well i went to measure the idle vacuum on my b motor, and i noticed that the gauge was jumping around but as soon as the motor was under load the gauge read a steady digit.
does any one know why this would happen
and what is regular idle vaccum level?

 

Idle too slow?

Possible imbalance in rotor sealing - - would show as compression variance.

Is it jumping in sync with engine rotation speed, or randomly?

 

its a bit random but also my idle is not super steady. im trying to fix the idle issue and a accel issue by measuring the the idle vacuum to get the right power valve for my holley. The my carb backfired and messed up my powervalve. ( i had the dist wrong)

 

vacuum leaks? brake booster good? compression test?

 

Idle speed, vacuum range #'s, porting and compression test. Vacuum doesn't always hold steady at idle, especially on a ported engine due to increased overlap.

 

Also, depending on intake and where he is measuring, he could get an unstable vac signal (for instance reading vac on one rotor on an ITB setup).

Although, I assume he is talking a stock engine, but he hasn't provided us with that detail.

 

Last week i tried tuning my S5 mildly ported on intake engine using a vaccume gage. (old schooler V8 guys use to do this) and my vaccume was shaking back and forth but once i advanced timing all the way locked it was steady at 18ish?

 

its a stock 86 13b running a holley, and measuring vac off the manifold.

 

If the vacuum source is only from 1 rotor, or 1 runner on the intake the gauge will always shake with the pulses of that rotor. Put a restrictor in the line or put a T in the line and connect it to both rotors and it'll smooth out.

 

its a racing beat intake manifold and it has a vac source right in the middle of the manifold....