Mechanical Advance

[NickRCarter]

On checking my mechanical advance with my dial back timing light it showed only 10 degrees of mechanical advance. The dizzy is a GM HEI with a MSD 6AL. A call to MSD Tech told me that the light will not work with the MSD system. So muggings here went and bought an MSD light. Yes you are ahead of me I think, you guessed it, this light gave exactly the same readings!
So the problem is, when I check the weights by hand, there is plenty of movement. Then I put the cap on, start the motor and all I get is 10 degrees of mechanical. I've tried all the springs in all the colours and obviously all that does is change the commencement of the advance.
Any ideas Guys as I have run out of them and the patience is getting a tad low too!!

Nick

[Big-Foot]

Nick - How many RPM are you reving it to when you are checking it?

Also - if the springs are weak, the advance may not be returning itself all the way to the -ZERO- line.

[NickRCarter]

Hi Randy,

Thanks for that prompt reply. I've revved it up to 4000rpm. I think it is settling back to its retarded position because each time I have the cap off and change the springs it commences at my idle speed timing and after revving returns to it. But whatever I have attempted it only ever has a mechanical range of just 10 degrees.

Nick

[Jack21]

I use a garden variety inductive timing light, and it works just fine with the MSD.

Idling, I adjust the initial lead to 14*. Then I check the total of 34* (14* initial + 20* centrifigal) all in by 2600 - 2800RPM.

If yours is not doing this, send it back to MSD and let them figure out why.

[Big-Foot]

Hi Nick,

There should be no slack in the mechanical advance springs at all - is this the case with yours? Usually there is a Pin that travels in a slot that limits the total mechanical advance. Sometimes there is a bushing on that pin and your bushing may be too big which would limit the total advance.

Jack - his distributor is not made by MSD.

[CSCHWER]

What color limit collar are you using? I seem to remember some confusion on my part when I installed mine... silver vs aluminum or something like that.

Chet

[flipper35]

I had the problem Big-Foot mentioned on another brand. Was supposed to be 13* at idle and 34* total, but was already at 22* at idle. Had it rebuilt and everything was fine from there.

[NickRCarter]

Randy,

The springs are tight and have to be stretched slightly onto their posts. If I remove the weights and the centre plate I can see how the range of movement is limited by the slot and pin mechanism. There is no retriction here and it moves through the whole range. Again, the advance movement can be demonstrated physically by hand pulling the weights out with your fingers. From this I can measure the movement in degrees using a cardboard disc. This shows roughly 15 or so degrees. Surely this should equate to double that at the crankshaft damper, as the Dizzy rotates at half speed. Or am I completely wrong here?
Nick

[Big-Foot]

Hi Nick - It sounds good so far.

Okay - Here's what you do to tell if you have too much spring rather than racing the engine to 6 grand to find out;

* Disconnect and plug vacuum advance (should always do this when checking timing)
* Set your timing at something simple like 10 degrees at a warm idle.
* Remove springs from distributor
* Start and check timing

With the springs removed, the timing should advance immediately to full mechanical advance even at an idle.
Bring RPM up to around 1500 which will keep the weights pinned to their outer limits and check the timing again.
What you should see is the full advance allowed by the distributor. That should be roughly 34-36 degrees total.
If you still only have 10 degrees of mechanical advance, there is something else that is limiting the counter weights from their full cycle of movement.

* Remember that when timing with a vacuum advance, the vacuum line must always be disconnected and plugged.

[392cobra]

Nick,
Have you checked to see if the cap or cap hold down screws are limiting movement ?

[NickRCarter]

Hi Randy,

You Guys are really on the ball!! Fred's comment was excellent as that is exactly what I have been doing this morning. And yes it looks like the weights were being restricted by the rotor cap. So I fitted other weights which appeared to allow more movement with the cap in place. Started the motor. First thing was different idle timing due to the previous weights not being allowed to return to their start (fully retarded) position. AND THEN on revving engine timing RETARDS !!!
Help Guys !!
Nick
PS Randy I'm just going to go and try it with weights out.

[392cobra]

Good going Nick.

I'm glad you understood I meant Rotor when I wrote cap.

[NickRCarter]

Thanks Fred, I knew what you meant.

Randy, Idle set a 10 degrees. Weights removed motor restarted. Rev to 1500rpm timing advanced to 35-38 degrees give or take, Vacuum line removed of course.
I have just noticed that when I secure the rotor cap to the plate via the two screws the whole unit becomes more difficult to moved as if something is binding. If I slack off the two screws lightly it frees it up. I think it might be a combination of two things. 1) Too heavy springs & 2) Probably more importantly the whole mechanism is binding.
Any ideas why it would bind and secondly why the previous run with springs in with these weights caused a retard in the timing?
Nick

[392cobra]

Nick,
I know you don't have a MSD dist but they warn to check the rotor hold down screws with their replacement rotors.

They can be too long and cause interference.

[392cobra]

I could be WAY OFF on this one.I haven't looked at a HEI dist in awhile.

Is it possible that the weights are reversed.Laying on the opposite of the cam plate that they should ?

[NickRCarter]

Thanks Fred. If you ask me I've always thought the damn things were back to front but they will only fit one way round.But there is definitely something binding but I'm not sure where exactly. It appears that when the rotor is tightened down the actual underneath of the rotor casing is fouling on something. But that does not explain as to why when I operate it without springs it shows me the full mech advance and then I put the springs on, rev the motor and it RETARDS !
Its got me.
Nick

[Big-Foot]

Okay - It looks like we are getting to the bottom of things here.

The distributor is most likely not the culprit in the retarding of the timing. What WILL cause the timing to retard is a worn / stretched timing chain. The rule of thumb on most street driven cars is 8 degrees of slop (which is +/- 4 degrees from zero)..

* With the ignition off
1) Remove distributor Cap
2) Put a socket / breaker bar on crankshaft / Hamonic Balancer bolt
3) Align timing mark with 2 or 4 degree spot on timing pointer
4) Rotate the crankshaft until you see the rotor move
5) Note the position of the timing mark to the timing pointer
6) Reverse rotation of the crankshaft while you are holding the rotor.
7) Note the position of the timing mark to the pointer.

The difference between the two is how much stretch or slack you have in the system.

This applies to gear-drives too. A customer brought me a gear drive to install in his SBC and it installed fine but had 6 degrees of slop in it from the factory.
Of course it was built by Hashimoto-Chang-Speed-Engineering in Taiwan - not Pete Jackson..

On your Rotor - make sure you have the correct year rotor for your distributor. They did make a number of electronically controlled HEI distributors that had no vacuum or mechanical advance and the rotors, as I recall, were different. Yes, the length of the screws (as you found out) is important as well.

[NickRCarter]

Thanks Randy, I really do appreciate your time. I would hope that there is no slack in the chain as it is a new motor, but I will check that.
I have made some progress. I have installed a third set of weights I have and selected a centre plate that seems to accomodate them the best. I now have the weights when installed with springs, situated with the point of the centre plate right in the apex of the weights, so they sit snug. With these weights I ran the motor without springs and it gave 30 degrees of advance. I fitted the springs and it gave me about 15/16 degees of advance at 4500 rpm. So I think we are getting there slowly. Its a bit late now for the neighbours, so tomorrow I will continue and try one lighter spring. Took it to the garage for fuel and she ran nicely.
Nick

[Big-Foot]

No problem Nick.
You really want springs of equal tension or it will not work correctly.
All your mechanical advance should be in by 3,000-3,200 RPM. With the light weight of a Cobra, you can get even more agressive and have it all in my 2,800 RPM, but I would not advise any quicker than that if you are running more than 9.5:1 compression - too much of a chance for detonation.

Keep us posted!

[NickRCarter]

Hi Randy,
Some success, put in Moroso weights. Now I get 24 degrees mechanical with an initial of 10 degrees, so total of 34 degrees which sounds safe to me for the moment until I can get the Bad Boy on the road to try.
This is all in by approx 4000 rpm which again sounds safe to me for the moment pending road test.
The engine is a 383 Stroker running Canfield heads and Comp @ 10.8 :1
Isky Mega Hyd Race Cam #201292

I may need to change the Vac Can for an adjustable one, but at the moment it appears advance is 54 degrees vac line attached.
Does this all sound OK ?

Nick