I have a 540 dart motor with a vacuum ploblem, @ idle it has about 7" when you open up the throttle it gets about 18" and maintains it. From what I understand this is totally backwards from what it should do. The guy who build the motor for me can't believe it and has talked to his cam people and his head guy, they say not possible. the cam company say this should have tons of vacuum. The car does not have any vacuum leaks, we tried 995 and a 750 carb, timing from 10' to 18' @ base, closing the clearance from .026 to .022 nothing helps. The cam specs are: intake 740 exhaust 722 lobe center 108+2
gross lift intake 430 274' @ .050
gross lift exh. 420 282'@.050

If any one has come across this before please help me out.
Thanks
Ron

that is a huge cam with a tight LSA , i'd suspect that 7-8 inches of vaccuum is about perfect.

when you open the throttle, as long as you aren't Wide Open Throttle, vaccuum always goes up.

if you crack the throttle wide open, then you should see the vcaccuum guage go to zero.

Unless I misread something, then your motor sounds like it is running right.

Yep, I agree. That cam even in that big of an engine won't make tons of vacuum as they put it. And slightly opening the throttle will raise it considerably.

Jody

Also, if you are measuring above the throttle plates (ported vacuum) you will get these results.

Thank everyone for your help, after reading the posts I went back to the carb it's not done but it'sthe right direction.