Droshki

OK here we go.

This started as a "glowing red header pipe" issue and as such I have read the timing sticky, Lar's paper, and also many posts on glowing headers. Oh, and for anyone not following my saga to date, this is 383, with a B&M 144 blower sitting on it. New, purpose built blower motor, forged everything yada, yada. About 8.3 static CR.

I think the glowing thing is under control for the moment, but it sure doesn't like being run with the vac can disconnected. Obviously it likes a lot more timing then the 12 initial it is set at. Headers still overheat with vac can disconnected at idle +revving. Now here is what I have:

Initial:12*

Initial and vac can at idle: 28* (wtf?, isnt this extremely high?)

Initial and Mech adv only 42 (12 Initial 30 mech)

Initial, vac can and mech adv (at 3000 RPM): 58 (12+16+30) (wow, not good!!)

The mech advance comes in late, starts at about 2000, all in at 3200 or so. I have a pile of springs to address that, but it seem I have bigger issues here.

The mech and vac advance both seem way high.

I dont know why I am getting 30 mech, I thought stock HEI was 20. Is there any way to limit this?

Also the vac seems high. It adds 16* at idle? Is this normal? The advance arm is marked with some letters and the number 10, so I am assuming its supposed to be 10*

Now the big number, 48. Reading the tech papers, they say to set the total with the vac disconnected and just ignore it, and never really discuss how this is going to impact yuor total number after its reconnected. My assumption was that there was no vac at WOT, so it wasnt a player, but it sure the heck is player at 3000 RPM.

I'd obviously like to run more initial and less total...

Help...

ps carb is Holley 750 with idle screws turned out one turn

63mako

Check Barry's timing post again. The paper you really need to refer to is Lars paper. I posted the direct link. Your Vac can is perfect. Total with Mech, vac and initial should be at 52 or under. Your running retarted @ 28 with initial and Mech. Should be 36 degees all in. This will make your heaers glow! Your right, your getting too much Mechanical advance @ 30 degrees. You have a Mech advance problem. Should be 18 degrees max mech advance all in @ 2500 to 2800. Sounds like a limit bushing in the mech advance is needed and a weaker spring set. Mech advance should be kicking in way before 2000 RPM all in @ 2500 to 2800. This will allow you to dial your initial up to 18 degrees. Follow Lars instructions below and all will be good.

Posted the specs below from Lars. Good luck.

http://www.lbfun.com/warehouse/tech_...t%20Timing.pdf

By Lars Grimsrud:

Lars’ Suggested Timing Specs for GM V8 Performance Applications:
• 36 degrees total timing (vacuum advance hose disconnected), all “in” by 2500 rpm
• 18 degrees initial timing at idle (vacuum advance hose disconnected). Note that it may not be possible to achieve the
18-degree initial spec with the 36-degree total without modifying the distributor advance stop system. It is more
important to achieve the 36 total than to hit an exact 18 initial. However, if your initial timing is very low (below 12
degrees) with the 36 total, it is important that you repair or modify your distributor in order to achieve correct engine
performance
• 16 degree vacuum advance control unit with a pull-in spec that allows the full range of vacuum advance to be pulled
in at the engine’s idle manifold vacuum level. Connect to manifold vacuum for most applications (this will allow
the engine to idle with actual timing at idle of 34 degrees).

Droshki

Yes, limiting the mechanical advance so that I can bring up the base timing is what I want to do. I have looked around and cant figure out how to do this. I see Lars talk of "modifying the distributor advance stop system", I see talk of limiting bushings, but I can find no instructions or kits for sale. I've also read an article that says it is the shape of the weights in an HEI system that limits the advance, (and another post by Lars elsewhere where at one time he actually manufacturing such kits), but again where/how do I do this?

On another note, the springs that came with my kit are in order of heaviest, gold, silver and black. Yes this chart from summit :


indicates that gold, black and silver will bring the timing in faster, in that order. Seems a mess to me. Am I correct in my intuitive interpretation that the lighter the spring, the faster the timing will come in?

Ironcross

Whats the plugs look like?

Droshki

Haven't looked yet. Are you kidding? It took me 2 hours to get them in there

I do agree that that would be useful information....

63mako

Lighter spring brings the advance in faster. Might need to have all in advance a little higher with boost, maybe around 3000. Might want to E-mail Lars at the address at the bottom of his link. Can solder the slot closed on the plate to limit travel on advance. Old school way to do it. Lars probably has a better method, limit bushings.

Droshki

I've emailed him but no response yet, unfortunately.

This slot in question, is it accessible without disassembling the dist?

yellow 72

There's a ton of HEI advance info out there...(Google is your friend ) Here's a quick example...

http://www.chevelles.com/forums/show...04&postcount=2

waterplay

Keep in mind that you have a blower motor! Most of that info is for naturally aspirated motors. You need to be very mindful of not getting to much timing. 52 total is a little high for your application. I would recommend no more than 50 depending on boost the 48 you have could be ok. Have it come in just a little quicker not much! Have more initial timing w/o vac can and limit the mechanical to give you the 48-50. BTW I am “old School” I welded the advance slot some to what I needed.

63mako

Good possibility you need a little less total with your blower motor. You need more initial, less mechanical, limit the mechanical to get where you need to be.

noonie

Here is a link to Norval's post. He ran a 540 charged and has many posts on timing and carb settings for it, obtaining some crazy mpg's as a result.

http://forums.corvetteforum.com/c3-t...he-lm-1-a.html

Do a search for his posts, or even pm or email him.

Droshki

Well I am pretty much at a loss on how to limit the mechanical advance. Many articles speak of a post in a slot that can be bushed, but this doesn't seem applicable to HEI dizzys. The articles on HEI seem to indicate that the shape of the center plate plate is what limits the weight movement, but give no indication on where aftermarket, differently shaped plate can be obtained. The photo in the link that yellow 72 put up seems like it would work, but how in the world would one know where to put the screw? Then I found an article that if you change the control module to Borg/Warner CBE22 or equivalent, there is an extra terminal that can be grounded to introduce 10* retard. Really? That would be a simple, if not cheap, fix. Anyone heard of this one?

I dont really understand even why my dizzy is putting out 30* advance if the factory setting was 20. I considered that perhaps the center plate had been modified previously, but I just replaced that along with new weights and springs from Mr Gasket kit 929.

While I appreciate the link to Norvals post, somehow, instaling a wideband O2 sensor doesn't seem to be the answer to my troubles. Perhaps I didnt read the thread far enough.

noonie

Gm had hundreds of different combos for the plate/weights.
There should be numbers stamped, but they are not available new from GM. If you want to do it that way then it's trial and error. Aftermarket don't have a variety.

You can get a stop limit kit from Crane that limits the total advance.


You are looking for a "5 pin module" stock GM hei from certain models in the early 80's.

Norval's many posts probably have some of the best real world info on blower setup and tuning on a C3, all tried and true, not just erroneous opinions. Yes, you will have to read many posts of his. Several other posters on here as well with good blowers.

yellow 72

Your dist may be giving too much advance because of worn parts. As far as where to put the limiting screw..I'd start with limiting the advance weight movement by half and go from there....trial and error.

yellow 72

[QUOTE
Norval's many posts probably have some of the best real world info on blower setup and tuning on a C3, all tried and true, not just erroneous opinions. Yes, you will have to read many posts of his. Several other posters on here as well with good blowers.[/QUOTE]

Helped me big-time when I started goofin around with mine

63mako

Norval would be a good choice for advice on blower tuning. Lars would also be a good choice on getting suggestions on how best to limit your mech advance and your options to do so, which I think is your whole problem. I try not to give erroneous opinions having 35 years of doing this under my hat, most of it on pre computer cars. Just trying to help out a fellow forum member.

noonie

That wasn't intended for you at all. I know you are well versed.
Just trying to make a strong point on going to a direct source. Sort of like going to a steak house for steak and not a seafood place although both serve good food.
Seldom will you find a 100% diagnosis in a long distance internet post on a complex install.
I've got as much time in this stuff or more as you and still am open to new info.

As I understand the posts, Lars' papers were consulted and not helpful in this case, to the op, so......

Droshki

Well, I admit that I didn't search on Norvel's posts. I am getting a little frustrated here, I would think this should be a fairly simple fix, and had to take a break from this to do my school homework.

I find it a bit of a reach that wear would cause a 50% increase in mechanical limit travel. 20-30% maybe, but 50%? That would sound more like a deliberate modification. In addition, if the limit is set by the shape of the center plate, this would be ruled out I would think, since it was replaced. Im surprised that no one knows if this is the case, that the center plate is what determines the limit, or there is actually a pin/slot arrangement in an HEI.

I've searched the crane site for a stop limit kit, and also Jegs and Summit, and found none, except for vacuum advance . Jegs/Summit have some for MSD dizzys, which do seem to use the slot/pin thing, which again, doesn't seem to be my case (but I still dont know, I would think not since they specify MSD only).

Again, reading Lars` paper, he mentions "modifying the distributor advance stop system", but gives no indication how this is to be done. I've emailed him twice in the last 2 days without an answer. He must be busy or out of town, I dont know.

noonie- Do you think the 5 pin module could be the ticket?

If not, I guess I am left with drilling holes and inserting screws into the thing, but that seems a very inelegant solution and would think that something better had been devised.

I suppose a solution would be to go out and buy a $600 ignition system, but.....really I guess I would rather understand why I have 30* instead of 20* before I just threw that kind of money at it.

I guess searching Norvel and emailing him is next, because again, I'd kind of like to understand the reason for the 50% variance from spec.

63mako

Good luck and many thanks! I went out into the garage to get a distributor to check it out and try to diagnose how you could be getting that much mechanical advance. The one I picked up was a points distributor. Been on the shelf for 30 plus years. Checked the number because I didn't remember where it came from. (this seems to be happening more since I turned 50!) The number is 1112019. Turns out it is out of my old 1970 Z28 Camaro. Replaced it with an electronic dist in the mid 70's. Did a little research and these are really, really rare. Used on 1970 Z28 LT1 Camaro and Yenko Duece coupe only. Turns out they are going for $750 to $1500. Got my E-bay listing in. Again, Many thanks! TTT for you.

Ironcross

What is the engine idle speed?

Its not necessary to be over 1K to run correctly

Also, by looking at the plugs will help identify lean, fat, or whatever conditions.....

Most distributors have 24 total built in and with 12-14 initial it should run. Plug off the VC can, you dont need it.