Bikespace

The GM HEI can is probably your best bet. I used an Accel 31035 adjustable vacuum advance, which according to the installation notes, can be adjusted to limit max advance from about 3 to 21 degrees.

Accel 31035

Its_hers

While you've received a wide range of opinions, my suggestion is to do this properly. Take the distributor out and send it to a professional who will will tailor the curve and limits so you know what your initial should be without guessing.

resdoggie

It doesn't matter what your timing is at idle as long as the car starts and runs smooth. As soon as you reconnect the vacuum advance canister the timing at idle will increase to whatever the max advance is for the canister e.g. 15 degrees. So yes, for example, you may have 16 initial, 5 mech'l advance at 800 rpm and 15 degrees vacuum advance for a total of 36* AT IDLE only. As soon as you step on the gas, initial remains the same, mech'l increases with rpm's and vacuum advance drops relative to the drop in vacuum because the throttle blades are open more. Follow what Lars is suggesting. The Accel canister mentioned above may be what you need for adjustability.

Greengear

Bikespace

1980 L48? You've either been running for a while, or your choke is stuck open, too.

A new vacuum can and a slight timing adjustment should really wake up your car. Let us know how it turns out!

Hamerdown

Lots of good feedback...and I read all the comments (thank you)
I have been driving around a little (building faith in my Chinese HEI) and what I've come to notice with my below readings;
I'm NOT doing 1/4 mile passes or looking to shave a 10'th off a second etc but driving my Vette around town as I normally do.
Idle quality, smooth engine operation and fuel mileage is all I want. This is a 1981 low HP emissions engine with a crappy intake and heads, I removed ALL the emissions and ECM BS and I'm very happy and pleased to be DONE with the ECM!

1) switching back and forth from 'manifold' vacuum to a 'ported' vacuum source I notice very little if any measurable seat of the pants difference and I honestly can't say one vs the other is giving 'me' more of a 'peppy' engine. HOWEVER, now the weather is cool and dry, that could all change when the heat and humidity return. (ping-ping was an issue before)
Does running manifold-vacuum really cool the engine more at idle, that would be a good thing on a hot day, in the city, with the AC cranking.
For idle, (and curiosity) I still want to dial-in the rebuilt Q-Jet with a vacuum gauge.
2) using either manifold.vacuum or ported-vacuum the engine instantly hot starts, zero drag on the starter...a good thing.
3) idle quality seems equally good with both ports except, manifold-source brings idle rpm up a little.
4) when using the ported-vacuum source the exhaust smell seems worse, a bad strong sweet odor...at least I notice it when in my small garage working on the car.
5) running manifold-vacuum at a cruising speed with a little pedal I 'think' I heard the slightest hint of ping...but I couldn't put money on it. Before ditching the ECM and E4ME pinging (at times) was very noticeable, especially under load going up a hill.
I haven't messed with vacuum advance distributors since the very early 70's...I don't recall if my Mallory Dual-Point Distributor even had vacuum advance in my 1969 SS Chevelle, But I did know how to properly set-up a dual point distributor (using a dwell meter)
With my 572 BBC Jet Boat (MSD RTR distributor) I'm use to just setting total timing at 35-degrees, all in by 2800 rpm, idles dropped to 12-degrees and it's a done deal. No stacking initial, mechanical/centrifugal and vacuum advance numbers.
6) in my head (and probably because what I have been used to with my boat) I seem to want and only want 35-degrees TOTAL advance at 2800-3000 and no more, and no higher. I'm not comprehending why I would want 35-degrees vacuum at idle when 12-15 degrees does not cause any engine stumble or tripping over itself so, what's the point/advantage of having so much more advance timing at idle?
7) I may even try driving around just using the HEI mechanical advance...to see how it performs. (why not, right?)
#8,9 and10...I'm just glad I'm finally done with that ECM, E4ME Carb and all the emissions crap.

I hook-up the distributor to 'manifold' vacuum;
800 rpm timing is at 35-degrees
3000 rpm timing is at 52-degrees

I try the 'ported' vacuum nipple at the carb;
800 rpm timing is 15-degrees
3000 rpm timing is at 36-degrees

Strokemyaxe

stingr69

3 Types of timing - Centrifugal, Initial, and Vacuum.

The Centrifugal advance needs to max out around 36 degrees for a typical wedge head like you have. You want the centrifugal mechanism to be inoperative at idle. Centrifugal advance needs to wait for the RPMs to come up to start working. Centrifugal advance needs to start advancing a few hundred RPM over idle speed, then be all in around 3000 RPM or a little lower. This is for best for idle, combustion and wide open throttle. Changing the advance springs and or weights in the Centrifugal mechanism will affect the beginning RPM, and the rate of advance. Vacuum advance is adding nothing at wide open throttle because there isnt enough vacuum to make the can move.

Your engine will probably like more Initial advance for improved idle quality. The proper way to increase the initial advance is to reduce the available amount of centrifugal advance. The sum of Initial advance plus the total available Centrifugal advance needs to add up be 36 degrees. If you want for example, 6 degrees more Initial advance because it improves your idle quality, you will need to remove 6 degrees of the available advance from the centrifugal mechanism to keep the total equal to 36 degrees. There is a slot in the base of the distributor that you can braze it up a bit to reduce the available centrifugal travel to achieve more Initial advance. There are other ways to do this but thats how I do it on old school distributors.

Vacuum advance is ignored for a race car that only operates at wide open throttle. If it has a license plate, it needs and benefits from a Vacuum advance, preferably attached to a manifold vacuum source. You can fine tune the idle and part throttle engine operation using a Vacuum advance can. The vacuum can needs to be selected to provide about 12 to about 16 degrees of Vacuum advance at idle and it needs to be fully advanced at a vacuum level that is around 2 inches of vacuum below your idle vacuum. For example, If you have 9" inches of vacuum at idle you want a can that is fully advanced at 7" of vacuum. If the can you have is not all the way advanced at the idle vacuum level, the ignition timing will move around at idle causing unstable RPMs. Unstable RPMs cause variations in manifold vacuum causing the advance can to move around causing further variations in RPMs etc....a mess. If you do not have a properly selected vacuum advance can that pulls all the way in at idle, you can try running it on a ported vacuum tap but you will not get any of the Vacuum advance benefits at idle. Still needs to be limited to no more than 16 degrees of vacuum advance max.

You want to run the vacuum advance attached to a manifold vacuum source if possible to get the most benefit. Idle quality and idle temperature are improved by the additional advance when the engine is not under any load. Once you load the engine, the manifold vacuum drops and the vacuum advance can will back off.

Remember - The sum of the initial plus all the available centrifugal needs to equal 36 degrees. The Vacuum advance adds 12-16 degrees to that but we only worry about it at idle or cruise under low load conditions. The total sum will be 36 degrees plus (12 to 16 degrees vacuum) for a total of 52 degrees (or a little less).

Best to just send Lars an PM and get his paper on the subject. He is THE MAN!

Hamerdown

Lars did email me is papers (great read)
See below...when hooked-up to 'manifold vacuum' I do have 52-degrees total at 3000 rpm
and, 35-degrees at 800 rpm.
Lars commented...it was too much and too long, I need to limit the advance travel to 12-degrees.
Sniffing around the internet last night I see Lars developed a (VAC) Vacuum Advance Corrector (plate) that maybe I should purchase from him? Lars, you reading this???
***************************
With HEI Vacuum distributor unplugged from a vacuum source;
I set total advance at 32-degrees at 3000 rpm...initial drops to 12-degrees at 800 rpm
(I hoped that was a done deal...no way)

I hook-up the distributor to 'manifold' vacuum;
800 rpm timing is at 35-degrees
3000 rpm timing is at 52-degrees
I can't drive the Car like that

I try the 'ported' vacuum nipple at the carb;
800 rpm timing is 15-degrees
3000 rpm timing is at 36-degrees
4000 rpm timing is 50-degrees and will continue to climb with more rpm
(how I drive, my engine never really sees 3500 rpm)

stingr69

You are needing to make actual distributor modifications, not just adjustments. Send it to Lars and let him curve it for you.

Hamerdown

No can do!
I have a 90 year old father (I'm the only one that is by his side) on Hospice...this Vette is my second vehicle (an emergency vehicle) that needs to be on the road 'if' my truck goes down.
At this time I don't have the luxury to send my distributor anywhere.

Bikespace

You can do it all yourself, from Lars' instructions.

Vacuum can limited to 12 degrees? Easy, either buy the Lars spec'ed, eBay sourced GM can, or the adjustable Accel can.

Mechanical advance maxed at 36 deg? Easy. The only hard part is making sure your dist maxes advance at 3000 rpm (not 2000, not 4000). For that, watch the timing, and change the springs inside with an advance kit to get full advance at 3K. Take measurements with the vac can disconnected You can mix and match springs to get the result you want.

Timing all over the place? Shims above the cam gear, even on a brand new dist.

It's all in Lars' papers. I had to buy a new dist, and then do all of the above, but it was all doable, and I didnt have to take the car off the road.

Good luck!

mobird

Ok I think you are misunderstanding a bit (unless I'm reading what you wrote wrong).

Ignore the vac advance for a moment, pull off that vac hose and plug it and the carb nipple. Now with the vac advance disconnected, set your TOTAL timing for 36 degrees at 3,000 RPM (I believe you said you already have it at 32 degrees, so should just be a little bit more counter clockwise rotation of your distributor).

Ok so now your timing is set correctly (judging by your numbers, you will most likely have something like 16 degrees at 800 RPM).

Ok so now for the vac advance. I would find a vac advance can that will get you 12-16 degrees of advance, This will give you around 30 initial timing, and around 50 degrees at cruise. That should work pretty well on your car. So your options (since you vac can seems to be giving you about 22 degrees) are to get a proper vac advance can (see Lars papers for the part numbers, you can get the right one from NAPA) or purchase a vac advance limiter (this is the one I am running: https://www.jegs.com/i/MSD-Ignition/...BoCYXIQAvD_BwE )

The limiter goes in the distributor and physically stops the vac advance rod from moving past a certain point (you have 8 different positions you can set it in to control the maximum vac advance). Only takes a couple minutes to install too!

Greengear

Hamerdown

Update...due to my 90 year old fathers failing health (rushed to ER last week) I have not had much time to mess with my Vette so, I'm still running it with 'ported vacuum'.

It appears my best options are;
1) purchase Accel #31035 adjustable vacuum can
2) purchase Lars VAC (vacuum advance corrector) plate (is this on eBay?)
3) purchase VC1838 AR12 1975 350 Buick 7-9 7 @ 10-12
EDIT #3 NOT AVAILABLE

Greengear

Hamerdown

Thanks for your reply... I'll answer your quote (I highlighted)
(1) no vacuum source, I reset total timing at 35-degrees at 3000 rpm (I went 1-degree conservative)
(2) at 800 rpm initial timing drops to 16/17-degrees (my light is not digital-readout but dialback)
*attaching to manifold vacuum source, @3000 rpm advance is 58/59-degrees, @ 800 rpm 40-degrees > my engine does NOT seem to like that amount of advance at idle (rough idle)
So, until I correct my can, I'm currently using 'ported vacuum' @3000 rpm 35-degrees and @800 rpm = 16/17-degrees
(3) trying to find a vac-can with 12 degrees is like Hens Teeth, they no longer exist (see #4)
(4) Napa no longer carries the AR12 (10-12) vac can listed in Lars documents...no one seems to have them "NA-not available"
(5) the limiter you used and posted the link to is for MSD distributors only, not GM HEI
(6) Lars email to me... Ray -Sorry, but my limiter (VAC-vacuum advance corrector-plate) will not work with HEI - it is for the points-type distributors using the short-arm vacuum advance only.
The "adjustable" vacuum cans (I suppose like Accel 31035) are only adjustable for diaphragm spring pre-load - they are not adjustable for length, and therefore useless. Paper attached with some info: You need to weld a dab into the limit slot and file to length using the arithmetic shown in the attached paper."
(7) I don't carry a welder around in my back pocket, but if the vac-can ROD travel needs to be limited I'll MacGyver it somehow lol
(8) > > > I need a fu*king DRINK!

resdoggie

Your data @3000 rpm isn't making sense. With manifold vacuum connected, timing is 58/59*. With ported vacuum connected @3000 rpm, timing is 35*. At 3000 rpm, ported vacuum is now manifold vacuum because the throttle plate has opened. So, you should be getting 58/59* with the ported vacuum connected. 58/59* is too much at cruise. With iron heads it should be ~50*. When you set your initial timing with vac adv disconnected, did you also disable the mechanical advance from advancing? Not sure what springs you have in your dist'r but lightest springs will advance the mech'l timing at 800 rpm.

Bikespace

A few comments:

59 degrees advance at 3000 RPM and presumably no load (transmission in neutral) is way too much. You have 23 degrees of vacuum advance. So you need a different vacuum can, or you need to modify what you have to get the advance you want.

Lars is a tremendous assest to this forum, and I'm reluctant to challenge his knowledge of distributors, but I believe he is mistaken about the Accel 31035 in particular, and believe it is correct for your application. Here's why the adjustment is not a simple diaphragm pre-load, but rather a hard adjustment limit internal to the vacuum can:

1. The vendor documents show that adjusting the internal screw adjusts the maximum travel of the lever that acts on the distributor to add vacuum advance. So at 4 turns back from fully adjusted counterclockwise, you should see 13 degrees of advance, maximum. The chart also shows for all adjustments the advance starting at more than 6 in hg vacuum, and maxing out at less than 9 in hg vacuum.



2. I have a brand new Accel 31035. When fully adjusted out (7 turns), it will pin the adjustment, presumably at 21 degrees advance or so. However, at 4 turns out, it will move the adjustment bar about halfway. Here's an image showing this state, 4 turns out adjustment, with 20 in hg on my vacuum pump (the adjustment was all-in at about 8 in hg).



So the production unit matches the spec in the instructions. I love it when that happens. Here's a link to the full instructions: https://www.jegs.com/InstallationIns.../110-31035.pdf

I can't argue that!

Hamerdown

Thanks for your feedback, I can only post what my Timing Light & rpm are showing me. Again my Actron dialback timing light isn't anything fancy.
*for giggles I hooked-up my old analog Dwell/Tachometer meter to the engine (it only swings to 2000 rpm) and the meter was giving a lower reading vs the vehicles tach.
(possible timing light, tachometer or harmonic balancer being off?)
*Unless you're referring to as in 'testing'-(which I did not do) or locking-out the mechanical advance for a race engine; I have never heard of disabling the mechanical advance from advancing.