adjusting vac can timing

[TimDart]

Guys, i'm in the final stages of timing my FBO custom curved distributor. I need to reduce the vac timing by adjusting the allen key in the vac can.
1 Do I turn the key in or out to reduce the vac timing?
2 what size key should i be using? I live in the UK and all our sizes are metric!

 

[GoodysGotaCuda]

http://www.sweptline.com/tech/engine2.html

 

[TimDart]

Thanks Mike, that article is spot on and gives me the final info I need!

 

[GoodysGotaCuda]

Notta problem.

 

[moper]

Interesting read. really good info. But I think it's a little off on the vaccum adjustment description tho. You are not adding or removing degrees of advance. You are adding or reducing tension on the preload spring. That means it takes a higher or lower vaccum signal to move the diaphram the same amount to get your desired number of degrees. The vaccum can always has the same amount of movement, and the degrees it adjusts is usually stamped into the arm itself. So by adding all the preload, you make it too hard for the engine to get the vaccum advance in. (like unplugging it at that point.)

 

[64ragtop]

So, the vacuum advance can acts like a switch??? When the manifold vac. exceeds the preload spring, the can adds its' fixed amount of advance all at once, not gradually???

 

[moper]

No, it's not a switch...It's a progressive resistance. You change what level of vaccum starts the can adding advance, and that in turn will change the resistance curve of the spring. You cannot change the total amount of advance unless the spring gets so preloaded, the vaccum doesnt reach high enough to move the arm that far. The more tension, the more vaccum required, and the less advance at a given load. I say load because rpm factors only in creating the vaccum signal itself. More tension leads to less advance at certain throttle positions, more as the vaccum rises as the throttle is closed (not closed totally, just closing a little from like 3/4 throttle), or as rpm rises with the throttle at a steady state (like when you crest a hill, but dont let off the gas). Basically, you want the least preload as possible without having popping or surging. More total timing with the vaccum working right means better milage, and cleaner running.