Ok friday my car was running like a dream! I have never been happier with it...
Saturday morning it started running bad.

I put a timing light on it and found that I have 40 degres of initial timing!
I reved the engine to 5000rpm and the timing stayed the same.

Today I decided to poke around and see what's broken. I assumed that the weights were stuck.

After taking out the dist. and checking for any binding I couldn't find any problems, the springs were in place, when I pulled on the weights it advanced smoothly, I put a vacuum pump on the vacuum advance and it appears to work properly, it starts moving at 10psi? and is finished at about 20.

-I put the dist back in and kept the vacuum pump on the advance.
-The dist. was very close to where it was when I pulled it out.
-I set it to 36 total, but at idle it stays at 36.
-I pumped up the advance and it moved the timing as I would expect.
-Another issue I discovered was that the vacuum port on the carb sucks 25psi all the way upto 5000rpm! (I need to double check when it start pulling vacuum, but it didn't pull much if any at idle)

I am using some sort of eletronic points, pertronix i think.
Could the pertronix cause these symtoms?

What could the problem be?

thanks,
~Jay

 

Your distributor's total advance (centrifugal + vacuum) should be around 30 degrees at 3000 rpm or so. Centrifugal advance can be adjusted by changing weights to change the curve or by limiting the stops. Vacuum advance can also be adjusted by using custom diaphrams made by accel, etc. But I think you first must understand the difference between ported and manifold vacuum. Manifold vacuum is just that, it hhoks up to the manifold, or the carburetor below the throttle butterfly. This provides very high vacuum at idle and decreases to zero as you approach WOT. Ported vacuum would be a port loacted on the carburetor just above the throttle butterfly which is zero at idle and increases as you accelerate. Set your initial timing with the vacuum disconnected. Watch your timing mark as you increase RPM to see that your centrifugal advance is working (a timing light with a advance dial makes this very easy for you to determine the total advance for each system). Normaly on performance cars, you hook up the vacuum advance to the ported source. (it's easy to check, it's the one that opulls no vacuum at ide.) Some cars have the vacuum advance hooked up to manifold vacuum (this advances the spark under light loads to boost fuel economy, ie. less fuel for the fire, light the match a little earlier). Even if you didn't have the vacuum advance hooked up at all, you should still get around 20 degrees of mechanical advance, so at 35 degrees total, you would be at 15 degrees at idle, which is better than what you're dealing with now. So I kinda got the feeling that maybe your distributor is hooked up to manifold vacuum. Hope this helps, Jeff

 

Thanks Jeff,

I do have the vacuum hooked up to a ported source. I tested it there is near zero vacuum at idle.

The timing does't appear to change at all with RPM. (except wht vacuum advance is hooked up)

As I said before I can't find any mechanical problems with the weights nor the vacuum advance.

This is how I tested it:

Hooked up timing light
put the clamp on the #1 spark plug wire
disconneted the vacuum advance
pointed the light at the harmonic balancer
kept raising the degress on the light until the timing mark was at zero
the light was set at 42 degres and the mark on the balancer was at zero at idle
I accelerated to 5000 RPMs
the mark stayed at zero
I hooked up the vacuum advance and the timing went higher...but i didn't take note of it.

I then took the distributor cap off and twisted the rotor to feel for binding- it felt normal

I then pulled the dist out to look for any problems and couldn't find anything.

Did I mention that the car ran like a dream the day before?

On the engine dyno the ideal total timing was 36 degres.

Any thought as to what change over night while i was sleeping?

thanks,
~Jay

 

Well, it seems to me you found the problem. The mechanical advance is not working. Either stuck at no advance or stuck at full advance. Check that again, please. G/L

 

Do you have any tips for checking mechanical advance? It feels normal when I twist the rotor, if I take the rotor off and fiddle with the weights they feel fine, I don't feel anything they could be binding on.

I'll try to look more carefully.

thanks,
~Jay

 

If you grab the drive gear at the end, and the rotor in the other, you should be able to rotate the gear opposite the rotor, what-ever the mech. advance stop lets you. The springs should return the rotor back once you release it. I bet it is jammed.

 

I did exactly that this morning... After the car cools down I'll double check.

....hmmmm that makes me wonder.... maybe it only gets stuck when it's hot??

~Jay

 

Just for fun, go ahead and remove the springs and pull the weights off. Make sure the inner shaft turns freely independent of the outer shaft, clean and lube if necessary. also put a little bit of lube on the weight pivot pins. When assembled, you should be able to pull the weights out by hand easily and see the mechanism advance relative to the pints cam, feeling only the resistance of the springs. There should a little up and down play on the dist shaft, but it shouldn't be excessive. The mechanical advance should not be affected by temperature. Since the car ran fine before, check the obvious simple things, distributor cap, rotor, dwell angle, points ground strap, etc.

 

I take it apart tonight. Hopfully I'll find something.
~Jay

 

Sounds like the advance springs are weak and are going to full advance on start up. Put heavier springs in it.

 

That appears to be the case, but doesn't it seem strange that they would become weak overnight?

~Jay