badazz84camaro

in gear at idle my vacuum is between 5 and 7in. what size power valve should i put in it? does this vacuum seem low for my combonation? the combo is in my sig. what effects will a wrong size power valve have on driveability and performance?

five7kid

iTrader: (14)

It seems a little low. You're on the edge of a "performance" cam vs. street cam.

You might try getting the vacuum readings at various cruise speeds, say 35, 45, 55 mph. Try a PV 2 inches lower than the lowest cruise vacuum observed.

Too low a number usually causes a dead spot off-idle. Too high a number makes it rich under cruise.

badazz84camaro

the needle on the vacuum gauge waivers a bit and it is hard to get a good vacuum reading at idle. when i turn the idle screws, its hard to tell if the vacuum is going up or down. what would cause a low vacuum condition, i talked to someone and they said thats probably all im gonna get because of the size of the cam and the compression ratio.

five7kid

iTrader: (14)

Well, those are the things the PV affects, so you need to decide whether or not it's worth messing with.

IHI

iTrader: (1)

My motor warmed up ideling in gear pulled 5", I've always cut the vacuum reading in half and run that size PV-2.5 in my case.

Great off idle, great crusing, no fumigation happening at the stoplights either

badazz84camaro

i tried it and it was loading up bad, it would barely run

IHI

iTrader: (1)

It was loading up bad at an idle? loading up while cruising?

What's your FP set at? 6psi-8.5 psi recommended

After the car is warmed up, where are your floats at? Once you get them adjusted so fuel either BARELY trickles out of sight pulg or is level with sight plug, give the throttle a good tap so the fuel level will drop in the bowls and have to fill themselves back up-this will tell you exactly where the floats are "resetting" to.

You could possibly have some contaminants in front or rear/maybe both needle and seats, this will cause excessivley rich conditions and want to flood the car out. Pull them out and just check them, very wuick and easy.

Where are your idle mixture screws set at? Turn them all the way back in until they seat, then back them back out 1 1/4 to 1 1/;2 turns, this will be VERY close ball park of where they need to be for proper idle mixture, you may possibly need to have some 1/8" holes drilled in your butterflies to allow more air to equal out the mix. If you have your idle screw turned up to keep the car running, it's opening the trasfer slots and allowing more fuel to be dumped into the motor by opeing another circuit in the carb, this is a bad thing. By turning idle screw back down to cover up the transfer slot it may make the car run very rough/stumble and deem the drilling of the butterflies.

With only 5" of vacuum a 2.5" PV is where you need to be.

F-BIRD'88

A 2.5" power valve is too low for this combo.

If the lowest idle vacuum is 5" then the power valve should be just lower than this. 4.5" is best.
While cruising, the manifold vacuum will be higher than 5"
The power valve will remain closed like its supposed to.
As you open the throttle the manifold vacuum will drop.
The engine , at some point will want more fuel as the throttle is opened toward WOT. At 4.5" manifold vacuum the valve will open. 2.5" is likely too late requireing a larg(er) primary jet to compensate.

A higher 4.5" PV will allow a smaller primary jet (better cruising) but switch (open) to full power mode when needed.

All depeneds on the sizes of the air bleeds and power valve channel restrictions as to when the transition point is required.

F-BIRD'88

Drilling the primary throttle blades for extra airflow at idle is rarly required not recomended. There is a much easier way if you need more idle speed/airflow. Its even adjustable.