bluethunder364

iTrader: (5)

I have a HP Holley 750 DP that has the stock jetting....75/76 . The Carb comes with a 80 on the secondary side but went to a 76 during tuning. During a roller dyno session the power valve. (comes with a 4.5)was changed because the mixture was lean on the top end. If I had the wrong PV installed would it make the car way rich in normal driving? I took a plug out today and damn the thing is way rich. Also had some exhaust work done Sat. and looked into the headers and ran my finger in and holy crap.....black soot. Something is messed up, the jetting is close to normal for that carb. Do I need a new power valve?? Thanks

bluethunder364

iTrader: (5)

Just checked the power valve....carb came with a 4.5, now has a 3.0 ?????

Doug G

Power valves open to richen the mix when the vacuum drops below the stamped number...so your PV shouldn't open until 3.0 or lower.

The power valve should be about 2 lower than your vacuum (check at idle) so if you're reading 10" at idle, a 7.5-8 would be correct.

Doug G

Straight from Holley's site....


Part 5: POWER VALVES and ENGINE VACUUM
There is a lot of misunderstanding concerning power valves in Holley carburetors. Many 4-barrels come with a particular power valve depending on the carb list # and application. Some carburetors have two power valves, while others only have one. The power valves are numbered by the amount of engine vacuum in inches at which they will open and add additional fuel to the power circuit. In other words a 6.5 power valve will open when the vacuum signal on the engine drops below 6.5" of vacuum and will remain closed above. One of the misconceptions is that they can't be trusted to work because an engine backfire can "blow out" the power valve. Many of the newer Holley performance carburetors now come with a built in power valve "blow out" protection which eliminates this problem. If you have an older model carb you can purchase a small, inexpensive, easy to install Holley kit #PN - 125-500 that will protect the power valves in case of an engine backfire in the carb. I use the CENTEK, "Power Valve Shield". See their website at www.powervalveshield.com , which takes about two minutes to install and does not require any drilling.

Many tuners will automatically remove the power valves and use a "plug" thinking this is the "hot" ticket. However, if the power valve is removed and plugged, the main jet size must be increased 6-10 jet sizes to make up the required fuel amount lost by the removal of the power valve. When the power valve circuit is plugged, part throttle fuel economy is lost and may become overly rich. Plug fouling may become a problem at part throttle.

Stock engines have high vacuum readings (10-18 inches at idle) and the Holley power valves with higher readings like 6.5 to 10.5 will work. Longer duration non-stock camshafts and other performance related parts can cause a problem, because engine manifold vacuum may be lower. The power valve, if incorrect, will always be open, even at part throttle, leading to an overly rich air/fuel mixture. The solution is to choose the correct power valve. To determine this, you need a vacuum gauge. On a manual transmission vehicle, hook up the vacuum gauge and take the reading with the engine at idle. Then use a power valve that is rated 1-2 inches below that amount. For example, a motor that shows 7" of vacuum at idle should use a 6.5 or 5.5 rated power valve. If you have an automatic transmission, take the vacuum reading at idle in "DRIVE" (with the emergency brake on and the wheels blocked) and chose the power valve 1-2 inches below that figure. You can get a little more detailed information by driving the car with a vacuum gauge hooked up with a longer hose so you can read it while driving. Drive the car at medium loads and while cruising and note the various vacuum readings. Then chose the appropriate power valve rating.

Holley makes a performance style "standard" flow or the "high" flow power valve. The latter has larger openings for more fuel flow with bigger engines. "Single stage" power valves are available in 1" increment sizes from 2.5" thru 10.5". Holley also makes a "two stage" power valve that is more for "economy" minded users rather than "performance" enthusiasts.


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bluethunder364

iTrader: (5)

Thanks for the info. I did go to the website also. Plan to put a vacuum gauge on it and get the stock 4.5 back in as a reference point to start.

Pop N Wood

iTrader: (1)

What carb number do you have? According to the Holley site they didn't make a stock 750 with a 4.5 PV. That is pretty low. The numerical listing say they put 6.5 or bigger in all 750's.

Plugging the power valve should make the thing run lean, not rich. If the power valve wasn't opening than that could cause you to put in jets that are too large to make up for the lack of a PV circuit.

The PV number doesn't control the amount of fuel being added. It just tells you at wht vacuum setting the PV will open. Going from a 4.5 to a 3.0 will make the PV open even later. Doesn't make sense you would go that way if the top end was lean.

I would try a new 6.5 PV at a minimum then reset your jets. The 6.5 should let the PV open sooner. If you are running a MAP sensor then you can read your vac setting by hooking up a lap top to the ignition.

I wouldn't read too much into sooty exhaust. That could happen when it was warming up and idling.

Also are you sure the carb came from Holley with 75 primary jets. Looks kinda rich. Check the Holley site for your carb and see if it really came with that jetting and PV number.

http://www.holley.com/data/TechServi...%20Listing.pdf

How are you tuning, a wideband will tell you exactly what is going on.

S10xGN

iTrader: (11)

Changing the PV will not make the car any richer, just "when" it opens. Put the original back in and either re-jet or enlarge the PV restriction orifice.

3pedals

iTrader: (3)

put the 80's back into the sec side. YOU leaned it out by changing them to 76's.

68 RS Convertible

iTrader: (1)

Is this a new carb or a used carb that is new to you that was tuned/jetted by prior owner. I bought a new in box 750 HP and it had 73H jets both primary and secondary with 6.5 power valves. I'm running the 73H's in the primary with 78's secondary and the 6.5 power valves. I don't know the difference between a regular 73 and a 73H though.

spdracer256

I agree that the power valve opens at too low of a vaccuum. put the stock one back in and check your idle. More than likely, the throttle blades are open too far exposing the transfer circuit. This causes a pig rich idle condition. (I thnk that has been mentioned before in this thread...) Anyway, just make sure that the idle is adjusted far enough down and that the idle mixture screws are adjusted properly then take it out for a drive and report back. Extended idling and tuning can cause the plugs to soot up so make sure you read the plugs after a drive with no/minimal idling at the end.