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 BRPV 
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Joined: Fri Feb 08, 2013 2:37 pm
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8-32 setscrews work well for this. Use a .0625" orifice in original PV cavity passage through body and base, usually .040" ~ .050" is the range for the upper. Use a normal power valve and select PV spring and upper orifice size to suit. Changing upper orifice size adjusts pressure in PV cavity. Change to larger opens PV sooner so that should be used for the first change in tuning. If you are using a WBO2 gauge or logging with WBO2 you will see a small change has a measurable effect.


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Mon Jun 16, 2014 2:00 pm
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Joined: Sat Aug 24, 2013 7:57 pm
Posts: 157
Location: Frostburg, MD
Tuner,

That's fantastic! Much simpler than external routing from hat/manifold.

Thank you.

Dave


Mon Jun 16, 2014 3:18 pm
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Joined: Tue Jul 30, 2013 1:48 am
Posts: 29
i might have missed something or 6 hrs sleep isnt doing it for me today, where is the lower orifice located?


Tue Jun 17, 2014 6:56 pm
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Joined: Fri Feb 08, 2013 2:37 pm
Posts: 1744
.0625" in original PV cavity passage through body and base, .040" ~ .050"+ in the upper. 8-32 setscrews work well. Select PV spring and upper orifice size to suit. Normal PV. The pressure between the two orifices is adjusted by changing either, obviously the upper for tuning. The pressure in the PV cavity will be higher than below the carb, and lower than above the carb, allowing the spring to act as it normally does and overcome bowl (vent-hat) pressure, so the boost pressure acting on the wet side of the diaphragm does not close the valve. However, because the pressure in the cavity will be higher than in the manifold, the valve will enrich at higher actual intake vacuum than the valve would open if the cavity was not vented from above. Because the pressure drop across the carb varies with mass flow, changing the upper bleed affects the fuel flow as the boost changes.

You can rig this up for test with some Tees, bleeds, an idling engine for a vacuum source, shop air for a pressure source, pressure regulators and pressure/vacuum gauges.

This works when the PV is new, but an obvious shortcoming and concern in this is the stiffening of an aging PV diaphragm will affect the apparent spring tension, so depending on how the fuel affects the PV diaphragm, it becomes a maintenance item.


Tue Jun 17, 2014 8:35 pm
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Joined: Tue Jul 30, 2013 1:48 am
Posts: 29
excellent, thankyou Sir!


Tue Jun 17, 2014 8:46 pm
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Joined: Wed Oct 30, 2013 7:46 pm
Posts: 34
Location: Peoria, AZ
I just have to ask, is this for a blow through application? I am struggling to understand for a draw through application. Thanks.

Paul


Sun Sep 21, 2014 3:38 am
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Joined: Fri Feb 08, 2013 5:50 pm
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Yes, either turbo or Procharger blow thru apps.


Sun Sep 21, 2014 9:27 am
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Joined: Wed Oct 30, 2013 7:46 pm
Posts: 34
Location: Peoria, AZ
OK, that makes a lot more sense. I have a draw through system I run on my parents boat and could not wrap my head around how it would work on a draw through. Then I realized it was probably for a blow through. Thanks. Just needed a quick sanity check.

Paul


Sun Sep 21, 2014 8:38 pm
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