Determining correct Power valve??
 

What is the best way to determine what power valve you need in you carb? I have hooked up a vac. gauge to the manifold and have between 11-13in. of vac. I have been told to take your vac. reading at idle and cut it in half, drop 2-3in. less from idle reading. What are you guys doing to get the correct pv?

That's a decent starting point for a NA motor. If you're supercharged/turbo'ed then a whole bunch of other factor come into play.

Assuming you ARE NA, then start with what you've said, and set yourself a throttle stop that keeps you from opening the secondaries. Then go run the boat and get your primary main jet sizes nailed down. After that, run at cruise settings and look for the spot where the PV closes by slowly reducing throttle setting a little at a time. (always helps to do ALL of this stuff at a trim setting that keeps the boat in the water, so you aren't dealing with porpoising or other wildcards). See if you can tell when it closes and if you get a lean surge. Adjust as required. If you are looking for BEST cruise fuel economy, then you will need to choose your PV to just begin to open a little faster than your favorite cruise.

After you do all of this, remove your throttle stop and work on your secondary main jets. Size them for WOT throttle. If you have bogs, etc, don't worry bout them yet. Get the mains set, then play with accel pump cams and vacuum pot adjustments.

MC

Take the vacuum reading with boat in gear at idle and under a slight load then go down 2-3".

It's a blower motor with two holley 750 blower carbs.

your saying run the boat with the secondaries closed,would that lean out the motor?

No I think he is saying limit how far the throttle will open so as not to open the secondaries. The difference is that the engine will be limited in rpm and not require the extra fuel and won't lean out. Just disabling the secondaries and running WOT would have a lean issue, but I don't think that's what he's saying.

run 2.5 valves, make sure the carbs are boost referenced for the power valve to work right...Rob

supercharged applications need to be treated differently due to the way vacuum is generated. With a roots blower, the carb(s) will sense vacuum almost all the way to WOT while the intake manifold will be running well into boost. Running a reference line from the intake man to the vacuum port on the baseplate (boost referencing) gives the carb a "view" past the blower into the motor.

mc