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Re: Mechanical Advance

To: Patrick McMullen <>
Subject: Re: Mechanical Advance
From: Bob Lang <LANG@ISIS.MIT.EDU>
Date: Mon, 19 Oct 1998 12:34:52 -0400 (EDT)
On Mon, 19 Oct 1998, Patrick McMullen wrote:

> Should I be concerned about using different strengh springs to control
> my mechanical advance?

To echo John's reply - no. The different springs allow for different 
"slopes" of the advance curve.. a weak spring will give you a little more 
advance at the lower end and a stiff one will give a flatter curve 
towards the top end. At least I think this is how it works. 

> I am using a combination of Triumph and Delco
> springs to get my current ignition curve.  Will this cause rotor wobble?

No. Inless the dizzy bushings are worn.

> I was thinking that if one weight starts to open before the other that
> the balance would be off.  Maybe cause ignition flutter at higher rpm's.
> Do you autocrossers use equal springs?

I actually have the stock distributor. Two of them, in fact. Mine have two 
different springs.

> If so, how do you fine tune your
> advance curve?  (trade secrets?)  

You find a shoppe that has a distributor machine and have them play with 
it. The springs in your Lucas 22D are quite a bit smaller than ones I've 
seen for the general distributor like those seen on the Big Three V8's, 
so before you can set this up, you should try to find a selection of 
parts that the distributor tuning person can use...

What I found out this weekend past is that many places that fix cars 
don't know a rats arse about recurving dizzys - or don't want to, or even 
worse are probably prohibited from doing so because of the E - P - A. I 
got a lot of "I can't touch that" responses. A lot. Like one shop of 15 
that I called... really.

I think that since everybody has gone to more or less distributor-less 
ignition systems, the art of curving a dizzy is going away.

FWIW, I did find a "race shop" that knows what to do. I'm bringing a 
distributor to them later this week.

> Or am I worried about nothing?  

This is a legit concern. I figured out this weekend that my advance 
thingie on my TR6 distributor was frozen to the lower shaft. Yep, no 
advance at all. Makes the power curve "real flat".

I also figured out this weekend that the built in limits of the advance 
curve of the stock TR6 dizzy are pretty small - like 10 degrees, more or 
less. If you are low compression (under 10:1) and you want the more ideal 
curve (the Kastner book says 23 degrees total advance for low compression 
performance cars) then you might have to reshape the weights or limit 
blocks to get a bit more "swing" in there.

Note: John's (and Chip Bond's) Electromotive unit is really nice. 
Drawback - relatively big money. You need to go to a crank trigger setup. 
Advantage - if the crank timing is off, you have way bigger problems than 
any ignition system can fix! That being said, I think the Electromotive 
setup is really nice. Drawback - def. not for the vintage crowd.


I've also looked into the MSD setup - the 6AL plus the "advance unit", 
module 8490, I think. I can look it up if you want to pull your Jacobs 
box out ;-). But between the two components, your talking about $400 even 
with a discount and you still need to trigger the thing somehow. I guess 
that means a billit distributor, but I haven't seen one for a TR6 yet. 
You could weld up a stock unit (or you don't oil the little screw 
thingie like you're supposed to - like I didn't - Doh!) to get a fixed 
timing point for your trigger - but anything driven off the cam is 
subject to the possible irregularities there... so a crank trigger is 
really the way to go if you want rock solid timing. And the crank trigger 
hardware is not cheap, at least that's what I've found from my inital 
foray into the topic.

> Pat in NC

Bob in lobsta land.
Bob Lang                Room N42-140Q          | This space for rent.
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