Gm hei question

[Bills64]

Hi folks... trying to setup a really nice gm hei. Its been cleaned up no slop in the shaft or bushings, weights and pins have no wear and were assembled with light lube. I'm using GM's 990 icm and the mechanical advance is limited to 18° and the va is limited to 12°. It's a Pontiac 421 with #13 cast iron heads (big valves), '68 cast iron intake, '67 ho exhaust manifold with Crower 60245 flat tappet cam... duration 248° intake 252° exhaust. The initial is 14° all in at 2800 rpm with total of 32°.I'm probably giving more info then what's needed but some might ask for it.
So to my question... It seems to be adding mechanical advance at idle (750 rpm) because the idle is varying a 100 rpm or so. I'm wondering how to get it not to advance until it's off idle say 800 to 850 rpm. My advance springs are where I want them to be for the total advance.
Oh and the carb is a 17057274 with ALL Cliff's recommended parts. Carb seems to be great in all it's circuits

[Cliff Ruggles]

Where did you source the springs?  Sounds like they may be from an aftermarket advance curve "kit"......

[Bills64]

Yes they are aftermarket I believe they're crane. My weights and center plate are gm 053 weights and 364 center.

[Bills64]

If the issue is springs does anyone know who sells gm springs? I've looked on the net and all I can see is aftermarket. The ones that were on this distributor made it come in around 34°

[Bills64]

I meant to say 3400 rpm not degrees.

[lightning boy]

Have you tried one light and one heavier spring? Sometimes this is what is needed.

[73ss]

I'm using a similar distributor, Original delco, but for BBC. I'm using one stock heavy spring and one medium aftermarket spring. Advance starts at 1100 RPM and all in at 2800~2900 rpm. Original GM 106 weights, 406 center cam. I'm getting 21 degrees total mechanical advance.

Anyone know where to find the plastic seal & felt washer located under the pick-up coil?

[Cliff Ruggles]

I've had ZERO luck in long term service with aftermarket springs.  They are poorly made and get "soft" over time.  The heavier wired diameter factory springs will hold their tension for at least 100 years if they don't get rusty.

You can "manipulate" the spark curve with the heavier springs by simply opening up the "eyes" in the ends of them.  Quite often I'll leave one side stock and open up the other one to allow the curve to come in a little quicker but still not add anything at idle speed......

[lightning boy]

that's a great idea. I'll have to try that. thanks for the tip Cliff!