ISO...even lighter distributor springs

[Gary S.]

working on getting all my mildly Isky cammed L36 centrifugal advance in by 3000-3500 rpm. TI distributor.
Have the MrGasket 928G spring set and even with the lightest, black, springs not getting the full 32deg centrifugal until 3750 rpm.
Have a couple year old Paragon/Moroso set, and those are even stiffer than the silver MrG's
Took some pics with a 5.5# weight pulling on the various springs. Pics uploaded sideways; center of the earth is on the right
Paragon black
paragon black.jpg
Paragon silver
paragon silver.jpg
MrG silver
Mr G silver.jpg
MrG black
MrG black.jpg

interesting that the difference between MrG silver and black springs was only about 1.5mm (5mm between the pen marks).

The Paragon set also came with a set of copper colored very thin wire springs that stretched about four times as much with the 5.5# tension before breaking. I wasn't quick enough to get a video.

Weights are 34grams for the pair, same as the replacements in the Paragon set. Same shape also.

[Mark E.]

Re: ISO...even lighter distributor springs

Gary,

This is odd. Try the Accel advance curve set. Their lightest springs come in well below 3,000 rpm (at least they did in the past). Using a combination of springs allows for dialing it in.

Also, make sure the advance mechanism moves freely.

PM me if this fails. I have a collection of springs I can share.

I like your test method- simple and visual.

[Gary S.]

Re: ISO...even lighter distributor springs

thanks, Mark!
seems to move freely; the vac does what it should.
I'll see if I can get an Accel set.

[Mark E.]

Re: ISO...even lighter distributor springs

thanks, Mark!
seems to move freely; the vac does what it should.
I'll see if I can get an Accel set.

Centrifugal advance is a separate motion from vacuum advance. Make sure the football and weights move freely and are lightly lubricated. Rotate the rotor clockwise by hand to the advance stop, then release it. It should rotate freely then snap back smartly when released.

[Duke W.]

Re: ISO...even lighter distributor springs

I don't recommend any kind of lubricant on the centrifugal advance mechanism. GM did not lubricate them, but I've seen some that had thick wad of grease than must have been applied with a soup ladle.

These are, lightly loaded hardened parts that have limited movement, so they don't need any kind of lubricant.

I do recommend wiping down the mechanism with WD-40 to provide corrosion protection.

Duke

[Duke W.]

Re: ISO...even lighter distributor springs

Use a dial back light to set total advance (VAC disconnected and plugged) at 40 at a little above the point of max advance, say 4000-4200. If the engine will tolerate that without detonation, I'd leave it alone.

With a 16 degree VAC total idle advance should be 24 assuming centrifugal doesn't start until above idle speed.

Go through the idle speed-mixture adjustment procedure with a target idle speed as low as possible commensurate with acceptable idle quality (your call) and tell us what you end up with... xx" Hg @yyy RPM.

I recall reading somewhere that the TI actually takes out a degree or two of advance in the upper rev range.

Duke

[Gary S.]

Re: ISO...even lighter distributor springs

Duke and Mark, thank you both. Yes, the centrifugal mechanism seemed free and clean, I'll recheck it when the Accel springs get here next week and post a pic of their stretchiness. The Accel 31042 spring set was not easy to find; I didn't find it on the Holley/Accel website and Summit shows it as not available. Found one on eBay and ordered it.

Currently have the initial at 7.5 deg and initial+centrifugal at 39.5. Always seems to change 0.5-1deg when I tighten up the bolt.
Seat-of the pants was mucho better 4500-5300 at 39.5 than 36 deg. No ping to my ear at cruise but need to do a couple uphills.
I do enjoy fiddling with it a bit so still would like to see if it gets a little more midrange punch with the advance in sooner.

The current vac is giving me 13-14deg (max) at 12" and the motor pulls 14" @700rpm idle with the Isky (21deg idle advance). I worry a bit about 40deg+16deg=56deg at cruise, so I'm ok with the 8deg initial and 14 rather than 16 vac advance. I suppose many fears are irrational.

I'll zing it up to 5200 next time when the timing light is on and see if the advance decreases a couple degrees or not.

BTW, the CR is 9.5:1

Gary

[Bill B.]

Re: ISO...even lighter distributor springs

I used the Mr. Gasket kit, P/N 928G. I tried the gold and silver springs with my stock dist. weights. This was the plotted results in crankshaft degrees from a few years ago: (VAC can disconnected and hose plugged)

Adv Curve.jpg

I ended up using the gold springs for drivability. With the silver springs, after I set my idle rpm, letting out the clutch on initial partial throttle launch (normal driving), the engine frequently stumbled and stalled if the rpm dropped below 1,000. (the silver 1500 rpm data point must have been a data read error on my part - suspect it should have been around 27.5 deg)

With the B28 VAC can and full-time Vac, as Duke recommends, (2" rule), I get some light "trailer-hitching" when cruising around town on side streets below 1,500 rpm. I just keep mindful that this ('70 LT-1) wasn't meant for low rpm ops like the ZQ3 base engine could deliver that I drove for many a year earlier.

It might be that your distributor weights are lighter or shorter than the standard weights.

[Mark E.]

Re: ISO...even lighter distributor springs

I ended up using the gold springs for drivability. With the silver springs, after I set my idle rpm, letting out the clutch on initial partial throttle launch (normal driving), the engine frequently stumbled and stalled if the rpm dropped below 1,000. (the silver 1500 rpm data point must have been a data read error on my part - suspect it should have been around 27.5 deg)

With the B28 VAC can and full-time Vac, as Duke recommends, (2" rule), I get some light "trailer-hitching" when cruising around town on side streets below 1,500 rpm. I just keep mindful that this ('70 LT-1) wasn't meant for low rpm ops like the ZQ3 base engine could deliver that I drove for many a year earlier.

It might be that your distributor weights are lighter or shorter than the standard weights.

Similar to your experience, my '70 LS5 didn't like full advance until 3,200 rpm. Best performance was close to factory spec advance curve.

Also, some engines don't like manifold vacuum for the VAC. This sometimes causes a stumble at launch due to timing first retarding with reduced VAC signal then advancing as centrifugal kicks in with rpm. Experiment using ported vacuum with increased static timing to correct this.