Power valve not opening

[DrCharles]

I took my 451 Dart to a cruise-in 38 miles away today, and noticed 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. It used to work...it's only sat in the garage for a couple weeks and I had last filled the fuel cell with alcohol-free premium, too. I did get fantastic mileage on this tankful (86/7.6 = 11.3 mpg!) but it was almost all 60-65 mph cruising.

I did check that the fuel pressure was still 5 psi and the level is 1/3 up the large sight glass. So, the next step is to pull the bowl and metering block. I figure either there is some crud preventing vacuum from reaching the PV, or the valve itself is stuck. (Since there are two PVCRs, both of them would have to be obstructed to see no enrichment on the wideband at all).

Before I start taking things apart... thoughts? I have heard of plenty of PV diaphragms being blown, but are they also known for sticking shut?

[Nick_nl]

Well i have pulled a PV out of a used carb that turned into rock. So yes, the rubber might not like modern fuel. Though you used ethanol free fuel. This would not match.

[DrCharles]

Thanks.
Sometimes I use the "up to 10% ethanol" when I know I'm going to burn it all and refill with alcohol-free, so it won't sit in the system for extended periods

I'll pull the bowl and block today, and have a look. I was going to lower the opening point by 1" Hg. anyway

[DrCharles]

OK, I took a look inside. 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

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 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. So what do I do now?

[Right hand drive]

[DrCharles]

Yes, I realized that and edited it just about the same time you posted
Anyway this was a new carb built by Brad H. last year, hopefully he can give me some tips. I've never seen one do this before!

[DrCharles]

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
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. 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?

[Right hand drive]

What’s the ‘950’?

[rgalajda]

What is your AFR reading at steady cruise ?

I don't know why people are looking for the AFR gauge to go rich when the power valve opens . What rich AFR are you expecting ?

AFR should be

(A) as rich as gave the best slowest smooth idle (12.5:1 -- 13.5:1 ?),

(B) then above idle as lean as would not misfire and gave least fuel consumption at part-throttle and cruise (14:1 -- 16:1 ? ), and

(C) increasing leaner as load is increased (14:1 --- 17+? ),

(D) up to the load percentage where very high or full power is desired,

(E) at which point it swings rich to the maximum power AFR or however rich it needs to be to protect the engine from damage (back to 12.5/1?). 

Of course proper ignition timing ( 40 to 50 degrees ) is required for leaner part throttle / cruise AFR

(C) increasing leaner as load is increased (14:1 --- 17+? ) is hard to achieve with an aftermarket Holley but was very apparent on some OEM Holley carbs such as
List 4346 Holley 1969 427/425 HP , that I have tested , and List 4803 Holley LS6 Corvette .

[DrCharles]