum advance can wouldn't change that.

Good point I agree...if the can has its full advance in by say 11- 12 "hg then coming off of a 20" hg pull at cruise will not drop out the vac advance with a light throttle application unless the HG pull drops below 12" or so, but i wonder about his 20" at cruise as I stated before it seems high, mine is about 14-15 at this cruise RPM of approx 2300 and any throttle be it just a touch drops it down enough to dump out some of my vac advance but each setup is different so it all depends...

[QUOTE]

So all that said, hopefully my earlier dismissal of Custom's testing being wrong will be forgiven. Fact is, it is right on. [/QUOTE

Thanks for that cobalt, nothing to forgive but I get what you are saying

I seems that we could almost start a separate category here on HRBB and call it tuning or something like that. I mean there is the base engine building and design considerations, variations, technique etc etc but when it comes down to making our engines work on the track or road its all in the tuning and ignition/carb setup could almost have its own header...food for though any how.

I am not sure if anyone has seen this spread sheet that is out there I don't know a bunch about it and it may be for ECU controlled spark but I was messing with it and changing some numbers around kind of cool, it shows the relation ship between initial, mech, and vac advance on total advance....

http://www.jimragtop.com/TC/spark_advance.xls

[oldbogie]

Originally Posted by kyle86

Great experiment!!!

I will definitely be swapping out my limter plates.

As for my problem, the tip in ping went away after I increased the jet size another step. Never would have thought it wanted this much fuel.

The only time it pings during cruise now is when I punch it. If I tip in or roll in, it accelerates fine but if I stomp on it, it will chatter. I'm guessing this is an accelerator pump problem and may be able to get tuned out with some cam changes.

You're probably more on the right track with richening the mixture than messing with the timing. Vacuum advance is maximized with high vacuum and minimized with low vacuum. High vacuum forms when the throttle is more closed and lower vacuum forms as the throttle is opened.

Yet you say you're getting detonation as the throttle is initially opened. So the manifold vacuum is going from high to low and the vacuum advance is going from a lot (advanced) to less (retarding). This would tend to show that excessive advance is not the problem, unless you have a very sensitive centrifugal that's coming advanced so quickly with increasing RPM that it puts in too much mechanical advance too soon. Could be, but I doubt it.

This sounds like the mixture is falling lean when the throttle is opened. When the throttle blades open the mixture flow from the venturis all the way to the valve slows as the greater available area thru the throttle plates allows the mixture to expand, this reduces velocity. The engine responds to this situation with a stumble that is partly caused by a momentary leaning of the mixture and a drop in the inertia of the mixture which is necessary to overcome the reverse pumping of the rising piston when the intake is still open. The accelerating pump on a carb is specifically there to at least overcome the momentary leaning of the mixture until the main metering can catch up.

I don't remember your intake configuration which has a major influence upon this event. Cold intakes on the street with high gearing and an automatic tranny make this a bigger problem. Exhaust heated intakes, as the OEM makes these things, have less of a problem because the hot spot under the plenum keeps the fuel evaporating into a "perfect gas" (as defined in your Physics book) which keeps the mixture closer to proper ratios without having to go to bigger and bigger jets or more and more accelerating pump stroke to smooth these throttle transitions. so much for the idea that cold intakes ipso-facto make more power, yes, but under very controlled conditions seldom seen on the street.

You have to find the music the carb and ignition will dance to. This can be that much larger jets with more and faster advance will run better and provide more fuel economy than leaner jetting with less advance. You have to play with these combinations rather than follow somebody's rules. The rules just get the motor running and provide comfort to minds that don't understand the science under these things. When you hit tuning problems, you have to experiment and to do that successfully you need to understand how the pieces play with and against each other.

Bogie

[kyle86]

Yea this thread is awesome, I've learned a ton from it. I'm now starting to think the problem is carb related as well, specifically the accelerator circuit getting air in it. Possibly related to a vapor lock problem I'm fighting.

Anyways, turns out the diaphragm in my AR-10 broke right before I had to make a 150 mile trip 3 days ago in 100 degree weather. I went to advance auto parts and bought the only can they had in stock. It's P/N V384Z. With the vac gauge, it appears to start advancing around 5" and fully advances at 13-15". It also has about 20 degrees of advance it it which is way too much, so I had to limit it. This time I used a home made limiter on the back side like the crane one and it worked GREAT. Just had to retard the timing a little to get it back to where it was. Here's a pic. Currently running 34 total + 12 vac.