Could someone please confirm that my distributor has both a mechanical advance and a vacuum advance. I can apply a vacuum with my MityVav to the diafram(sp?) and I see the XR700 optical disk turning, but I don't know if its turning in the advance or retard direction. It rotates CC when applying vacuum. And I think the rotor turns in the CCW direction when running. The mechanical springs rotate the rotor back CW when I rotate the rotor CCW by hand.

 

To advance timing the timing disc/plate would be pulled in the direction opposite of normal rotation so, yes, your diaphragm is advancing the timing when vacuum is applied.

The mechanical advance uses weights and centrifugal force to move the timing disc/plate in the direction opposite of normal rotation.

Cheers
DD

 

Why two advances? Are they additive to each other?

 

Vacuum advance is used to enhance partial-throttle response and fuel economy.

In the old days it wasn't unusual to have centrifugal advance only. And some high performance distributors use only centrifugal advance...because partial-throttle driving isn't the primary concern.

Cheers
DD

 

Yes. And add initial advance ('base timing') as well

XX-amount of base advance + XX- amount of vac advance + XX-amount of centrifugal advance =total advance

Here's a pretty good article

How to Set Ignition Curves and Create Optimal Performance - Hot Rod Network

Cheers
DD

 

Great article Doug. It perfectly compliments Grant's Ignition Timing tutorial. Thank you both.

 

Where can I find the refered to Grant's initial timing article refered to?

Yes rhat other article was enlightening. Thank you.

 

Here is the link to Grant's most excellent article on setting ignition timing.

https://www.jaguarforums.com/forum/a...ing-engine.doc