I'm still on my non-stop quest to tune this carb correctly. Been a year and a half now, and still not quite right. lol

Here's how I kinda have it adjusted now:

I set my timing until I got the highest vacuum reading with the throttle blades closed and adjusted to Demon specs at .020 transfer slot clearance. I end up with 21 degrees BTDC. The engine also seems to rev quicker at this setting and run smoother.

Then I adjust each A/F mixture screw in until I get the highest vacuum reading from each one. I end up with a max manifold vacuum of around 14". It runs smoothly, but the idle is way too high. Around 1100 RPM's.

I have the "Idle Ease" feature on the carb, so I try to adjust that in order to decrease the RPM's down to where I'd like to see it idle (around 850), but I can't seem to get the RPM's down enough using idle ease alone. But I don't like idling at 1100 rpm's - even if it is smoother.

So what should I do next?

- Decrease initial timing (back it off) until I acheive 850 at idle?

- Close down on my throttle blades? Keep in mind I'll lose the supposedly required .020 transfer slot clearance...

The Demon manual says you want to end up with, and I quote:

"The goal for best quality idle and throttle response is to have the engine idle with the butterflies closed, at the correct RPM, with the highest manifold vacuum, and the mixture screws adjusted between 1 and 2 1/2 full turns out from fully closed."

Then it adds a disqualifier that says "your particular combination may function correctly ouside this range."

So I ask you, the carb gurus here - and I know you're here....what am I doing wrong??

 

What is your total timing at?

 

Straight Mechanical HEI. Total advance is in at around 2500 RPM's and I end up at 40-41 degrees.

 

Why so high?

Even when you set the trasition slot...I would still turn them down after the mixture screws are set to get the rpms down.Just make sure to go equal amounts.I have found no other way to get my Demon to idle down.

I used to run straight mechanical.Cured a ton of tuning issues/problems on my Demon 750dp by going back to vacuum advance as well.

 

I would have to agree. 40 degrees total advance is too much. The best way to get the total advance you should have along with the idle advance the engine/carb seems to want is to use manifold vacuum advance.

 

I know optimal, or so I've heard is 36-38 degrees total. And it is high. But it's just where it ends up after adjusting for best vacuum at somewhat of an idle.

I suppose there's some way to limit the total advance in the distributer- maybe an adjustment or a bump stop or something, but I have'nt fooled with that.

OK, but after I do that, should I readjust all the A/F screws again for best vacuum or not?

I guess that's two votes to get a vacuum advance distributer...but how will it help me? I guess what I'm asking is - how will that allow it to idle better, etc??

 

A.No..I don't and my mixture is fine and the car runs great.

B.I asked this same question when I ran mechanical.40-41 is too high.34-38 is optimal depending on the combo.Right now you are over that just to get the idle and part throttle timing you need to make the car run well down low.With manifold vaccum you can run an easy 14 initial and an additional 9-10 vacuum to make up 23-24 degrees at idle.Great for throttle response and part throttle.But when your on it at the top end..the vacuum goes away leaving you with the optimal 34-38 degrees.All depends on where you set your initial and the curve in the distributor.Vacuum advance has no ill effects on performance..but is an excellent aid in ANY street car.I used to disagree till I learned how to set it up and tune it right.Now I won't live without it..lol.Get yourself a vacuum advance distributor before anything else.You will understand why once it's tuned.

 

Thanks again onebad82z -

OK, I guess I'm gonna try a vacuum advance distibutor then. I've stubbornly held onto hopes this would work out with the straight mechanical advance, but I give up!

That brings just a couple more questions if ya don't mind...

1. Would I port it to full manifold vacuum or above the plates?

2. So if I get a vacuum advance, should I adjust the timing where it runs and idles best at now, leaving the vacuum hose hooked up, or adjust it to give me 34-36 degrees at around 2500 rpm's?
- How will I end up with 21 degrees at idle (where it does best)??

- Any recommendations on a decent vac advance distibutor? Will a stock one do?

OK - that was 3......

 

 

Read these:

My original problem when I didn't beleive in vacuum advance:
https://www.thirdgen.org/techbb2/sho...vacuum+advance

Assorted vacuum can specs:
https://www.thirdgen.org/techbb2/sho...vacuum+advance

And this article is what finally made me realize why I needed to go back to vacuum advance:
https://www.thirdgen.org/techbb2/sho...ghlight=ported

And this is a great tech article on how to modify a stock vacuum HEI to wrk great on a modified car:
https://www.thirdgen.org/newdesign/tech/hei.shtml

 

Yes, even the original HEI GM distributors are really good. Pick up the proper Crane vacuum can kit to go with it and set it according to instructions, usually something like 9 or 10 turns counterclockwise with the included allen wrench. Then setup the springs to work. I find that the stock weights and one light and one medium spring give the best advance curve for me. Your's may differ.

Get a good cap, coil and module, too. Proform stuff is good and not expensive. Good luck!

Even a junkyard HEI unit is good to start with. Only thing that goes wrong is usually the bearings.

PS Crane has a new distributor setup out that does everything with a computer chip; the "vacuum" advance and then the curve. Has a number of different settings built in for both ends so you can experiment. It looks really slick. Saw it on Horsepower TV this last weekend.

 

WOW!!!

THANKS ALOT! I can see that you really have "been there done that" onebad82z! It looks like a lot of good reading I need to do.

By just glancing over it all so far, I'm seeing some of the same issues I've been having since day 1 with my setup. Don't get me wrong though...except for being able to get enough advance at idle in order for it to run right, it's got gobs of power at WOT.

But - I can tell you it sucks down gas like no tomorrow on the street, and the engine is hard to turn over and start with my timing set at 21 BTDC at idle where the engine likes it. It's like the compression is working against the starter. I'm on my 2nd hi torque starter already.

But like i said, my total advance ends up at 41-42 degrees which I know isn't right.

I've even tried backing the timing down from 21 to 16 degrees initial BTDC at idle, which actually gives me 38 degrees total, and I adjusted the carb so itl would still idle OK(although rough), But it wants to run hot when I do that. Even with a Griffin aluminum radiator and 180 degree T-stat.

Again, I thank all you guys that responded, and I garauntee I'll have a few more Q's after I read through all this great info - I'm a little slow... - if you guys don't mind a painfully long post... lol

 

 

OK - I've read all the posts you kindly referred me to onebad82z, and it's alot of good reading!!

But the part that confuzes me (pun intended) lol, is you mention briefly above about running a ported vacuum to the canister, and it may work best for me.. Now, I think ported vacuum comes from above the throttle plates, so there would be no additional "advance" after the engine starts, correct?

So-

I should be able to run manifold vacuum to the canister (below the throttle plates), and I would then have additional advance after startup - right?

So -

If setup with full manifold vacuum, at idle my canister will be advancing the distibutor plate as far as it can correct? Then if I give it part throttle (like going up a slight hill) the canister would slightly retard the timing correct? And if I gave it WOT all at once, the canister will go to full retard until RPM's increased and I start regaining vacuum, at which point it would start advancing the timing again.

So-

If I set the timing up right, I could get away with say 14-15 degrees BTDC static (hose disconnected), and then set the canister to provide my additional 6-7 degrees of advance at idle (to make 21 BTDC), then i can limit my mechanical advance to 32-38 degrees at around 3200 RPM's - correct?

Just wanted to get your take on this, and to make sure I have the whole vacuum advance concept right in my head..

BTW..I think I need to get the MSD 8365 instead of the 8361, since I have an HEI distributor. Besides, I like the idea of the built-in rev limiter.

 

First off please let me correct some wrong info I stated here..

quote:
------------------------------------------------------------------------
I run ported.Seems to work best for me.Try both and see what works better..every car is different.Although I venture to say it will like ported better.

I run MANIFOLD.Anywhere is says ported replace it with MANIFOLD.Very sorry for that misight.Duh!

 

Yes, it has bushings just like the others to set mechanical advance according to thier website. I ordered it yesterday (the 8365), so I should get it in a couple of days.

Thanks for all the assist/advice onebad82z - and others!! I'll most likely post back on this thread when I go to tune the advance curve. Never done that before, but with the info you gave me, it SHOULDN'T be too big of a deal...I hope!

 

Not a problem!

Let us know when ya get it in and start tuning.

 

Allright - It's in. Nice looking distributor!

Too bad the one I'm taking out is also a nice looking Proform billet mechanical.

I set it up with the black bushing to limit mechanical to 18 degrees according to the instructions. I went with the two (light) blue springs (slightly aggressive) for the weights. The distributor cap is a pain though. I have to remove my wires to get the cap on/off.

I set the rev limit to 5800 for now. I started it up, and with the vacuum hose disconnected from the advance, I set max timing to 36 degrees. That left me with about 18-19 degrees at idle while disconnected. And I pull about 13 -14 inches of manifold vacuum at 750 rpm's. Idles smoothly but seems to surge just slightly.

Now get this:
When I plug in the vacuum advance, It's suddenly 32-34 degrees advanced, and idle jumps to 900-1000!!

So I slow the idle down to 800 rpm's, and I'm still at 32-34 degrees while idling. Does that seem excessive??

I may end up stepping down to one light blue spring and one heavy silver spring to lengthen the curve a little. I get 38 degrees at around 3000 Rpm's. But it starts up fine.

If I tried advancing my straight mechanical distibutor that much, it wouldn't even crank over.

Am I going in the right direction here so far as tuning?? I'm open to "constructive" critisizm.....

PS: I gotta feeling that what's happenin is the inital is 19, and the vacuum advance supposed to add up to 10 degrees advance at 15" vacuum. Since I have 13" at idle, I think the vacuum advance is adding about 8 degrees, but I'm just guessing.

 

Sounds right to me.

If you are getting some starter kick-back, then put more mechanical advance in and retard the intial advance so that you maintain the 36 total w/o vacuum.

 

Nope, no starter kickback.

I just drove it around the block, and it definatley didn't lose any power. Seems to run strong still, but I have idle issues...every once in awhile it dropped below setpoint and stalled when rolling up to a light. I may have to open up the transition slot a little on the primaries and get idle up to 800-850. It's around 750 now.

I don't hear any pinging, or I'm deaf. I almost expected it with the curve I have in now. But I do think I hear a lifter ticking. I gotta adjust them again I guess.

I do have a question though...when it was dynoed, the torque was at it's highest by 3400 rpm's, with 38 degrees total. Should I change springs around to try and get all the mechanical advance in by 3400?

 

Where's it coming in now?

Unless you have sky-high CR, earlier is typically better, as long as it isn't advancing at idle. Mine is all in by 3000.

 

 

Thanks for the assistance!

About 2800. I have 9.5:1 compression.
Now I'm confuzed...since my car seems to want to idle best at 21-22 degrees initial even with the straight mechanical dist. installed, if I used the blue bushing I'd end up with 45 degrees total if I didn't limit it right?

That's why I chose the black bushing which limits the mechanical to 18 degrees - and I want total advance at 36-38 degrees max correct? -or am I missing something? I can't even keep it running with only 15 degrees initial.
My vacuum advance isn't adjustable. I'd have to get the Crane unit for that. But it's something I may need to look into.
Yeah, I think I'll try that next, but the bushing thing still baffles me.
Yeah, makes sense. They get dynoed with open headers etc. -Just asking.

- But I'm getting closer to getting this right....I can feel it!!

 

 

OK - I do understand what the bushings are doing then. I guess the only problem is that it won't idle at 15 degrees initial - or doesn't that matter since I'm hooking up the vacuum advance anyway??

 

Vacuum advance will add the necessary additional timing needed to allow the car to idle.

 

Ok, That's basically the way I started out - I adjusted timing to 21 advanced with vacuum hooked up and adjusted idle.

But this time This time I revved to wherever the mechanical advance stopped and set it at 36 degrees. Then I let my initial fall where it may, which happened to be 18 before. Plugged in the vacuum advance and ended up idling at 30 plus degrees.

I can try the blue bushing and see what happens, but since I set timing at 21 initial advanced + 21 (blue bushing limit) - won't I end up with 42 degrees total advance? I thought I actually need a thicker bushing than the black one that could limit mechanical to 15 degrees, and that would make 21 initial + 15 mechanical= 36 right? Or am I missing something?

I'll try the light blue and heavy silver springs together too and see if it's better.

 

 

I think you need to get over the hurdle of the differences between timing adjustment and timing in operation.

You set/adjust the timing without the vacuum attached. You set/adjust the idle with the vacuum advance operating.

The initial timing and the timing needed to get a good idle and proper idle speed having nothing to do with each other. You may need to screw the idle speed up during the timing adjustment operation, and there's nothing wrong with that.

If using the 21 degree bushing gives you 42 degrees full mechanical advance, set the initial advance back 6 degrees so it's back to 36 degrees. There really isn't any reason to have 21 degrees initial advance (no mechanical coming in, no vacuum attached). 15 initial and 21 degrees mechanical makes a lot more sense to achieve 36 total mechanical than the other way around. If the engine wants more initial advance than that to idle properly, give it manifold vacuum for the vacuum advance after you've set the initial advance, and adjust the idle mixture and speed after that.

 

I could not have explained it any better than five7.That should clear it up a lil more for ya..lol.

I think I may have been confusing you as well.Not the best with words.

 

Thanks for the explanation 5-7!...it's a bit clearer now.

I didn't pick this screen name for nothin...

There are reasons I'm asking though. If you go by MSD's instructions for the distributor, it tells you to hook up vaccum advance to the ported (above throttle plates).

But I realize that won't work. Then it tells me the mechanical advance should not start to come in until after idle. Then this also kinda "confuzed me:

With a 21 degree bushing, that would have to be 15 degrees right? Well, I can't get the car to hold an idle there.

So - anyways, I guess I need to listen a little more and quit tryin to nuke this. I'm overthinking this it seems.

Anyways, I re-installed the blue bushing, and changed springs to 1 heavy silver and 1 light blue.

Gettin too late to run it today, but tomorrow, I'll try setting it up as you both have suggested and see what happens!

 

 

OK - did that, and seems to run great with the bushings/springs you guys recommended. I still have an issue with it stumbling below idle ocassionally and dying. I think I'll need to tweak the transfer slot adjustment a little and I might be OK.

Idling at 850 with my vacuum line hooked up, I still run 31 degrees advanced, but it seems happy there, and during the test run it didn't seem to ping under light load, and has tons of kahunas!

I'm gonna go ahead and put my new spark plugs in and check them after awhile. I'll be happy with any color other than black! lol

Thanks for the help!!

 

 

Hey i was wondering i also have a demon 650 with 4 corner air/fuel screws.

is there a certain sequence for adjusting the air fuel like fronts first or rear ones first or it does not matter

 

Set all four screws at 1.5 tuns out (CCW) from lightly bottomed.Then with the car at a hot (fully warmed up) idle adjust the screws in equal amounts to the highest vacuum.I usually start at driver side primary screw,move onto passenger side primary screw,then passenger side secondary screw and then driver side secondary last.Keep track of how many turns out each screw is.Once you get highest vacuum you can lean them out 1/8 to 1/4 turn.I can't seem to lean mine out any otherwsie I get a tip in stumble.Experimentation will tell you what works for you.I then shut the car off and turn all 4 screws back in and set them all to the leanest screw (least amount of turns on whichever screw) and set all 4 to that number of turns...more or less to equalize all 4.You may need to tweak them a lil after starting it bac up and doing some test driving.This should get you close to optimum.

 

Just to check in on this post one last time...

The 400 is running better than ever I believe!! It seems to have better part-throttle response. Again, thanks for the help everyone....and especially onebad82z.

 

sounds good!

 

Wait 'till you get your 400 project together .....You'll love the torque!

I think I'm ready to see how it'll run in 1/4, just to see what it'll do, just for fun!

 

I hear ya!

The big upgrade this winter is trying to hunt down an LS1 series T56 trans to swap in.I need a 6 speed.

Then hopefully next winter I can get the 400 in

Get here to the track...it'll suprise ya.