Vacuum advance adjustment

[texasmike]

I have a an '84 LE9 engine the original re-built Qjet carburetor and new CARDONE HEI distributor #84-1845 for a 1982 model (no electronic spark control like the 84 and 85 models). All of the emission systems have been removed and I've been trying to time it for maximum performance. I even broke down and bought a pricey dial-back timing light, which I really like. I've read all of the articles and decided that manifold vacuum is best for me. My problem is that my 19-20" hg produces 25* advance. With 12* initial, 18* mechanical and 25* vacuum, I'm at 55* max at highway cruising speeds. I understand that I should shoot for 34-36* for total time so that means I still need about another 5* of advance, making max advance at 60*. I've read to never exceed 54*.
I've seen a video where a guy sticks an 1/8" Allen wrench in the vac port of the can and adjusts the vacuum produced by turning the wrench one way or the other. I tried it and turned it four times clockwise and it stopped. I then turned it counter clockwise and after ten turns I stopped for fear of backing the screw completely out and losing it in the can. I decided to put it back in the position I found it and seen if anyone has any experience adjusting this particular distributor.

 

[Camar068]

I'll let others chime in on your issue, but for the degree symbol on a windows pc you can hold Alt and type 0176 and release alt.

nothing meant, just thought I'd share some trivial info.

 

[Ricko1966]

There are 2 types of adjustable vacuum advances,1 limits total advance by twisting the allen,the one that crane sells adjusts at what vacuum readings the diaphragm moves and uses a limiter plate to limit total vacuum advance. The le9 has higher compression than earlier 305s which is why they went to knock sensor ignition,egr also allows you to run more timing without ping,anyway did you get your timing light out and see if turning the screw limited how much advance you got by removing and replacing the vacuum line. If it's not limiting it making a stop plate is simple. I',ll try to find you a pic rather than trying to explain it. My gut says you're probably going to end up fixing your egr and your knock senor ignition, but we'll see. Here you go read this,look at the pics.Also look at the pic I put at the bottom it will show you where the tubing go's or a bushing to limit advance.

Vacuum Advance Stand Alone Degrees Stop Plate information - DAVE's small-body HEI's

www.davessmallbodyheis.com

 

[75gmck25]

As mentioned already, there are different brands and types of adjustable vacuum advance. Many products that use the Allen wrench adjustment will adjust the vacuum point where it comes in, but don’t adjust the total advance.

If you look at how the advance works, it easy to see how total advance can be limited with a stop in the distributor. There is a slider that moves the advance, and you just limit how far the slider can move by using a stop plate. Most stop plates have an eccentric shape, or a stair-stepped area on one side, so that installing them in a different orientation will change the point where the slider stops.

 

[fast 99]

Back when they started to take octane out of fuel, we changed out the vacuum advances to stop pinging complaints. If you look at the area where it attaches to the advance plate, there is a slot. Longer the slot the more advance it provides. Does the same thing that is explained in the article Ricco1966 provided.

 

[Matt69olds]

Read this: https://www.460ford.com/threads/timing-vacuum-advance-101-good-article-by-gm-engineer.117504/

The 34-36* rule of thumb is good for max power at WOT, you can get away with a lot more timing at cruise. Your vacuum advance canister might be fine, or you might need to tweak it a little to limit and delay advance. The article describes how to modify it.

Part throttle timing was explained to me like this: At part throttle, the cylinder isn’t completely filled with air/fuel charge. To get complete combustion, you need to start the ignition sooner. It takes more time for the flame front to ignite everything when it’s spread out instead of packed in like it would be at WOT.

I spent alot of time tweaking the Q-Jet and HEI on my olds. In my case, the engine liked 34 total timing, with about 20 additional vacuum advance. I spent most of the time playing with the Allen wrench adjustment and advance stop plate. When it’s right, you will know. Cruise on the highway, start to crowd the throttle, you might hear a tiny amount of spark knock, then the vacuum advance will start to fall off and the pinging will go away.

 

[Ricko1966]

You still need to determine what your allen screw does as I mentioned before.
Okay,home so not as busy now what I was trying to tell you but not wording it well was get out your timing light at idle with vacuum advance connected to manifold zero your marks with your dial back. Now adjust your screw in 2 turns did it change your timing reading? Back out 4 turns check again. That way you can see if the allen screw is a stop. If it is a stop now you know how much 2 turns gives you and how much 4 turns gives you. Keep posting and checking in,we'll help you get it where it needs to be.

 

[fast 99]

Cruise on the highway, start to crowd the throttle, you might hear a tiny amount of spark knock, then the vacuum advance will start to fall off and the pinging will go away.

That's when an EGR cools the mixture. Need to understand the ramifications when removing emissions equipment and how it effects drivability.

 

[texasmike]

Thanks, that’s all good information. How many counter clockwise turns will it go and will it just stop? I had mine ten turns out and it would have gone further but I stopped for fear of it dropping out. I checked the timing with it all the way in, four and six turns out and saw no change to the 25* it was adding to my base. Should I keep turning it out until I begin to see it decrease?

 

[Ricko1966]

Thanks, that’s all good information. How many counter clockwise turns will it go and will it just stop? I had mine ten turns out and it would have gone further but I stopped for fear of it dropping out. I checked the timing with it all the way in, four and six turns out and saw no change to the 25* it was adding to my base. Should I keep turning it out until I begin to see it decrease?

No that's a can like I mentioned earlier,the screw alters the vacuum required to operate it,put it back where it was
Make a stop plate,read the articles and look at the pictures I posted for you

 

[fast 99]

RIcko1966 is saying it changes the spring tension applied to the diaphram not limiting travel.

If you don't want to use/make a stop plate, get a stock replacement with a smaller cut out area.

 

[Ricko1966]

@fast 99 and I have told you that deleting egr is not a good idea. It does nothing at wot but allows more timing and less detonation. Here's a good read.

Tech Overview of the 2013 Domestics | MOTOR

Has the exhaust gas recirculation (EGR) system been replaced by variable valve timing (VVT)? Are EGR valves gone for good? Not really. The Generation 3 Prius has an EGR valve, as do other late-model vehicles. With EGR still around, it seems worthwhile to revisit why and how EGR affects...

www.motor.com

 

[Ricko1966]

With functiong egr you can add 4 more degrees of vacuum advance.

 

[texasmike]

I decided to turn the Allen screw and find out how far it would go and then check to see if it would make a difference to the advance. It went 11-1/2 turns before it stopped and changed the vacuum from 25º (4 turns out) to 16º. My mechanical advance looks like this:
1,000 rpm 2º begin
1,500 9º
2,000 16º
2,500 18º all in
3,000 18º
Here are my current settings: Base time 16º @ 590 rpm
Vac adv 16º
Idle 32º @ 680 rpm
Mech adv 18º @ 2,500 rpm
Total time 34º
Max time 50º
Manifold pressure 17" hg at idle and 22" @ 1,200+ rpm
The truck starts quick, mostly smooth idle, accelerates well for the LE9 (9.2:1 compression ratio) and has no ping or rattle at WOT. My only concern is that cruising at 65 mph in 4th (OD) with stock tires and 2.73 : 1 axle, I'm only at 1,475 rpm with about 9º mech advance. Am I at max performance or is there anything else I need to do? Thanks.

 

[Ricko1966]

Back your initial off 4 degrees,find 2 land marks that you can pass the second land mark about 3/4 through second gear. Check your speed at the 2nd landmark. Bump the timing 2 degrees try again,keep bumping 2 degrees until trap speed drops or you get detonation, then bring it back 2 degrees. Let us know where initial is then.