Vacuum Advance Issue

[69GMCLonghorn]

Hey guys, haven't been on here in ages. Now working on a 70 Custom/10 longbed. It has a stock 2 bbl on a 307. When I bought it, the vacuum advance diaphragm was blown out, so I just advanced the distributor a bit and capped it off. Worked fine. Well, through the course of several carb rebuilds and now finally upgrading to HEI, I reconnected the vacuum advance. It is connected to the port on the carb that is midway down and pointing at a 45° angle toward the front. It is basically getting full constant vacuum instead of ported vacuum, resulting in an instant 25° of advance as soon as I hook it up at idle!! No wonder the old diaphragm blew out. I've tried the other 2 vacuum ports on the carb. One is the exact same and the other one doesn't have much vacuum at all (I think it went to the flapper valve on the original air cleaner.

I'm not at a point where I want to do a manifold/carb swap to a 4 bbl. I just want this to work for the moment. Any ideas on what's going on with this thing? And more importantly, can it be fixed??

[geezer#99]

Nothing wrong with that.
Two of your ports are manifold vacuum (full time) and the one with little vacuum might be ported vacuum.
Full manifold vacuum won’t blow out the vac pot. Old age does that.
The one that shows little vacuum should show more vacuum when you open the throttle.
You might need to reduce your initial now that you have the vac pot working.

If you would like less vac advance you can buy an adjustable limiter.
What’s your initial timing at?

[Myoldtruck]

I have my hei vacume advance connected to manifold vacume. i have about 27inch at idle. no problems.

[HO455]

There are plenty of folks who advocate for constant vacuum over ported vacuum to the vacuum canister. Both sides make valid points, so it is mostly up to your personal preference.
A possible reason you may not be able to find the ported source is your butterflies maybe open to far at idle and the ported source is "open" to the venturi.

And to Myoldtruck. Please don't take this wrong but I do question having 27 inches of vacuum. That is an awful lot. Have you checked your guage against a known source?

[geezer#99]

I think he meant degrees.
As in an added 27 degrees from vac advance.

And there’s a possibility the op’s carb doesn’t have a ported source.
If he posted a pic of where he thinks it is, we could tell him exactly what type of port it is.

[69GMCLonghorn]

A little more info: I set the base timing (sans advance) at 8° "by the book". I was always taught to do it this way. When I bought the truck, it was at 8° WITH vacuum advance (until it blew out). It had zero power.

So do I just dial the base timing WAY the heck back until it's back to normal? Something just doesn't seem right.

Also, I'll try to post a couple of carb pics either tonight or tomorrow to show the ports.

[geezer#99]

Vacuum advance is only a bonus type of thing. It adds more timing during slow lazy burn times in the cylinder. Like when at a steady cruise or when idling.
While 8 btdc might be factory, with today’s low octane swill you gotta start the burn earlier. You should have no problem running 12 initial.
This initial numbers are only theoretical. They’ll only be accurate if you’ve proven tdc #1 is dead accurate on your timing tab. You said with 8 degrees you had no power. That might suggest your timing tab is incorrect.
You can run 8 with your vac advance giving 24 more but you loose a good bunch of your vac timing as soon as you hit the pedal to move.
Or you can run 12 and your 24 vac advance or limit your vac advance if you want to.

Is the port you think is a ported source on the back top of the carb?

[68 P.O.S.]

Geezer is correct. The purpose of vac advance is to add some extra advance when its needed at lean conditions; idle and cruise. Lean mixtures burn slowly, so the fire needs to be lit earlier in the compression cycle to ensure a complete burn. Rich mixtures burn quickly, so the added vac advance isn't needed during acceleration and wot. Seeing your timing at idle in the upper 20 degrees is just where it should be. Way back in the day GM engineers figured out most SBCs run best at 36 degrees total timing with vac advance connected to manifold vac. Ported vac is for running emissions equipment such as egr, air, etc. Vac advance should provide 10-15 degrees. No more or you will get detonation. With vac advance connected to manifold vac, you will see improved throttle repsonse, better idle cooling, increased mileage, and better overall driveability. Which HEI do you have, GM or aftermarket? What is the amount of mechanical advance and vac advance it has? This also determines how you set your timing. As Geezer said, make sure your timing marks line up with TDC on the balancer, or everything will be off.

[LH Lead-Foot]

I agree with HO455, Geezer & 86 P.O.S.
When doing a tune up, the under hood decal was the bible. Engine warm, at idle, A/C off, then remove vacuum hose to advance & plug. Adjust timing to 8 degrees or what ever it said.
When you stomp on it, intake is flooded with atmospheric air pressure. With a accelerator pump shot, it goes rich. The vacuum advance hooked to intake vacuum goes back to base timing to prevent detonation of ping. As engine vacuum catches up, the advance adds timing to the total ignition advance.

So set base timing with advance plugged, plug in advance and all should be well if the weights are not stuck or have worn pivots. We had to pull distributor and place it on the SUN distributor machine to inspection, clean, lube, install points, test for continuity then adjust dwell. The machine would allow you watch proper timing advance wit ha strobe light. If needed, we would drill out and install new bushings, cleaned points contacts if resistance was high, as many had like a coating on them. (Damm I am Old / Fill her up with Eythl sir?)

I miss the smell of tetraethyl lead!!

With HEI, points go away of course, but the weights still have to work correctly.
So, Base timing first, then advance kicks in and weights swing out (Depending on springs) add the rest to get your 36 degrees going down the road.
Just make sure at idle you adjust A/F mixture to get the highest idle speed, then back off 1/2 turn or less. Set idle screw at carb, set base timing with known good harmonic balancer, the test drive.

Old School...
One way with the unknown under the hood with camshaft from where? Install a good vacuum gauge to intake vacuum, advance distributor to highest vacuum, the back off 1 inch of Hg. Then re-test drive while listen to the engine for ping.

[Greasey Harley]

The following is just my opinion, and is worth exactly what you are paying me for it

First it is important to note that, on older engines, it is VERY common for the outer ring of the harmonic balancer to slip, causing inaccurate timing readings. I recommend doing a TDC test to verify the balancer is still accurate.

Secondly, if you set your base timing, without taking the mechanical advance into consideration, you will have awesome performance
...at idle. I don't drive around at idle that much.
Total mechanical advance is (in my opinion) way more important than initial timing.

Most V8's perform best when the mechanical timing (without vacuum advance) set to about 36 degrees
Sometimes a little less for hard working engines.
Sometimes a little more for all out performance.
I usually set boat motors to around 34 degrees, as they are under constant load.
You might set a 383 in a Vega to 38 degrees, due to the excellent power to weight ratio.
Fact is, all engines are different, and vary slightly, but 36 degrees mechanical timing is a solid starting point. This reading needs to be taken with the VACUUM ADVANCE DISCONNECTED.

To set mechanical advance for optimal performance:
Use a "dial back timing light" like this one:
https://www.summitracing.com/parts/ino-3555/overview/
or this one:
https://www.tooltopia.com/actron-cp7...oaAkLWEALw_wcB

If you don't have a dial back timing light, you can make a new "36 degree" mark:
Using a sewing tape (or simalar flexible tape measure), measure the circumference of the balancer.
Do this as accurately as possible.
Divide that number by 10 and this is the distance to 36 degrees.
Measure this distance clockwise from the "zero" mark on your balancer and scribe a new mark. This will be your "36 degree" mark.

Disconnect the vacuum advance , start it up and slowly rev the engine while observing the timing marks with the light.
You will notice that the new "36 degree" mark will move up towards the timing plate as rpm increases. Continue to increase rpm until the line does not move any further (mechanical advance is “all in”). Once the mechanical advance is “all in”, the new line on the balancer should line up with the “0” mark on the timing tab. Adjust the distributor as needed to achieve this.

Plug your vacuum advance back in, take it for a test drive.

Note:
You usually want too see your mechanical advance "all in" between 2500 and 3000 rpm. (Different engines respond better to different timing curves.)

On stock to mild engines, vacuum advance usually responds best to ported vacuum.
Some engines, with more radical cams, work better with vacuum advance plugged in to direct manifold vacuum.
Again, all engine/vehicle combos are different, try both ways.
Vacuum advance is only utilized under low load situations, when there is a high vacuum (cruising speed). It adds additional advance, on top of the initial and mechanical advance. making your "Total timing" somewhere around 40-50 degrees while you are cruising down the road.

Advance at Wide Open throttle = Initial Advance setting + Advance from mechanical advance only
Advance at Close or Part-Throttle = Initial Advance setting + Mechanical advance + Vacuum advance

That's what I think
-Russ

***Credit to Lars Grimsrud, for a whole bunch of this information***

[HO455]

Originally Posted by geezer#99

I think he meant degrees.
As in an added 27 degrees from vac advance

Ahh... That makes more sense.

[68 P.O.S.]

You ever get this figured out?

[68 P.O.S.]

Originally Posted by HO455

Ahh... That makes more sense.

I read that as he has 27 degrees of advance at idle