Topic: Vacuum Diaphragm Adjustment

[ClevelandKid]

Last summer I was playing around with my 70 351C Mach 1. Setting the idle and checking the timing. It runs like a Swiss watch. But I remember reading an article several years ago about adjusting the dual diaphragm vacuum advance so I put a crescent wrench on the snout of the advance and gave it a slight turn and noticed a slight increase in RPMs. I took it out on the street and did a hard launch and about cremated the tires and the car took off like it was in after burner. It was scary fast. I had marked the exact location before the adjustment and returned it to the original position. Can anyone explain what took place ? I did not know that turning the advance one way or the other would have such an effect on timing. I did not turn the distributor at all just the snout of the vacuum diaphragm.

[67gta289]

More importantly, why the heck did you turn it back

[ClevelandKid]

Because not understanding what was happening mechanically I didn't want to run the risk of damaging something down the line. 

[J_Speegle]

Just a guess (not a tech) that you adjust total mechanical advance for the distributor. Since the diaphragm is connected directly to the plate the points are mounted to the adjustment would adjust a greater or short arm length

Didn't recall that factory diaphragms were adjustable - thought it was aftermarket ones but what do I know about the subject

[ClevelandKid]

I forgot to mention I have a Pertronix unit installed. No points. That really stumps me. If the weather here in Upstate NY EVER warms up I'm going to try it again. Ii have a small mark on the vacuum diaphragm and will set it to that mark and then move it a little more to see how much she will take. I'm in a rambunctious mood after the longest winter I think I ever remember. 

[65pon]

At full throttle the vac advance doesn't do anything anyway.  It is only active at part throttle.

[dave6768]

The only damage you could cause (other than shortening your tire life  ) is if you are pinging at WOT.  If not, you are good to go.

You probably adjusted the timing at idle...this may cause the engine to run a bit warmer and may make it slightly harder to start...but shouldn't cause any "damage."