Hi all.
I decided it was about time to look at the mechanical advance on my aftermarket HEI distributor. I am getting the beast ready for the early summer.

I placed the shaft in a vise and measured the "rotation" of the advance by attaching a pointer to the movable piece, I then marked one 'end' of the rotation on the housing, rotated the moveable piece and marked the pointer position.

The linear measurement between extremes is 1.44 inches. The OD of the dist housing is 5.165" If my math is correct, 10 degrees equals about 0.4507 inches along the housing OD. If that is correct, the 1.44 inches equals about 32 degrees (at the distributor!!!!).

Comments please. I think what I want is about 11 degrees at the distributor. I don't think I have room to bush it that far.

Thanks in advance for your feedback.

Lang

 

You're over-thinking this Just get some timing tape or better yet a dial-back timing light and set it where you want it. See this sticky for details:

http://forums.corvetteforum.com/c3-t...long-post.html

 

I appreciate the amount of calculations that went into that, but for $100 a dial back timing light will give you the detail information you seek.

 

I have a dial back light. The distributor is not in the engine currently. I can set the initial timing and check the total once it is reinstalled. I am working on the mechanical advance piece right now and I don't have access to a distributor machine of any kind. just trying to work from the basics.

Thanks,

Lang

 

The mechanical advance cannot be calculated by simply measuring the angular movement of the weights. They act as a V shaped lever on the cam or 'football' at the top of the shaft. The distance from the pivot point to the contact point of the cam and the angle of the 'V' must be considered, as well as the distance from the contact point of the cam to the centre point of the shaft to be able to calculate the relationship.



Might be easier to make a mark on the shaft and see how much it turns with respect to the housing

 

Thanks Mike.
Do I understand that the weights themselves provide the limit in advance degrees? I have the weights off as I have assumed that the limit is the pin in the slot. I know on the older points units you use a bushing on the pin to limit the max centrifugal (mechanical) advance.

 

I stuck one of the weights back on. It did limit the movement but not that much. I will still need to physically limit how far the star can rotate if I want a maximum of 20 degrees crank (10 degrees distributor).

 

I'm not familiar with HEI units to that degree so can't help much.

 

I think you'll drive yourself nuts trying to work this out on the bench...only to have to do it again once it's in the car.

The limit is not the weights - it's the pin on the advance plate that goes down into the plate on top of the distributor shaft. If you need to limit mechanical advance, then it's done by crimping on a limit bushing onto that pin...all covered in the sticky previously referenced.

...and you'll still be well short of the recommended 34-46 degrees total advance, and without in-car testing you won't know how quickly the advance comes in. Seriously - I'm just trying to save you effort...

 

If you don't have one of these your wasting your time unless the distributor is in the car and running. http://ctci.org/gilsgarage/SunMachine.php

 

You can't determine mechanical advance by moving the weights through their travel distance in a vice - the data is not relevant. The weights will not move through that arc distance in a running distributor - the shape of the weights and the center cam make the weights run out of mechanical advantage before they reach their travel limit, and this determines the length of the curve. You have to have a distributor machine or install the distributor in the engine and test it with a dial-back light. You're going to find that the distributor probably has about 20 degrees-or-so of advance.

Lars

 

Thanks everyone. I guess I'll go back to the engine and determine the curve using the dial-back light.

 

What I wouldn't give to find one of those in good shape sitting around somewhere...

 

Used and reconditioned distributor machines are available but they are too costly for one or two distributors. If you are in the business, maybe you can afford $2,000 plus.

BTW, I remain convinced that one can set the maximum centrifugal advance on the bench. Followed by on-the-engine setting of the advance timing (springs - when it comes in) and the total advance (initial plus centrifugal). I just haven't figured out how to apply a physical limit.

Lang

 

I sold one a couple of years ago, didn't use it much any more.
Now just use a digital Inovate adj timing lite, gives advance, rpm, and voltage. Better results than the machine if used with a hand vacuum pump.
The biggest misunderstanding with the machine is that you at best can only set the curve for an ideal comparison curve, not the actual engine. Both are good enough for most users.

langg
You can set the limit properly as Lars described in an hei with the center cam. Rob some spares from a junkyard and play with the shape by adding or removing metal.
For the vac advance, just make a sheet metal stop for a limit.

 

Your math is correct; it's a good method which I also use to set up the HEI advance. However, one must have the weights and springs installed also, and as someone commented, measure the rotation of the advance plate not the flyweights themselves - it does sound like you did it correctly.

It is notable that on most HEI distributors that I've worked with - the pin never limits in the slot like it does with the pre-1974 small diameter distributor. Instead, the max mechanical advance is limited by the short arm of the advance weight reaching to virtually the center of the center cam ("football") and not being able to advance any further due to insufficient leverage. One can determine this point by pulling outwards on the weights and following the action. CAUTION: I've seen some aftermarket weights which have still shorter arms which can still have enough leverage to advance even farther after their inner tip is tangent to the football. In this case, you will probably need to limit the slot or bush the pin.

I dislike cheap, light-weight, aftermarket advance kits, and instead, use the heavier GM weights with stiffer springs against the GM football. To adjust the curve, alter the shape of the football with a bench grinder and adjust its quickness with spring tension. Finally, test it on the engine with a fixed timing lights and a timing tape on the balancer for both length and quickness of advance.