Power valve not opening

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DrCharles

DrCharles

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Recently I found that the power valve on my 950 carb did not appear to be opening at all. I have it tuned for a lean cruise, but now the AFR readings on the wideband do not richen up when the manifold vacuum drops below the PV rating. In fact it gets even leaner and sags. I did punch it once to get the secondaries open and it took off so fast I didn't have time to watch the AFR! So it's getting fuel.

It used to work perfectly...it's only sat in the garage for a couple weeks and I had last filled the fuel cell with alcohol-free premium so I know it didn't gum up. I did check that the fuel pressure was still 5 psi at idle, and the level is 1/3 up the large sight glasses.

So, today I pulled the bowl and metering block. Everything is pristine, the passages in the metering block appear to be open, and the PV operates smoothly when I apply lip vacuum to it
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If for some reason the manifold vacuum is not reaching the PV chamber, then it would be OPEN all the time, which it isn't. The idle AFR is in the high 13's and cruise near 15 where it's always been, so I know the gauge is working. The only thing I can think of now - the gasoline is not getting to the PV somehow. But I could see gas in the PVCR's when I removed the PV and the chamber in the main body looked like it had been wet. So what do I do now?
 
15" at cruise. Currently a 7.5" PV.
Vacuum varies greatly with throttle, of course... but the point is, when the manifold vacuum drops below 7.5" on the primaries only, the AFR does not richen. It used to. (71 PMJ, .066 PVCR). So something changed and I'm not sure what.
 
“But I could see gas in the PVCR's when I removed the PV and the chamber in the main body looked like it had been wet.”

So you’re saying you have gas on the vacuum side of the power valve? If that’s the case, my thoughts are bad power valve, I know you said it operates with lip test but does it hold under vacuum?, metering block to main carb body gasket leaking, cracked or warped metering block, power valve gasket damaged or power valve improperly torqued (loose). As I said that’s my thought process. Maybe not in that order but somehow you’re getting fuel where it doesn’t belong on the vacuum side of the power valve. That manifold vacuum reference is a straight shot to the base of the carb. I also can’t explain why it occurred after just sitting.
 
Yeah, mine too... of course it could have been a little wet just from taking the (emptied) bowl and block off too, gas does tend to run around. Will take a closer look and maybe sub another PV as recommended.

This carb is brand new from about a year ago and I've put maybe 500 street miles on it since then. Everything looks great, the PV just isn't working. I also suspect a clump of crud somewhere that may be fun to locate. Was hoping to avoid pulling the carb.

Re: 10.5" PV, I did try one last year but that comes in much sooner than needed for low-speed or even highway cruising. I have a [email protected] cam that idles at 8" and makes 12-15" depending on cruise rpm. Just wiggling my toe will drop it below 10"... As has been pointed out here by @Mattax and on racingfuelsystems, the mixture actually needs to get leaner until approximately 50-70% of max power, then start richening up. Even an 8.5" was a little too sensitive.
 
I agree on the gas moving around. If it’s newish I know that Holley “products” are very good for not cleaning burrs and things from the ports and passages after machining. I’ve seen this particularly where the mixture screws seat in the metering blocks. Brand you’re running really doesn’t matter, may come down to, as stated previously by others, a good thorough cleaning and look over.
 
Do you have the proper metering block gasket and is it in the right orientation so the PV passage isn't blocked??? If they are put on backwards, the passages may get blocked....
 
krazykuda said:
Do you have the proper metering block gasket and is it in the right orientation so the PV passage isn't blocked??? If they are put on backwards, the passages may get blocked....
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Well, it worked for the last year so...
Easiest thing to do is just try a new PV. If that doesn't help, it's time to pull the carb and get more into it.
 
I ran a line for a 2-5/8" fuel pressure gauge to the cowl (I already had the gauge and the tubing kit in my spare parts collection). But I don't have a gauge cup so I ordered one - meanwhile it's duct-taped to the cowl
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Close to 5 psi at idle, 4.5 psi while driving. Open flow is close to the rated 110 GPH on a brief 10 sec. test.

Today I removed the 7.5 PV and installed a 8.5. New blue gasket too. Drove 10 miles to town for a cruise-in - seemed to be working until the engine was fully warm, at least... after the cruise-in, I could see the enrichment working (about 1.5 AFR) and it did so all the way home!

I think I'll try a little more fuel pressure, and maybe raise the float level slightly (it's about 1/3 up the large sight glass). Another 1/2 - 1 psi will raise the level a hair anyway. Probably won't hurt to change the HPG-1 filter cartridge too which may be causing a slight drop.

The AFR seems to swing a lot even at 3000 rpm on a smooth paved road - sloshing in the bowl?
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Vacuum holds it shut. Two things, Ruptured diaphragm, hole under carb stopped up. If there’s fuel in there it’s ruptured.
 
What I found that works best "for me" is the cruise vacuum method. Mine had about 14hg cruise vacuum and I have a 10.5 PV now. I like it there. It picks up almost immediately when I get in the throttle at cruise. I can feel the difference. My idle vacuum is around 6hg.
 
Thanks. I agree with the "X inches lower than cruise vacuum" selection, definitely not the half-of-idle vacuum which is only for SBC owners who don't understand how the carb works ;) My idle vacuum is 8", cruise is 15".
My carb is set up for a very lean cruise (71 PMJ) with about a 15:1 AFR, and a relatively large PVCR. It's very easy to drop the vacuum below 10.5" during low-speed cruising and on the highway, just by moving a toe on the gas pedal.I actually tried one, but I get too much unneeded enrichment at relatively low power. The 8.5 seems to be a reasonable compromise.
 
What is the afr at wot? With 15:1 cruise and your PV adding 1.5 afr points you are lean. When the pv opens you should be down around 12.5:1 as a starting point. If you want to keep the cruise that lean I would up the pvcrs.
 
“the mixture actually needs to get leaner until approximately 50-70% of max power, then start richening up”.

This is true, to an extent. From your cruise afr when you lightly lean in to the power it should get a little leaner. 2 or 3 tenths. What I dont agree with is putting a % (50-70%) on how much throttle it takes to make power enrichment come in and make a change. Every combination is different and hard fast rules shouldn’t be followed. I’ll tell you this, on higher output stuff, at 70% power you better be in power enrichment with a load.
 
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Those numbers are approximate... and a bit high because I had to fill up at a station that didn't have alcohol-free premium. PVCR's are .066 with the 71 PMJ.

I didn't say % throttle, I said % power... and at 70% throttle the secondaries are probably half-open too.

Also I really can't test at WOT - with the secondaries open which should be plenty rich enough (86 SMJ, no PV) the car accelerates so fast that I can't watch a gauge and the road, nor is it safe to stay on it for more than a few seconds!
 
Very true you did say power. And that’s what I was responding to. I edited my post to reflect that.
 
Noted.
Anyway since I street-drive the car (and won't be at a strip until I get a trailer, since the nearest one is 90 miles away), just about all the driving is on the primary circuits. I'm too old for the kind of stunts that attract the attention of law enforcement. I'd have to look it up but I seem to recall that the secondaries start to open at 40% throttle?
Tomorrow I'm going on an 80 mile round trip (most of it at 60 mph) so will have run about 8 gal of premium through the carb. No E10 this time.
 
I would buy a power valve tester and a mity vac.
I have found the advertised and actual on alot of power valves to be not accurate.
 
Good advice, as it turns out ;) My Autometer dash vacuum gauge agrees well with two underhood-type large gauges.

During the 80 mile round trip to the car show, I had an opportunity to watch the gauges more carefully. I filled the fuel cell with alcohol-free pump premium.

Idles around 13.5-14 AFR. The lumpy idle from the long duration and overlap already smells bad enough, I don't want to richen it more if I don't have to...

Cruise 14.5 @ 15" Hg.

This PV starts to open around 7-7.5" (it's clearly stamped 8.5!)
Fully open at more like 6", where the AFR drops rapidly to 12.5 and stays there as I keep opening the throttle more to stay at 5-6". That does not last long even in 4th gear :)

I know it's not a true binary switch - it has to open far enough so that my relatively large .066 PVCRs are not restricted by the PV seat. Now I'm wondering what exactly the PV rating is measured at - where the valve starts to open? Or fully open? Either way the one I picked does not seem to be quite rich enough. Time to try the (alleged) 9.5" I guess!
 
DrCharles said:
Good advice, as it turns out ;) My Autometer dash vacuum gauge agrees well with two underhood-type large gauges.

During the 80 mile round trip to the car show, I had an opportunity to watch the gauges more carefully. I filled the fuel cell with alcohol-free pump premium.

Idles around 13.5-14 AFR. The lumpy idle from the long duration and overlap already smells bad enough, I don't want to richen it more if I don't have to...

Cruise 14.5 @ 15" Hg.

This PV starts to open around 7-7.5" (it's clearly stamped 8.5!)
Fully open at more like 6", where the AFR drops rapidly to 12.5 and stays there as I keep opening the throttle more to stay at 5-6". That does not last long even in 4th gear :)

I know it's not a true binary switch - it has to open far enough so that my relatively large .066 PVCRs are not restricted by the PV seat. Now I'm wondering what exactly the PV rating is measured at - where the valve starts to open? Or fully open? Either way the one I picked does not seem to be quite rich enough. Time to try the (alleged) 9.5" I guess!
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How do you know when the PV opens? Richness is accomplished with the holes behind the PV, it does nothing but open and close those holes to the main circuit. You want gas millage lean the main jet 4 steps then drill the PV holes out 10% and use a 4.5. Now it opens at 3/4 to full throttle only, putting around town don’t need it open.
 
From my original post: "the AFR readings on the wideband". How else would I know? :poke:
I know what a PV does. That's why I'm tuning it... your formula is just a shot in the dark without a wideband AFR gauge and knowing the particulars of my combo. In fact my primaries are a smaller than stock #71, PVCR are a larger .066, and secondaries #86 as I mentioned in my post #17.
 
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