The car is a 1966 mustang with a 289 4 bbl engine and C4 automatic. Engine was recently rebuilt. It has a slightly larger cam measuring duration @ .050 at 204 intake and 214 exhaust. Rest of rebuild is pure stock. I have the timing set at about 12 degres BTDC. I can get 18-19 inches of vacuum in Park at an idle speed of 800-900 RPM. When I put it in drive vacuum drops to 13 inches and RPM to 600, I tried cranking up the RPM with it in drive but no increase in vacuum. I tried disconnecting and plugging the trans modulator hose but that did nothing. Is this normal?

 

What stall converter? And 18-19 idle is pretty tame.

Regardless, does it idle well in gear and run right? If yes, then don't worry.

 

The transmission was rebuilt with a stock converter.

 

Im gonna guess its normal...Engine rpm drops due to load when in D adding load always drops vac. I get 13-14 at idle 0 at WOT and 25" coasting at high rpm.

Again if it runs fine dont worry.

-Gun

 

don't forget that when you drop it into drive you probably stand on your brakes, when you have a servo this can change the vacuum reading as well.
like 67m302 said 18-19 is very good vacuum reading. and I think 12-13 in drive at 600rpm is a good signal.
there's probably no need for worry

 

If I had 18" vacuum at idle, I'd suspect that my timing chain jumped teeth and advanced the crap out of my cam.