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Manifold Vacuum Increasing not Decreasing with Throttle??

Will-Fonkjam

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FBBO Gold Member
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I am curious to know what would is causing my manifold vacuum pressure to continue to climb as engine RPM increases from idle.
I am only tuning the car in the garage at this point and have not tested while driving. All information I have collected (internet) says manifold pressure should began to decrease as you apply more throttle. Yet, my mine continues to go up!
Initial vacuum is 9-11 In Hg @ 900-1030 rpm & vacuum climbs to 20 In Hg as throttle increase to ~3500 rpm but never drops down. All measurements taken at the intake manifold port.

Build details:
Motor: 535CI Stroker,
Cam: Hughes Cam SER5458BL3-9, Duration 254 int., 258 exh. with lobe Separation@ 109 degrees.
Carb: Holley 750cfm with vacuum sec. (this is a rebuilt carb for engine startup & test. Planning to go to EFI);
Timing: 25 degrees initial; all in at 45 degrees at ~1700 rpm (dist. vacuum port not connected);
A/F ratio: Gauge reads 14.6-15.3 at idle & drops to ~13.5 at throttle.

Any help would be appreciate FBO.
Thanks, Will
 
Manifold vacuum increases w rpm in park or neutral. It takes load on the engine to make vacuum decrease. Sort of like a turbo in park will not build boost w rpm, it must be under load.
 
Snap the throttle open & closed and watch the gauge... It'll drop then recover... Slowly adding throttle on an engine under no load vacuum will increase cause you get past the reversion due to cam overlap..
 
Car is a manual shift.
Yes, I did check for vacuum leak via propane, visual & the process of elimination but nothing was found. I don't even see a difference in the idle when I intentionally create a vacuum leak at the front port under the carb. However, if I unplug the large port on the rear pass side on the manifold itself, it will stall out completely.
It seems I will have to drive the car. This just makes it dial in the idle mixture screw setting... good thing I install the A/F gauge & O2 sensor. It does react to the screw adjustments.
 
Most likely scenario: you aren't holding the throttle completely wide open. You won't get low vacuum readings with part throttle and high rpm.
 
All above sounds correct. Just the same, check gage to be sure it’s PSIG and not PSIA.
 
All-in timing seems a bit much. Should only need about 34-36°
 
Agreed that you have to put the engine under load.
 
My 500 stroker with a slightly smaller cam would pull 8.5 inches of vacuum at idle in gear and 17-18 inches cruising on the highway. The high overlap of the cam bleeds off vacuum at low speeds.
 
Idle mixture is likely too lean try around 13.8 - 14.2.
You need to drive it.
 
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