skinny z

Anyone here have any experience with this particular distributor?
Looking through the paperwork that came with this distributor I see that up to 34° of mechanical advance is available. I'd like to limit that amount to about 14 (crankshaft degrees). I run 20° of initial timing and am looking for 34° total (initial of 20 plus 14 centrifugal).
A conventional (non-HEI) Chevy distributor uses a limiting bushing to tailor the total advance available. I see no such bushing or hard stop on the FAST unit. Only the limiting plate for the vacuum advance.
Ultimately I'm looking for a curve that starts the advance at 1000 RPM and is all in before 3000 (2500-3000 depending on the tuning process).
What are my options? Old school was to weld up the advance slot and grind it to an appropriate size.

NoEmissions84TA

iTrader: (1)

MSD uses bushings to limit the mechanical advance.
I wonder if they will fit your distributor?




From the manual:
21. Optional tuning of the distributor's mechanical advance curve is possible with the various springs included with the distributor. Lighter springs allow the mechanical advance to come in at an earlier RPM, heavier springs come in at a later RPM. For best performance, the mechanical advance should come in as quickly as possible, while still avoiding spark knock.

You are right - no mention of limiting the amount of centrifugal advance, just the rate.

skinny z

It may be that there is something that could carry over between the two types and brands of distributors.
I have an MSD distributor presently and am using the largest busing to limit the advance. And as a side note, it still isn't enough of a limitation. I'm looking for about 7 degrees max mechanical (dist degrees) and the largest bushing allows 9. That's too much for my target total.
Back to the FAST HEI, there are pins travelling in a slot that make up the centrifugal mechanism. I'll have to take this new unit apart and compare pin and slot diameters.


FAST HEI centrifugal advance slot which may or may not be the same as the small cap MSD.



I wish I still had access to the old distributor machine. I've spent many hours dialing in curves and it's far and away the easiest way to get the job done.

NoEmissions84TA

iTrader: (1)

I wish I still had access to the old distributor machine. I've spent many hours dialing in curves and it's far and away the easiest way to get the job done.


Yep, I remember.

skinny z

So, getting a little deeper into this, I find that the MSD bushings don't fit the pin or the slot in the FAST HEI.
It looks like the advance can be limited but unless FAST comes through with a method (I've got an inquiry on the tech forum), I'll be building something on my own. Not a big deal except that anything I make will have to be tested and that means pulling, fitting a bushing, reinstalling the distributor a number of times and and spinning the engine up to check on the curve. And that's a PITA.



The shoulder on the advance pin prevents the bushing from fitting in the proper place. It would appear that said shoulder is the bushing here.



And the MSD bushing itself has a diameter that's too large for the slot.

NoEmissions84TA

iTrader: (1)

Looks like you will have to make a custom bushing.
How is that pin held in there - staked or welded?

skinny z

The pins are welded in place.
Looking as to what's available for HEI advance kits, several brands offer pretty much the same thing which consists of advance weights, weight bushings (not an advance bushing)and springs. I could see how the profile of the weights contributes directly to the advance total. By how much I haven't determined.
And no support at all from the FAST tech forum.

skinny z

Seek and ye shall find!
As I suspected there really isn't anything out there that allows the mechanical advance to be limited. Now at least one of the kits I've come across has a centre "cam" plate with a profile that's said to reduce the total available to about 14. That would work however I have no idea which kit it is.
Further searching showed one example that I had originally thought might work. It involves placing a screw beside the screw that holds the rotor in position. It would still be a trial and error process determining the total advance however it's totally reversible should it prove to be a failure and may just be the solution I'm looking for.


Rotor screw hole in blue. Available hole in red. You can see here that the advance plate is rotated about halfway (as evidenced by the pin being midway through the slot and the edge of the hole is even with the cam plate).


White arrow indicates screw installed as a limiter. The size of the screw head would determine the maximum amount the cam plate can rotate.

I might be on to something.

NoEmissions84TA

iTrader: (1)

Adding that screw is exactly what I did to a GM distributor to lockout the centrifugal advance.
Like you said, it all depends on the size of the head of the screw. You can grind it into a cam shape, and depending where you lock it down, you can "dial in" your limit.
BTW, I have played around with a few advance curve kits. Only one, MOROSO, has glass hard weights. All the others are soft and wear rubbing against each other quickly. Then you are pulling what's left of your hair out trying to figure out why your engine is acting "funny".