[joes455]

Had an issue with low power take off, trying not to activate the secondaries .like as heavy throttle from a red light The power valve crossover point governed by the pv spring tension was not happening even with vac down to the rating of the pv,had very lean off the scale readings.After further test Ive come to learn the pv ratings based on eng vac that holley recomends is foolish.I SET MY PV 2 INCH BELOW IDLE OR CRUISE VAC.Even with a 10 is was still not opening at heavy pt.It wasn't untill i removed spring and sttetched it out that fuel enrichment was being added aprox 13in pv is what it took.fwiw

[steve25]

It’s times like you have where if it a 4 idle mixture screw carb its a good test to swap ft and rear metering blocks , tripple check gasket and see if the issue follows the block.

[PAUL K]

I agree with choosing a power valve a little lower than the cruise vacuum. Bigger cams with a tight converter pulls the engine vacuum considerably lower than the cruise vacuum.

[Tom Vaught]

Originally Posted by joes455

Had an issue with low power take off, trying not to activate the secondaries .like as heavy throttle from a red light The power valve crossover point governed by the pv spring tension was not happening even with vac down to the rating of the pv,had very lean off the scale readings.After further test Ive come to learn the pv ratings based on eng vac that holley recomends is foolish.I SET MY PV 2 INCH BELOW IDLE OR CRUISE VAC.Even with a 10 is was still not opening at heavy pt.It wasn't untill i removed spring and sttetched it out that fuel enrichment was being added aprox 13in pv is what it took.fwiw

1) In the old days, Holleys PV Valve selection point was:

a) Read Idle Vacuum. Say your idle vacuum is 12"
b) Divide Idle vacuum by 2 gives you 6"
c) Add .5 to the number in Calculation "b" and you get a 6.5" Power Valve
setting which is stock in 95% of the sold Holley Carbs.

2) That calculation was based on Vehicles with gear ratio numbers much higher than the gear ratios of today. In the 60s many street performance vehicles had either 3.55 or 3.73 (Chebby) axle ratios. That is where most of Holleys Carb sales came from.

3) If you have a gear ratio that is a lower number (say 3.23) vs the 3.55
gear ratio, I typically would install a HIGHER Number Power Valve like a 8.5"
Power Valve vs the 6.5" to get the enrichment started Earlier under vehicle load conditions. Some work vehicles actually had 10.5" power valves installed to get the fuel from the Power Valve circuit started sooner. THEY ALSO INCREASED THE SIZE OF THE PVCR RESTRICTIONS FROM THE STOCK .059" DRILLINGS TO IN MANY CASES TO .073" DRILLINGS.

4) What mods did you make to the PVCR drillings, joes455?

5) Without the drillings (along with the TIMING of the Power Valve opening points) you have limited chance of success.

6) Higher Load on the engine required more fuel enrichment, that is why the PV Valve and circuit is there.
So you are trying to not open the secondary barrels (not add enrichment from the secondaries) but have an issue with
getting any fuel under load to the primaries. The PV Timing does little without understanding how the circuit actually works.

Tom V.

Your post about being lean makes perfect sense.

[joes455]

I have a billet block and now that i can see the kickin point of the pv spring via wideband and vac gauge .Im in the process of testing different pvcr's .I had a .089 with a 73 jet turned out to be way rich.Went down to a .050 way lean,current .073 but have not tested it

[joes455]

Its tough to get this right when striving for a lean cruise 14-15 afr.and not get too lean or rich at wot.I think I nailed it .tested how lean this eng will run by turning off fuel pump while cruising as the float was getting lower i could see the afr going higher.It got like 16.5 afr with no surge or anything strange happening.turned pump on and afr came up to normal

[Tom Vaught]

16.5 to one would be considered a Lean Burn engine.

Lean Burn Engines will run fine under that air/fuel ratio under a light load with no engine mechanical issues. As far as Emissions Issues go, depending on the engine, a 16.5 AFR might cause higher NOx emissions (Chrysler and the EPA had issues with the Chrysler Engines running too lean and causing the NOx emissions to be high and fail testing).

In your case, you should still be safe, as far as getting better fuel economy under light loads. The trick will now be dialing in the OTHER circuits for acceptable Acceleration, Tip-In Performance, and Passing modes on the highway, as well at WOT modes.

Good Luck.

Tom V.

[joes455]

Thanks Tom,I'm striving for 14.5 cruise.Its nice for once to know and verify when pv circut is supplying fuel and see it happen in real time at the vac where it was required,before sec application.

[Tom Vaught]

We used to use $3,000 Horiba exhaust gas analyser systems to do what most hot rod systems today can do for a few hundred dollars.

But the point is to get very good at where the different carb systems are and then how to manipulate them for your best drive and performance.

Very Good Carb Tuning is rapidly becoming a lost art. Have fun!!!

Tom V.