im tring to set my timing on my 1920 holley. i get it set at 10 btc but when i take of the vac advance hose it goes way off the marks . so i ask whats happeing here never had this problem before

I think your problem is that you're trying to set the timing on your Holley 1920 carburetor instead of on your distributor!

Seriously, if you get a bigger-than-normal jump in timing when connecting/disconnecting the vacuum advance, it's an indication that the idle is not set correctly; the throttle is too far open at idle. Could also be worn throttle shaft/bushings.

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Too many people who were born on third base actually believe they've hit a triple.

then whats the best way to get things set to the right idle ? and how can i tell if it's the throttle shaft/bushings.right now i have to set my idle a little high aorund 1000 just so it wont die wehn i put it in gear.

i know i have a vacuum leak that i have yet to deal with but it has never been that much of a problem untill now. the bolt on the center of the intake and exhaust just turns cant get it tight and cant get it out.

Well, first everything has to be in proper repair (vacuum leaks fixed, carb and distributor working right, etc.
Spray carb cleaner at both ends of the shaft where it passes into the carb body. If the engine idle speed or quality changes significantly (or you hear the carb cleaner fluid get sucked in "SLURP!") then you've got this problem.
That will interfere with your ability to set the timing correctly. Timing needs to be set *at the specified engine idle RPM*.
That's how most things break (they don't make a problem 'til they make a problem).
Cut/grind off the head of the bolt, remove and rework the manifolds, then deal with getting the remains of the centre bolt out with everything all apart.

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.