Can I fine tune the timing on my carberated sportster using a vacuum gauge. After lowering the idle to its lowest running point, (preventing vacuum advance) adjusting the timing plate to highest vacuum and then backing it up a bit ?

 

This will possibly give you good timing at its "lowest running point (preventing vacuum advance)" but have absolutely no correlation to proper timing at any other RPM and load.

 

Isn't it true that after modifications ( Cams, heads, carb & exhaust ) the timing requirements will be different than stock ? Timing using vacuum is commonly used on automobiles. Why not here ?

 

Yes, it is true that the timing requirements will be different, need more initial and low RPM advance but usually not more total centrifical advance at higher RPMs. This requires internal distributor work such as springs and advance plate stops. Using your vacuum advance method will move the timing at all load and RPM points, potentially damaging to the engine. "Timing using vacuum is commonly used on automobiles"??? This proceedure is totally improper for setting the timing on, especially, a modified, performance engine, there is less room for error on such engines.

 

OK, I can see that. Thanx.

 

I may be way off...thinking that using a vacuum gage is best for setting the idle mix?

 

Using a vacuum guage is a good way to set the idle mixture, turn mixture screw until maximum vacuum then back it out 1/8 - 1/4 turn for a slightly more rich mixture.

 

a vacuum gauge is used to check vacuum strenth for the advancment of the timing . A timing light is used for timing. the vacuum line should be plugged for this procedure.