Dizzy – Mechanical Advance Weights

[Atrus]

As I had posted previously, the primary spring on my mechanical advance was completely ruined somehow. I’ve been reading up on how to tune the 25D, and figured for the $5 it cost for a “universal” spring kit, I’d play around and give it a whirl. While playing, I’ve thrown a couple of different springs on there just to see how tight it makes the advance.

What I am noticing is that my weights have some play in them when there’d be no advance applied. Is this common? They can’t advance outward nearly as far as when the actual advance arm moves, but oscillating the dizzy shaft clockwise and counterclockwise, you can hear them rattling. Using a small screwdriver, I can move them without resistance.

 

[Morris]

I can't answer your question, but if you are going to be playing around with advance curves, you may consider getting this:

https://www.viatrack.ca/

 

[Atrus]

Very cool, I was wondering about putting in a knock sensor. I'll have to check this out at home when I have more time. Thanks for the link!

 

[Bruce_B]

Can anyone answer there the actual knock sensor unit itself goes? I see that Knocksense sells a used Bosch unit, but where is it mounted?

thanks...

 

[Trevor Jessie]

Knock sensor in an old clanky pushrod engine? I have my reservations.

I'm feeling skeptical today!

 

[Atrus]

it's not "old and clanky", but I have one in my 3.8L pushrod engine . False knock (and knock retard) is common in the L36/L37 GM engines. PCM senses knock, it pulls timing. In the A-series, however, it'd be good just to monitor it. No detriment to having one, other than your wallet /bcforum/images/%%GRAEMLIN_URL%%/wink.gif

 

[Atrus]

don't mean to be a pain, but anyone have any idea on these weights? Is it OK to be a little loose?

 

[DrEntropy]

Define: "loose."

 

[Trevor Jessie]

Where's Jeff from AdvanceDist when we need him?

 

[Atrus]

I guess I don't have a quantitative measurement...standby, I'll try with the camera to get a pic.

 

[Trevor Jessie]

Here is Jeff's website:

https://www.advanceddistributors.com/

 

[Atrus]

OK, I see no way to get a good pic...let's put it this way, the advance arm gap = 13*, I'd say it's about that much rotational play out the outward corner of the one near the primary, about 2/3 that much on the secondary. Secondary spring is loose when at rest, primary has some tension on it.

 

[racingenglishcars]

From what I understand then, it doesn't sound completely wrong. the weights should be loose enough to move freely. The light spring should be tight and the heavy spring should be loose, when at rest. What is probably wrong is the advance curve. This is difficult to get right. Trial and error might do it, but for good results use go to advanceddistributors. Or use a dyno and many hours of work.

 

[healeyboz]

I agree! Deciding how much time is worth spending on getting it right through trial and error is the question. I sent mine of to Advance Distributors and can rest assure that it is darn near perfect! Hopefully one less factor during the initial start-ups.

 

[Morris]

It may be a bit late for this, but I found at this at my new favorite parts source

https://www.bpnorthwest.com/.sc/ms/dd/Midget--Engine%20Electrical/8240/Distributor%2025D4%20New


I am not sure what it costs to have your old dizzy recurved, but if it is close in price, it may be worth getting the new dizzy.

 

[Atrus]

Well, I sent a lengthy e-mail to Jeff, and he said that's a common debate. He stated that Lucas didn't put any specs out for a stock dizzy for a reason - there are HUGE variances. So, basically, what I have right now is very normal.

He's given me two options. He'll completely rebuild my stock unit - new bushings, lessen the advance weight play. new cap/rotor (mine are cheap junk), recurve it and he'll put in the pertronix unit there as he stocks them. The other option is that he sells the pertronix distributors and he'll recurve that for my needs for the same price. He says that he truly feels his rebuilt stock units have much better components than the overseas built pertronix units. He's going to charge me the same for either option.

So, I have some thinking to do...looks like the completely rebuilt stock may be the way to go. /bcforum/images/%%GRAEMLIN_URL%%/wink.gif

 

[healeyboz]

Rest assure that Jeff knows what he is talking about. You will be happy with having him do the work!! Hap uses him and recommended me to him. The distributor looks great and everything is relatively new. Can't wait to see how well it works when running!!!

 

[Sarastro]

I'm a little surprised that they offer the same distributor for sprite, MGA, MGB, TR3/4 and so on. Is there really no difference between the stock advance curves for all these cars, or are they just close enough that you can get by with a single replacement distributor?

 

[jlaird]

I would be willing to bet a beer that they were all different. Heck, the engines were different. Stands to reason. But then they are LBCs so who knows.

 

[Atrus]

according to Jeff and several online sources I have read, that's the problem. These cars all do have different advance curves desprite using the same housing. In fact, Vizard states this - the car doesn't care what kind of dizzy it has, as long as the curve is set up. Basically any dizzy can be curved to fit any car.

Regardless, when I rebuild the engine next year I will have a distributor custom curved. My #1 goal right now is to get the car running decently enough to drive this spring/summer/fall. I had it going well enough to marginally drive it, although it wasn't correct last year. Makes total sense that I could never dial it in correctly with the spring missing on the primary. I have two brand new (not rebuilt) HS2's on the car, so I am thinking the dizzy is it.