Can't account for all of my timing advance, please help.

[Gregski]

So not a total newb here, I've been running this Proform HEI distributor for almost 5 years, not looking for the Basics 101 just an explanation, ha ha

1974 Chevy C10 350 SBC with a 4 spd OD trans, 2WD short box in lime green

here it goes....

at 780 RPM base timing is 17° (no vacuum connected) and 27° with vacuum advance of 20 inHg (the highest it gets)

so far so good, this tells me the vacuum can is advancing the timing a Max of 10°

at 1,600 RPM timing is 29° (17° initial + 12° mechanical) and I'm still down with that

now this is where it gets Crazy...

at 1,600 RPM with the vacuum canister connected timing jumps to 43° (17° initial + 12° mechanical + 10° vacuum = only 39), so where did the other 4° come from?

This is exactly why I bought the MSD 6AL-2 Programmable 6530 ignition box, cause these mechanical specs are just whacked

help me understand please

[geezer#99]

Where’s the 12 mechanical come from?
I thought you locked out the time in the distributor.

[Gregski]

Originally Posted by geezer#99

Where’s the 12 mechanical come from?
I thought you locked out the time in the distributor.

Thank you for the rely, you may be referring to my other thread, I apologize for any confusion. In that thread I am using a different distributor (GM brand) for demonstration purposes only.

My truck has a regular Proform HEI distributor still in it with not locked out mechanical advance and a regular vacuum canister.

[Gregski]

I just found an old email from Proform Tech Support where they tell me the distributor specs and say the vacuum can pulls maximum of 14°, which would explain the additional 4° however why do I not see that at idle wouldn't it still pull the max

I think a vacuum can pulls the max already before it even passes 10 in Hg so what difference does it make if it's 11 inches or 21 inches, shouldn't it pull the max

[geezer#99]

Gotcha!
So 10 degrees from the vac can at 1600.
Does it increase at higher rpm’s?

[Gregski]

Originally Posted by geezer#99

Gotcha!
So 10 degrees from the vac can at 1600.
Does it increase at higher rpm’s?

that's the mistery? i think at 1600 RPM it is pulling 14° from vacuum advance can, but if that's the case than why does it only pull 10° at 780 RPM

will test higher RPMs today, good advice, thank you very much, it really helps to be able to bounce this off of someone, I really do appreciate it

[geezer#99]

Adjustable vacuum can perhaps.

[Gregski]

Originally Posted by geezer#99

Adjustable vacuum can perhaps.

Glad you brought that up but an adjustable can should still abide by the laws of physics, adjustable can effects when the advance kicks in (ie at what inches of vacuum) not how much it should advance the timing, the how much is controlled via the additional limiter plate which screws on to the end of the vacuum advance bar and has little teeth on it, each tooth reduces the advance by 2° I do not need such a plate since my setup is already 14° max unlike the factory GM ones which could go North of 20° advance hence needing the limiter plate on our amateur "performance" rigs

[Gregski]

ah what the hewk, let's meet this little guy (who's been loyal to me for nearly 3 years now) ha ha

Proform distributor part number 66941BK $125

[Gregski]

this is my rifle, this is my gun
this one's for fighting, this one's for fun

... well actually this one's for timing! lol

I am sharing this with you because although I really like my Mac Tools TL90 digital timing light I now realize it's limitations or maybe even design flaw, you see you can only look at one or the other, either the degrees of advance or RPMs, but not both simultaneously, and that is fine and dandy at a constant idle RPM of say 800 revs, but try and catch the exact timing at 2,500 or 3,500 and you need a second display

the struggle is real, ha ha

[rpmerf]

Some thoughts.

Disconnect the vacuum advance, plug the port on the carb. Hook the vacuum advance up to a vacuum hand pump and crank it up to like 22" to see how far it goes.

Tee a vacuum gauge into the line going to the vacuum advance to see what the vacuum is at each scenario.

Temporarily lock out the mechanical advance to narrow down whether this mystery advance is coming from mechanical or vacuum.

Have someone connect / disconnect the vacuum advance while you watch the advance on the crank. This way you can see the change as it is happening and not several minutes apart.

Are you all in at 1600? Might it be worth trying at an RPM you are all in? the spring rate can change due to heat.

Is is possible this is due to play somewhere? Worn down distributor teeth, play in the distributor or timing chain? Would think that would show up as unstable timing readings, but it might be worth looking into.

Trying to think of any way to ways to 'single out' different parts

[Gregski]

Originally Posted by rpmerf

Some thoughts.

Disconnect the vacuum advance, plug the port on the carb. Hook the vacuum advance up to a vacuum hand pump and crank it up to like 22" to see how far it goes.

Tee a vacuum gauge into the line going to the vacuum advance to see what the vacuum is at each scenario.

Temporarily lock out the mechanical advance to narrow down whether this mystery advance is coming from mechanical or vacuum.

Have someone connect / disconnect the vacuum advance while you watch the advance on the crank. This way you can see the change as it is happening and not several minutes apart.

Are you all in at 1600? Might it be worth trying at an RPM you are all in? the spring rate can change due to heat.

Is is possible this is due to play somewhere? Worn down distributor teeth, play in the distributor or timing chain? Would think that would show up as unstable timing readings, but it might be worth looking into.

Trying to think of any way to ways to 'single out' different parts

brilliant thank you, just a follow up question on the part below

Originally Posted by rpmerf

Disconnect the vacuum advance, plug the port on the carb. Hook the vacuum advance up to a vacuum hand pump and crank it up to like 22" to see how far it goes.

is the truck running at this point or is this with the truck off? what am I looking at to see "how far it goes" I think last time I played with that I had to remove the distributor cap and watch the metal rod on the vacuum advance bar slide over roughly half an inch

I ask because the rod slides all the way over by the time I reach 10 inches on my Mitty Vac pump and after that I can just keep sucking until the cows come home

not being difficult, just a bit confused here

[geezer#99]

Originally Posted by Gregski

Glad you brought that up but an adjustable can should still abide by the laws of physics, adjustable can effects when the advance kicks in (ie at what inches of vacuum) not how much it should advance the timing, the how much is controlled via the additional limiter plate which screws on to the end of the vacuum advance bar and has little teeth on it, each tooth reduces the advance by 2° I do not need such a plate since my setup is already 14° max unlike the factory GM ones which could go North of 20° advance hence needing the limiter plate on our amateur "performance" rigs

It’s voodoo!!
I too have never figured out the why’s and wherefores of a vacuum can
That’s why I always plot the timing curve of both the mechanical and vac advance.
Your timing light is like mine. Shows only one thing at a time. I have an old dwell/tach I hook up for rpm reference.

[Dead Parrot]

When I played with one of those adjustable vacuum pots with the dizzy out of the truck and connected to a vacuum pump, I saw they aren't a perfect binary switch. If the magic point is say 11", then at 10.5" I would see the advance start to pull. It wouldn't reach max pull until say 11.5" or 12". If the magic point on yours is right around 10", it might not be full in at idle but is full in at an unloaded engine speed of 1600 rpms and higher vacuum, giving you the extra 4*.

Also possible your timing tab isn't stamped perfectly accurate.

[Gregski]

Originally Posted by Dead Parrot

When I played with one of those adjustable vacuum pots with the dizzy out of the truck and connected to a vacuum pump, I saw they aren't a perfect binary switch. If the magic point is say 11", then at 10.5" I would see the advance start to pull. It wouldn't reach max pull until say 11.5" or 12". If the magic point on yours is right around 10", it might not be full in at idle but is full in at an unloaded engine speed of 1600 rpms and higher vacuum, giving you the extra 4*.

Also possible your timing tab isn't stamped perfectly accurate.

I hear you on the can, mine is not adjustable it's stock, I do own an adjustable one but I don't think its on there now, also I get that it can be 1 or 2 inches off, but at 19 or 20 or 21 it is way passed the 10 inches it needs to pull all in at, hence the head scratcher, but hang in there we shall finger this one out

[rpmerf]

Originally Posted by Gregski

brilliant thank you, just a follow up question on the part below

is the truck running at this point or is this with the truck off? what am I looking at to see "how far it goes" I think last time I played with that I had to remove the distributor cap and watch the metal rod on the vacuum advance bar slide over roughly half an inch

I ask because the rod slides all the way over by the time I reach 10 inches on my Mitty Vac pump and after that I can just keep sucking until the cows come home

not being difficult, just a bit confused here

I was thinking with the engine running and watch the timing change as you apply vacuum to the advance. This gives you a known level of vacuum rather than whatever that vacuum port is pulling currently. Make sure you have a vacuum gauge on it so you know what vacuum you are at, and you can see if it is dropping off.

[Gregski]

Originally Posted by rpmerf

I was thinking with the engine running and watch the timing change as you apply vacuum to the advance. This gives you a known level of vacuum rather than whatever that vacuum port is pulling currently. Make sure you have a vacuum gauge on it so you know what vacuum you are at, and you can see if it is dropping off.

Makes, sense, Copy that!

[Gregski]

well decided to put these to a good use, and with AutoZone price matching Summit, scored this bad boy for $80 out the door

INNOVA 3568 Digital Timing Light

now we're cooking with Propane gentlemen!

[Gregski]

Originally Posted by rpmerf

Some thoughts...
the spring rate can change due to heat...

Speaking of heat kind sir, after I bought the new light and plotted the mechanical curve only, without vacuum, I decided to take the truck for a spin just to see how it would run without the vacuum can connected, and I barely made it around the block, something wasn't right, did I really hear pinging, what?

Well the heat made the PVC rubber grommet expand and it was loose like a goose in it, MAJOR VACUUM LEAK!!!

For the record I checked it with the rig cold and I could not even twist or wiggle it it was solid, but after it heats up, oh Nelly

figure I report on that, now I will fix this thing once and for all, and start collecting my metrics all over

your quote led me to finding the culprit, thank you kind sir

[rpmerf]

I had something similar happen, but the opposite. Used a generic PCV hose from my local auto parts place. One day, I am driving and the truck stalls out. I manage to get it started again and notice the PCV hose is collapsed under vacuum. I replaced it with a piece of fuel hose and it's been good for 6 years now.

[Sawlog]

Originally Posted by Gregski

I just found an old email from Proform Tech Support where they tell me the distributor specs and say the vacuum can pulls maximum of 14°, which would explain the additional 4° however why do I not see that at idle wouldn't it still pull the max

I think a vacuum can pulls the max already before it even passes 10 in Hg so what difference does it make if it's 11 inches or 21 inches, shouldn't it pull the max

Do you have your vac can plugged into port or manifold vac?

[Gregski]

Originally Posted by Sawlog

Do you have your vac can plugged into port or manifold vac?

manifold vacuum

[Gregski]

100% Humidity, I think tunning will have to wait!

[Gregski]

picked up these Mr. Gasket Valve Cover Grommets Part No. 6376 at Oh Really's and one of them seems tighter than what I had so we shall see (Made in China, so they gotta be good)

this should not be this difficult, that's why I keep on sharing, it's going sideways, lol

[geezer#99]

Lubricate the grommet with wd40 and it should be easier to install.