SB

iTrader: (1)

Definitely let us know any updates.

I have this feeling that in the end the rear PV will be removed and secondary jet size up about 6 sizes.

IMHO You want to slowly go richer and richer from secondaries just open to target A/F ratio at WOT.

Bill 3

Makes sense to me.

On a dyno, if only checking max hp and tq, the throttle is going to be wide open which opens PVs. They apparently didn't care about tuning for any throttle position in between - kind of disappointing. These engines like their fuel and obviously need more before the PVs are open.

SB

iTrader: (1)

Yeh, a PV is an on off switch. So having two of them, is, wow.

I forget exactly why some carbs have two powervalves, I want to believe, but could be totally wrong, it was for appilcations that used a bigger than normal carb (say, an 850 on a 350) for a certain cid and road racing or etc. IE: a lot of different loads for same throttle setting (think manual trans around corners, downhills, uphills, brakes used at same time as throttle to settle front end) or etc. Again, it was something like this I believe.

Baja Rooster

iTrader: (1)

Well there you go - disconnecting the secondaries turns out to be a bad idea, lol.

What exactly does a power valve do? I've read up on QF's website and they say go with one set at half the vacuum at idle (#12 at idle use a #6 PV) but what exactly does it do and when? It seems like it riches the mixture?

mike tkach

iTrader: (3)

when the power valve opens it richen,s the mixture,lets say your car is cruising and the throttle blades are barely opened,it does not require a lot of fuel at that point and the power valve is closed because the intake is in high vacuum,now you step down on the gas and the manifold now goes to low vacuum,now the power valve opens and feeds the xtra fuel the engine needs.most engines are happy with a power valve in the primary and a plug in the secondary.because the secondary has no power valve you jet it 8 or so jet sizes richer to make up for not having a power valve.

Baja Rooster

iTrader: (1)

Got it. Kinda like an automatic tunable accelerator pump?

mike tkach

iTrader: (3)

the acc pump is only for a shot of fuel so the engine does not get a lean spot when the blades get opened.most carbs have adjustable acc pump.

fbc25el

Make sure your base plate is drilled in the rear like in the front for the vacuum to operate the power valve.

Baja Rooster

iTrader: (1)

So would you jet a carb for cruising (light load/steady throttle) and then size the PV (heavy load/low vacuum) for a suitable AFR? It seems like the PV would be more critical on the primaries when trying to get these pigs up on plane.

SB

iTrader: (1)

Yup. You got it pretty good.

The PV is opened/closed from intake vacuum, so it works the same if on primary vs secondary assuming the PVCR (power valve channel restriction) is the same size.

However, the PV is mounted a little higher than the jets and faced the same way as the jets.

Uncovering a rear PV from fuel is alot easier on the sec side vs primary when accelerating. More of a car issue vs I/O boat, but...

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Depending on boat and powerplant, not all need secondaries to open for planing........but I would venture to say well over 95% + of singles do !