MelWff

The man who wrote the paper on the link, Lars, is an expert in his field. Follow the instructions in the paper and forget about trying to set initial timing.

Priya

Well, I put both medium springs in and wrapped a rubber band around the centrifugal weights so no advance could take place and then set the timing to 12 BTDC. I removed the rubber band and checked the timing again and it was still at 12 BTDC. The timing was steady in both cases, not jumping around like when I first tried to set it.

I made marks on the harmonic balancer for 24 and 36 degrees BTDC so I could get a rough idea of the total advance. I was by myself so I couldn't tell how fast I was revving the engine but when the timing maxed out I guess I was turning 4000 rpm. The total timing at that time was roughly 28 BTDC so the instruction sheet MSD gave me was off significantly. They said 8 degrees of centrifugal advance at 1000 rpm and max centrifugal advance of 22 degrees at 4500 rpm. So instead of the roughly 28 total advance I got they said I should have gotten 34 degrees total advance.

I'm going to try driving it for a while and see how it goes and if there's no "too much advance" problems I'm going to try the lightest springs or perhaps adding more base timing.

Thanks everyone for your help, you've all been a big help. I feel confident I know how to handle this now.

scottyp99

Those figures that came with the re-curve kit are a bunch of non-sense. Take the weights that came with the kit, and make a pair of earrings out of them, and put the stock ones back in. Then install the medium springs and tell us what you get for timing, we'll go from there.

Scott

Priya

I'd be open to that but the bushings for the factory weights were mostly gone and the weights were flopping around. I'm not sure the bushings for the MSD weights would work with the factory weights and it took a lot of pressure to get them in place so I'm not sure I can get them out again without destroying them.

When we first replaced the cap and rotor hubby showed me how the centrifugal advance mechanism had gotten stiff with time and wouldn't return on its own to the starting position. We put some oil on it and he worked it back and forth several times and it seemed to respond okay then but I wonder if maybe its still a bit stiff and that has something to do with it.

I think I'll try the light springs with the MSD weights next, see what happens with that and decide where to go from there if need be.

It came as a surprise to me that the centrifugal advance I got with the medium springs was so far off what MSD said.

scottyp99

You could put the factory weights in and see if they work. I don't see why they wouldn't.

Scott

Priya

I assume the timing was eratic because the bushings were gone on the factory weights and they were flopping around. When I removed the factory springs they didn't seem particularly weak. I think I'll try the lightest springs first and see what happens with that and if that doesn't work out then maybe I can try putting the factory weights back in.

DucatiDon

And I would add in another 4* of advance as well.

scottyp99

Yeah, that sounds like a good way to go, always try the easiest/cheapest thing first, right?

Scott

Priya

Yes, I was thinking along those lines myself. I see the factory service manual says there is 16 degrees of mechanical advance at only 2000 rpm. So with the medium springs in I have the same amount of total advance as the factory put in but it comes in at perhaps 4000 rpm instead of 2000 rpm - so this "recurve" kit so far isn't as good(yet) as what the factory had.

Priya

That's what I was thinking

scottyp99

OK, according to this document (page 91),

http://www.gmheritagecenter.com/docs...t-Corvette.pdf

you are correct, your stock distributor should have 16 degrees of advance available from the mechanical advance apparatus. Set it at 20 degrees at idle, there's your 36 degrees all in. All the springs do is govern the rate at which the mechanical advance happens, so yeah, try the lighter springs. I would set the timing at idle, then change to the lighter springs, and see if the timing at idle changes due to the lighter springs. If it stays the same at idle, you should be good to go. Let us know how it all works out for you, ok? BTW, your vacuum advance should provide an additional 10 degrees of advance, so you should see about 30 degrees of advance at idle with the vacuum advance hooked up to a manifold vacuum source, don't be alarmed, this is a good thing. One more thing: don't get all wrapped around the axle about factory settings. Set your idle as low as you can get it, while still having good idle quality.

Scott

Scott

Priya

Just took it for a quick drive and its running fairly nicely. Prior to changing the advance mechanism it would miss between 5000 and 6000 rpm. Traffic was heavy so I was only able to get it to 5500 in first gear but there was no missing and it pulled nice and evenly with about the same power it had before.

I had set the idle to 900 yesterday but when driving today it settled at about 1100. When I was adjusting the idle yesterday I thought its still a little rough for what I think an L82 should be. I know it was the "high" performance motor of the day but I think it should idle almost completely smoothly. The exhaust fumes while it was idling were burning my eyes so I think the idle mixture is rich and I'm going to have a go at adjusting that later today.

Priya

I put in the lightest centrifugal advance springs today and they were pretty flimsy compared to the medium springs so I was expecting too much advance. Once again I was surprised there was no centrifugal advance at idle. The total advance was aproximately 26 @ 2000 rpm, 30 @ 2500 rpm, 32 @ 3000 rpm, and 34 @ 4000 rpm. I'm pretty happy with that but I'll still have to see how it drives. We're broke until a week from Monday and the car is on "E" so it'll be a bit till I drive it.

Prior to this I got the idle mixture sorted out and its now idling nice and smoothly like I thought it should. It took a full 8 turns out on the idle mixture screws to get it right.

scottyp99

What's the timing at idle? What rpm does it begin to advance at? Are you using vacuum advance hooked up to a manifold vacuum source? The stock weights may be a little heavier and allow your advance curve to come in faster, give them a try and see what happens.

Scott

Priya

The timing at idle is 12 BTDC. To be honest I didn't watch the timing light as the rpm climbed so I didn't notice when it began to advance but it was at 26 by 2000 rpm. I don't know what the vacuum advance is hooked up to but I believe it is not a ported source as unhooking it causes a drop in idle speed. I think I'm comfortable with this advance curve, at least I'd like to give it a try before I look at adding any more advance. The stock weights look smaller to me than the ones I put in.

scottyp99

Well, your ignition curve could be better, but if your original problem has been solved to your satisfaction, then I guess that's something to be happy about. The one thing that I find a little weird is that before, you only had 16 degrees of mechanical advance, which made sense according to the specs for your car, but now you seem to have 22 degrees of mechanical advance. Strange.

Scott

Priya

Yes, I've resolved my original problem to my satisfaction and the timing curve is pretty good if not perfect so I'm pretty happy. My original goal was simply to improve the idle quality so the better timing curve is just a bonus. According to the MSD sheet (which obviously wasn't accurate) the max advance for the heavy spring was 18, the max for the medium spring 22, and the max for the light spring 24 (all at 7000 rpm) so maybe it makes sense that I've got more max timing now with the light spring than I had with the medium springs. Also, I only took the medium spring to what I guess is 4000 rpm, if I had taken it to 6000 rpm I suspect it would have added even more mechanical advance.

The other thing is that any timing above 14 degrees is an estimate. I put a string around the harmonic balancer to calculate where to put marks for 24 and 36 degees. The string almost certainly wasn't perfectly centered and so my measurement was longer than it should be and my timing marks for 24 and 36 degrees were farther away from 0 than they should be and my timing estimates above 14 degrees were much more likely to be underestimated than overestimated. So, better to be on the safe side and have my estimated timing curve a little lower than would be ideal.

Thanks for your help Scott.

Priya

Well, I finally got the car out on the highway today. Its idling real smooth, there's no sign of too much spark advance, the car pulls smooth and strong to 6000 rpm, and it feels noticeably more powerful than it did. So, I'm thrilled!

scottyp99

A dial-back timing light would let you adjust timing accurately, using just the factory timing mark. They're a little on the pricy side, though. I put a timing tape on my balancer, so I could keep using my trusty old timing light.

Scott

Priya

Maybe I'll ask hubby for one for christmas, he's never sure what to get me.