Smitty

iTrader: (1)

I need a little edumacation on power valves. I need one to come on (or kick in sooner) so do I need one with a low number or a high number??

Any help is greatly appreciated !!!


Dave

MESABALANCING

The Number Needs To Be Larger Be Careful Because Sometimes They Will Open Up Around Idle And You Can Get To A Point Were You Develop An Idle Problem. If You Want We Do Lots Of Carb Work Here In The Shop And I May Be Able To Help You Through A Problem Your Welcome To Call Anytime Sincerely Laz Mesa

Cord

iTrader: (2)

Higher number. They are based off the vacuum level of the engine. I believe you want to be two sizes below your idle vacuum. A number 8 is very common because many engines idle at 10". A higher number powervalve would thus open sooner on a engine with a lower vacuum. The problem of course would be the powervalve opening to soon and the engine running rich.


Guess we were typing at the same time...

Smitty

iTrader: (1)

Ttt

KNOT-RIGHT

Yes. What has been said is correct. The # on the Power valve is the measurement of the vacuum in "HG" meaning a 6.5 would open at 6.5 HG and remain open anything below 6.5. If you connect a vacuum gauge under the carb (normaly Asperated) and you read 12 HG of vacuum at idle a starting point would be 10.5. However Heavier boats and or different setups play a role. The Idea behind the power valve is it enrichens the carb circuit when the engine needs more fuel. Hence low vacuum (Load). Obtaining the correct
PV in a boat is a little different then the street. The Ideal situation would be
to have the power valve closed at minimal planing speed that way when
you give it throttle the vacuum would drop opening the power valve. Some
choose not to run power valves by installing plugs and upping the jet sizes
by 7-10. Please Remember to check plugs accordingly. Not to confuse you
anymore. Some Supercharge applications Boost reference the power valve.
meaning they block the signal from above the blower and reroute it beneath
the blower so the carb see's the engine vacuum not the Blower.

Hope this helps
Gerry

Cord

iTrader: (2)

I do not run a power valve on the secondaries. By the time the secondaries open, the powervalve would be open, so you might as well add fuel with the jets.

Smitty

iTrader: (1)

Thanks for the ideas. I am going to get some different numbered ones and try them out. I have qiuck change setups on the carb so I should be able to do it right on the water.


Thanks !!!

bpine

Gerry, can you elaborate more on this boost referenced thing! not sure I still quite understand! I have two boats with blowers! 1 has the merc sc600! B&M 420 Mega with 2 Holley 700 carbs! The other a Blower Shop 12-71 with 800 demons?
The Question? I realize you said "SOME". For the carbs to be Boost referenced to the manifold pressure. there would have to be some line or tube from the manifold to the carb. Neither of mine do! Also both boats are almost always under boost, measured at the manifold below the blower???
so the power valve would never open. am I making any sence or am I totally confused?????????

Bryan

KNOT-RIGHT

Bryan,Let me try to clarify this. You said "Both boats are almost always under
boost. So the powervalve would be closed. This statement is incorrect. Because actually the power valve would be open.
Think of the powervalve as a valve that allows more fuel to enter the engine.
It will open on and after the # stamped on it. Meaning a 6.5hg will open at 6.5 HG or under meaning closer to 0. So in simple terms this scale.
7hg-6hg-5hg-4hg-3hg-2hg-1hg -0-1psi-2psi-3psi-4psi-5psi. One side is
vacuum other is pressure. The PV is held closed by vacuum. The PV in this example (6.5HG) will be open from 6hg on the scale to as much boost as
it will see. Still With me?

A engine will make vacuum at low load, Idle situations. The butterflies
in the carbs are closed and the engine is trying to fill the cylinders, Creating
vacuum. When butterflies are opened (whacking the throttles) it allows air
and fuel to fill cylinders droping vacuum. If a supercharger were present it would start making boost and moving to the psi side of scale.

Text book Blower calibrations on carburetors "boost reference" the carbs.
This is accomplished by blocking the PV signal above the blower and adding
a vacuum hose beneath the blower so the carb see,s the engine vacuum not the blower. Without this mod there could be a time where the engine is
demanding fuel but yet above the blower it doesn't open the pv creating
a lean condition.

There are many who run without boost referencing they have compensated
by adding additional jet sizes to overcome the leanout.


This has been discussed in this forum and others in great lenths.
here http://www.offshoreonly.com/forums/s...oost+reference

Here is a How to.
http://www.offshoreonly.com/forums/s...oost+reference

From my own situations I can say that there are actually times where a
power valve is not practical meaning it is alway open.
One example is two 900hp supercharged 540,s spinning 32inch props with 1.33 to 1. It will produce boost as soon as the throttle plates are open
where there is no minimal plaining speed in which I can get the P/V to close
In this case. I am also running the lowest pv available a 1.5HG. Now with this same setup I can throw my small props on 28inch and then I can get up to minimal plaining speed and actually get enough engine vacuum to close the p/v. This is the optimum p/v situation. Because as soon as I increase throttle the vacuum goes to boost and now Pv is open.

Sorry Im so Dam long winded, I have had way to much Coffee this morning

bpine

Gerry, Thanks for taking the time!, I'm with you.

Bryan