bluedawg

If I need a lot of initial advance say 24* and have 23* mechanical advance but only need 34* total then I either got to limit the mechanical by welding the slots up some or some distributors/curve kits have different colored bushings. Or i can use part of the curve as initial by using the lightest weight springs made in a country were the steel is low quality so that the springs start to come in early, which is what I do. I set the total were I want it then I set the initial/curve by playing with the springs and weights.

chevymans 77

mechanical advance at idle will almost always create an unpredictable and inconsistent idle.

Use what ever combination of springs it takes to make sure the initial advance is steady (advance should not start until above idle rpm) and the total is around 34* to 36* all in by around 2500 to 3000 rpm.

Neal

Neil B

Ah, my mistake. You have the 8428 kit for GM HEI. I was referring to the 8464 advance kit for MSD distributors. The tuning principles are still the same though. You need to set your 'all in' timing to 36 degrees either with a dial back light or timing tape. Then, you can play with the provided springs until you get the timing in as soon as possible without affecting idle stability. From my experience, if you're idling on the advance curve, the engine won't always return to full curb idle speed. It will tend to idle a few hundred rpm higher when fully warm. Good luck.

Priya

The corvette service manual says idle speed for an L82 manual trans should be 900 rpm, I referred to 1000 rpm as "pretty much my idle speed" to simplify what I was saying as the MSD instructions refer to the amount of advance at 1000 rpm rather than 900.

Priya

Thanks for confirming that. I suspected that was the reason why my idle was inconsistent.

There's the problem. If I use the light or medium springs I get centrifugal advance at idle. If I use the heavy springs (one of which I buggered up so can't use) I get no centifugal advance at idle but I don't get max centrifugal advance of 18 degrees until 5500 rpm so to get 36 total even at that rpm I'd need base timing to be 18 degrees which I suspect would be too much for the motor to crank easily.

Priya

As I said to chevymans77 both the medium and light springs give centrifugal advance at idle and with the heavy springs to get 36 degrees total advance at 5500 rpm I'd need 18 degrees base timing which might be excessive. I also appreciate your confirming that the engine sometimes hovering at 1300 rpm idle and then settling gradually to 900 rpm is due to idling on advance curve.

Priya

Maybe I need to buy a different brand of advance kit. When I removed the old centrifugal advance mechanism the bushings on the advance weights were mostly gone and the advance weights were somewhat floppy. The advance springs didn't seem particularly weak though so that makes me wonder a bit if the erratic idle and timing at idle was really due to the original centrifugal advance mechanism although I can't think of anything else that would cause that (I'm by no means an expert though).

When I was looking for a recurve kit I went to one place and they sold me a set of springs for $10 and when I got to the car I saw the package said "For Delco points distributor" so I took it back. It wasn't an MSD brand and the springs looked the same as the ones I got for the HEI distributor. Would those springs work on my HEI distributor? Should I go buy those springs and experiment with them to see if I can get no centrifugal advance at idle but 36 degrees total at 3000 rpm?

Neil B

Yes, if the engine is hovering at 1,300 even though you set curb idle at 900, you are definitely idling on the curve. Your engine will probably like 18 degrees initial. It will probably feel a bit snappier with the extra initial timing. If the idle hunting isn't something you can live with, you really don't have a choice but to use the heavier springs or lower the idle speed. 900 rpm is pretty high - my DZ302 with a 254@.050 camshaft will idle at 900.

DucatiDon

I ran my L82 at 18 initial.

MelWff

You seem to be stating you dont want to recurve your distributor but on the otherhand you purchased a kit designed to do that. So what are you really trying to accomplish? If you just trying to correct for a worn out stock spring buy the correct spring from a Corvette vendor otherwise read this link, use the Mr Gasket kit.

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

Priya

When the timing was originally set at 2 BTDC the idle was very erratic and quite rough. Now that I've bumped the timing up (with the original centrifugal advance mechanism) the idle is less eratic and significantly smoother. I could probably live with the idle hunting as it does now but it still idles a little rougher than I think it should and I'm a lot less keen on living with that. I think more base timing would smooth that out.

I'm just going by what the GM Corvette service manual says, idle for a '79 L82 4 speed should be set at 900 rpm. If I set it any lower the idle quality deteriorates too much.

Priya

Good to know, thanks for that. Knowing that I'm less concerned about setting it there.

chevymans 77

Priya, the instructions are saying if you use both the heavy springs your advance will be as advertised. You need to use a combination of the springs that works for your set up. Use one heavy and one medium and test to see what your curve looks like, then if it is not what you are looking for use one heavy and one light spring and do another test to see what the actual advance curve is.

Tune for your needs with a combination from the kit.

Neal

Priya

Yes, I bought it just to correct the worn out stock spring. Last time I had this happen I went to the GM dealer and they said if I wanted to get the new springs I had to buy a whole distributor. I was unaware that Corvette vendors sold just the springs, I thought my only choice was to buy a recurve kit so thanks for that and thanks for the link to Lars paper - I looked for it but couldn't find it.

While initially I just wanted to replace the stock spring once I bought the recurve kit I thought maybe it wouldn't hurt to modify the factory advance curve while I'm at it. Really either way is good for me.

Priya

As I buggered up one of the heavy springs I currently have one heavy and one medium spring in there so I'm going to check it today and see how that looks. I spoke to hubby about it last night and he didn't think it was a good idea to used mixed springs as this would cause uneven pull on the advance weights and uneven wear. I know most, perhaps all, of you disagree with this, but he's the boss and I'm probably going to go with what he says.

chevymans 77

I understand about the boss, keep at it and with trial and persistence you'll get it.

Neal

Priya

Thanks Neal.

Neil B

I have never had an issue using mixed springs. MSD even gives graphs of the different advance curves associated with mixed spring combos in the 8464 curve kit.

DucatiDon

Factory idle specs 900 because of the low 12* initial advance. Advance it to 18* and you can reduce the idle to 750 and still be smooth

Priya

Very interesting.