[67drake]

Had my distributor out to work on a few issues.
My car idles at 750 rpm or so. I have 2-3 degrees mechanical in already at idle that I want to clear up. Not having another GM points distributor to steal springs from, I used the only thing available at the time- the “stiff” springs from a Mr. Gasket advance kit. JUNK! Made the situation no better at idle, and detonation under full throttle at any speed. I had no detonation before the spring change, and initial timing is exactly the same as previous-10 degrees. Distributor is putting in 26 degrees mechanical, BTW.
So I went to a local junkyard and found a few old 60’s points distributors to pull the springs off of.
Question is, do these look like factory advance springs? They look beefier than the ones in the “Mr. Gasket detonation in a blister pack” do.
Thanks

[steve25]

Yes, to me they look like factory stock ones.

What you experienced with the Mr gasket springs might happen again with the stock ones you now have but certainly try them out even if you use one stock one and a light aftermarket one at the same time.

The reason for the detonation you had taking place is this .
For every 1 degree of additional timing that a motor may be lacking to run its best At or near full throttle , the cylinder temperatures will go up a whole darn lot, and this then makes for ping and detonation also, just as with having too much timing for any given engine rpm.

The short of this is that not enough timing kills the peak cylinder pressure and the motor then can not rid itself of the exh gasses it’s making fast enough.

Also it’s not a given dad thing that having your mechanical advance start to kick in around where your motor wants to idle.
This can be of use if your motor is hard to crank when hot since you can then run less initial timing by a few degrees.

As long as your total timing is not excessive and your carbs throttle blades are still in a good way with the fuel transfer slots then this type of set up can be made to work for you.

[67drake]

Originally Posted by steve25

Yes, to me they look like factory stock ones.

What you experienced with the Mr gasket springs might happen again with the stock ones you now have but certainly try them out even if you use one stock one and a light aftermarket one at the same time.

The reason for the detonation you had taking place is this .
For every 1 degree of additional timing that a motor may be lacking to run its best At or near full throttle , the cylinder temperatures will go up a whole darn lot, and this then makes for ping and detonation also, just as with having too much timing for any given engine rpm.

The short of this is that not enough timing kills the peak cylinder pressure and the motor then can not rid itself of the exh gasses it’s making fast enough.

Also it’s not a given dad thing that having your mechanical advance start to kick in around where your motor wants to idle.
This can be of use if your motor is hard to crank when hot since you can then run less initial timing by a few degrees.

As long as your total timing is not excessive and your carbs throttle blades are still in a good way with the fuel transfer slots then this type of set up can be made to work for you.

So you’re saying my detonation may be lack of advance?

[67drake]

Well anyway-
I put one of the factory springs in with 1 heavy aftermarket spring. WAY better. Timing mark rock solid. I was able to actually take another 50 or so rpm out of the idle speed.
On a test run only a hint of detonation. I then went home and moved initial advance from 10 to 9 degrees. That’s the factory spec on this ‘69 400 anyway. Took it for a short cruise before the rain hit, and detonation was even less noticeable. I think with 10.5 compression ( well, factory advertised) this is what it is for now. MUCH better than where I started for sure.

[TCSGTO]

Those don’t look like factory springs to me. It hard to tell wire thickness from a picture but stock springs I’ve seen have more coils and the looped ends are shaped different.
The only aftermarket springs I’ve found that are decent and don’t add timing at idle are the heavier ones in the MSD kits. Moroso, Mr Gasket etc. are made from piano wire and aren’t heat treated. They’ll lose tension after they get exposed to a few heat cycles and cause the weights to swing out too soon.

IMO the old rule of thumb that started in the 70’s about getting full advance as early as possible was meant to crutch low compression smog motors and isn’t necessary with high compression performance mills. It’s also probably the biggest cause of auto trans park to drive idle issues and run on/dieseling.

[67drake]

Originally Posted by TCSGTO

Those don’t look like factory springs to me. It hard to tell wire thickness from a picture but stock springs I’ve seen have more coils and the looped ends are shaped different.
The only aftermarket springs I’ve found that are decent and don’t add timing at idle are the heavier ones in the MSD kits. Moroso, Mr Gasket etc. are made from piano wire and aren’t heat treated. They’ll lose tension after they get exposed to a few heat cycles and cause the weights to swing out too soon.

IMO the old rule of thumb that started in the 70’s about getting full advance as early as possible was meant to crutch low compression smog motors and isn’t necessary with high compression performance mills. It’s also probably the biggest cause of auto trans park to drive idle issues and run on/dieseling.

Wherever they came from, it’s a lot better now than with those 2 Mr. Gasket springs in there.

[Shiny]

Glad to hear you are making improvements!

Below are dimensions of springs SunTuned (Bob Davis) sent me and which I assume are "stock". But I don't know how many "stock" spring PNs were used in Pontiac points distributors, nor have I ever seen any published specs.

After cleaning and lubing my distributor bushings, these springs held the weights home and prevented any mechanical advance up to about 900 rpm, well above my idle.

I measured these with a caliper:

[Shiny]

Here's a used distributor I bought off eBay. It's PN 111208, correct for my 70 350 and same as my original. Being used, I can't confirm the springs are original but they have 4.5 turns like those sent by SunTuned.

You can see they have some stretch when homed. I measured the extension on mine to be 0.150" when the weights were parked. I didn't measure that on this used distributor.



Mike

[Formulas]

todays octane on yesterdays compression i would run 2 steps colder plugs 160 thermostat couple jets sizes bigger on primaries block heat crossover on intake .... I did all this plus used stock heavy springs on mech advance but wait theres more i also cut the ends of the mech advance weights a touch to slow it down... this was on a 440 short block with #48 heads ran great after all that

on your build you probably wouldnt have to do all of this and as i remember stock points springs are around .040 - .045 coil dia.

[67drake]

Originally Posted by Formulas

but wait theres more i also cut the ends of the mech advance weights a touch to slow it down... .

Hmmm…never thought of that one.
Yeah, today’s gas. Around here that’s a whopping 91 octane non ethanol that I run.

[Shiny]

I don't recall the source but this chart suggests there were multiple springs used across different distributor PNs. So "stock" may vary and need to be qualified by application.

" Weight Spring" I interpret as the spring PNs.

This may have come from Kenth. He shares a lot of this type of info.

Would be truly helpful to see all the specs for these PNs!

Mike

[67drake]

Yeah, I would have spent time measuring mine, but I kind of had to get that spring in there when I had time to work on it. Time is limited sometimes.

[steve25]

So what does your overall timing curve look like now as compared to what it was?

If you want to be on the safe side of Spark plug induced detonation then use of a heat range of plug that during normal cruse shows signs of the color change over on the ground electrode being just before the 90 degree bend on down to the plugs main body.

This will then be a plug that’s right on the tipping point of being too cold and as such give you more latitude for tuning errors that would have you on the lean side.

[67drake]

I’ll pull a couple today and see. One thing that I didn’t think about till now is that the guy that sold me the LeMans said it had a ‘73 400 in it. After looking up all the casting numbers and stamps, I determined it to be a ‘69 400! Who knows what plugs he put in as he probably just guessed what to run as the ‘73 plugs would have been taper seat.

http://forums.maxperformanceinc.com/...d.php?t=865618