Comanche3Six

This is a potentially lethal situation for an engine with a roots type blower. Because the carburetor sits atop the blower and can't get a accurate vacuum signal. It is possible for the power valve to close while the engine is in a throttle up position. This would create a lean condition while the boat is under load and destroy the engine. Always boost reference the power valve to the intake manifold.
I know you have stated it has had no ill effect on your engine, but I am thinking you were just dodging bullets.

tomcat

This is how I think about the boost reference thing. With the throttle blades partly open a naturally aspirated engine can only get so much air. With a blower under the carb you can get more air with the throttle blades in exactly the same position. Why? Because the blower creates a lower pressure under the carb than the engine could on its own. This higher vacuum could cause the power valve to close. The condition is most likely at cruise RPM. Once the throttles open a lot, there's no vacuum anywhere.

Holley's blower carbs are boost referenced and many people do it this way. It's foolproof, since it requires no testing and tuning, and even if the external line gets cut it will fail safe because then the power valve will always be open then.

But you don't have to boost reference if you measure the highest vacuum that can be produced under the carb and pick the power valve accordingly, meaning a lower number so it will not close under cruise conditions. This is no different than the way you would select a power valve in a naturally aspirated engine, as described so well by others above.

Hey caveman: Good to hear you holding forth on this subject. Did you get your fuel system sorted out?

KNOT-RIGHT

Yes I did Tomcat I ended up running returns to all the tanks.
I run a claysmith mechanical Also backed by Comp 140 electrics
to the fuel rail. Then at the end of the fuel rail I run the mallory
return style regulator back to tank. It was a plumbing nightmare.
I have 8psi Rock solid.

What I also Did was removed the #6 lines that feed each bowl. 4 to
be exact put these in a gallon container and started the engine it filled
the gallon in just under 11 seconds.

Now thats some fuel..
I made a note with the date so If I ever need to refer back I have
baseline.

Gerry

FloridianSon

I just went through this on my N/A motor but with the new BG carb.I first made shure I had the base timming right (with my BG you need 18-24* base ,I run 24*.Then I warmed it up on the water full oil temp. and water temp.Then I tweeked the Idle jets to the highest Vaccum and reset the rpm in gear to 750.Then I took a vaccum reading at idle and it was 12".BG says to split that number in half.My carb came with a 6.5 power valve.Then I took it for a ride and just like it has been said in this post I ran it up to 3800rpm and the power had just opened. From a dead stop right into full WOT she is running flawless no spitter no spudder and the plugs reading for what it is worth is right on.I changed nothing just took it out of the box bolted it on and tweeked.Bottom line you need a vaccum gage and 1/2 on the water.