MP distributor - reducing mechanical advance

[1MeanA]

I always knew it as you weld the resting side of the slot, the bottom/base, not the top or advance side..

It seemed that it was done both ways and I picked one...kinda like when you pull a distributor and forget to note the orientation of the rotor and then go to reinstall

I've had the distributor out and apart so many times that it shouldn't take me more than an hour or so to weld the bottom and grind back the top. Hopefully the extra 0.100" tension makes a difference.

ps The engine doesn't seem to be pinging with VA at least I don't hear it.

 

[1MeanA]

Fail. I welded up the inside slot about 0.120" and ground back the upper end 0.120". Now I have no advance below 2000 rpm. It didn't seem right installing the slot bar. I think that the heavy spring is coming into play right away. I'll try 2 medium springs. I guess this is the trial and error part. On another non-Mopar engine I had a programmable distributor that looked pretty stock but you could plug your laptop into it and give it any sort of advance curve that you wanted. I'm not sure if there is anything out there like that but I am sure its $1000 or more up here.

 

[1MeanA]

I made some progress using 2 medium light springs (10 coil, 0.025 wire, 1/4 od, 0.7 long approx). Is all in timing by 2400 rpm too soon? I might try and buy some Mr Gasket springs and have another go. Slots are pretty ugly after all the welding and grinding.

• initial 20 deg
• advance starts at 1500 rpm
• 25 deg at 1800 rpm
• 30 deg at 2000 rpm
• 35 deg at 2400 rpm (all in)

 

[crackedback]

You can tweak the gold tabs the springs hang on out a little to increase tension to raise the all in point or find a heavier spring. Swap one to a slightly higher spring rate to slow up the curve.

Not pretty but as long as there isn't rotational slop in the tower (slots too much clearance on sides for pins) and it doesn't bind up anywhere, run it. Just my 2 cents

 

[1MeanA]

Bending the tabs didn't make a big change maybe a 200 rpm max. Maybe I should live with it. There is not much for springs in stock around here to look at. I can order the Mr. Gasket 925b Mopar springs and get them a few days but I can't return them. Does anybody know if these are heavier than than what I have? (10 coil, 0.025" wire, 1/4" od, 0.7 long approx.)

• initial 20 deg
• advance starts at 1500 rpm
• 25 deg at 2000 rpm
• 30 deg at 2200 rpm
• 35 deg at 2600 rpm (all in)

 

[qkcuda]

What happens if you put the heavy red spring back on one side? It's free, so worth a try.

 

[1MeanA]

What happens if you put the heavy red spring back on one side? It's free, so worth a try.

I got one really heavy spring from one of my old distributors. It wouldn't advance with it on (slot welded on the inside). That spring has some slack in it at rest when the slot is not welded and I believe is designed to bring the advance in late and slow at the top end with the long T slots that give you 34 deg mechanical advance.

 

[qkcuda]

Any of those medium springs from the extra distributors in your first post a bit heavier than what you have in there now?

 

[1MeanA]

Any of those medium springs from the extra distributors in your first post a bit heavier than what you have in there now?

Nope. I already robbed them.

 

[George Jets]

Not sure why people are having so much trouble with the advance springs?

I go by what @halifaxhops says and sells, as he has the experience on what works.

A good original setup, and a good simple ignition can solve a lot of problems.

Never floated the valves once, on the '89 Roller 318 with an original Mopar Distributor with the HEI conversion and the e-core coil. Just keeps on pulling... run out of straight road first before I have to back out of it.

Really runs out nice.

.050 ths. spark plug gap running the 60,000 volt e-core coils.


☆☆☆☆☆

 

[1MeanA]

I picked up some MSD 8464 springs that looked to be close in size and measured them up (on the left). The light silver or blue one should be heavier than what I have (blue is heavier than light silver). The other two springs on the right are the stock springs out of my 1970 factory points distributor. What is installed now is in between.

 

[92b]

I picked up some MSD 8464 springs that looked to be close in size and measured them up (on the left). The light silver or blue one should be heavier than what I have (blue is heavier than light silver). The other two springs on the right are the stock springs out of my 1970 factory points distributor. What is installed now is in between.

View attachment 1716546015

There are at least 5 varibles ( that I can think of) that will determine how the spring will affect your curve. Wire diameter, number of coils, diameter of coils, spring length eyelet to eyelet and wire material. Looks like you are getting closer to what you want. Good job.

 

[Mattax]

There are at least 5 varibles ( that I can think of) that will determine how the spring will affect your curve. Wire diameter, number of coils, diameter of coils, spring length eyelet to eyelet and wire material. Looks like you are getting closer to what you want. Good job.

You're describing spring rate. Spring rate in turn is one of the parameters for determining spring force at a length. Spring force at a various distances is one of the variables controlling the start of the advance as well as the shape of the curve.

 

[92b]

You're describing spring rate. Spring rate in turn is one of the parameters for determining spring force at a length. Spring force at a various distances is one of the variables controlling the start of the advance as well as the shape of the curve.

Exactly. The eyelet to eyelet measurement would be spring preload. The other varibles would be spring rate. I know it sounds like I'm being picky and I know you know the difference but this might help somebody else. I think it helps to make the distinction between spring rate and spring preload when trying to understand how a spring change might affect the curve. A long loop spring is a good example of this. It has no preload and until the weights have moved enough to engage the spring it will not influence the curve. Also a longer eyelet to eyelet measured spring with little preload and a higher spring rate can potentially give you a faster curve than a short eyelet to eyelet spring with a softer rate and lots of preload. In the end you just have to try it and see what happens.

 

[Mattax]

A long loop spring is a good example of this. It has no preload and until the weights have moved enough to engage the spring it will not influence the curve.

When properly used it has no effect on the starting rpm of the advance.
But it has a key if not critical effect on the curve.

think it helps to make the distinction between spring rate and spring preload when trying to understand how a spring change might affect the curve.

Yes. I agree.
I've pretty much explained this all with photos and graphs in previous threads.
Here's how the advance works and can be adjusted
Earlier in the same thread links to videos illustrating why the heavy secondary spring is important with electronic ignition even for drag race only applications.

Everyone learns differently. I've walked some people through it, and often suggest looking at those threads as examples.
How to example with Rock_Able, and another with Rick as he works through and posts the effects of his changes here.
And an example of how timing changes a bit with different combustion characteristics of a later LA engine.

 

[1MeanA]

I think my distributor is worn out from being welded, disassembled and assembled so many times. I ended up re-welding the outside of the slots and grinding back the inside. I still can't slow the advance down with the springs that I have and the best I can get is full advance at 2500rpm. After crunching numbers on spring rates I know what springs I need now. A guy on Ebay sells a set with 4 different springs for Mopars with curves so they are on the way.

 

[M_Body_Coupe]

I think my distributor is worn out from being welded, disassembled and assembled so many times. I ended up re-welding the outside of the slots and grinding back the inside....

So here is the thing, why not pick up one of the adjustable aftermarket distributors instead?

Someone mentioned in this post the Mallory piece and then get yourself the matching 'advance curve change kit' and go play!!!

If you still want to tune the factory Mopar stuff take a look at the Summit kit.

I've attached the PDFs to the email for ease of reference.

 

[1MeanA]

So here is the thing, why not pick up one of the adjustable aftermarket distributors instead?

Someone mentioned in this post the Mallory piece and then get yourself the matching 'advance curve change kit' and go play!!!

If you still want to tune the factory Mopar stuff take a look at the Summit kit.

I've attached the PDFs to the email for ease of reference.

Because a) I bought an MP distributor thinking it would be adequate b) I wanted something stock looking c) I don't think Mallory is available even if I did want to spend over $600 USD d) the Summit kits are for Mallory style distributors which the later MP is not. As mentioned I bought a spring kit which should allow me to get a decent curve.