Yes. Stock jet size is 56 and I'm running 54s. I've tried all the way down to a 50, just to see what would happen. It still ran incredibly rich, but was super lean when I'd accelerate.
Plus, when it would randomly behave normally, cruise became so lean I started surging.

 

Can't be the jets and get the inconsistent AFR he's getting. Something is changing and apparently opening/closing the PV at various times.

 

So I got around to doing some diagnostics this weekend.

I was still hoping to figure things out before having to shell out $30 and ship the carb back to QF and hope not to get a "doesn't seem to be anything wrong with it."

However, here's what I found.

First off, I have some very whispy thin wire that I used for restricting fuel circuits on older carbs. I think it's like 32ga or something. I couldn't get it to fit around the checkball. I might try to find something thicker to see if I can just keep it open.


However, I started playing around with a vacuum pump to see if there actually was a problem with it.

I completely disassembled the carb and applied suction on the bottom of the base plate (as it would if engine vacuum was pulling on it). As it pulled air through the passage and around the checkball, the vacuum dropped like it should (although slowly).
I then applied vacuum from the top of the base plate (reverse of how it should flow) and the checkball snapped shut and it held vacuum.
Lastly, I applied vacuum from underneath (engine side) again with my finger plugging the hole on the other end. As expected, it held vacuum this time since no air could flow through. No internal leak or the like.

So, the baseplate seems okay. Only thing I noticed is that air passage was very slow, and it took about 2 - 3 seconds to drop from 20 down to 0 hg.


Next, I bolted the baseplate up to the carb body and repeated the process.



This produced the same results. Vacuum from below pulled air through. Vacuum from above snapped the checkball shut. Vacuum from below with my finger over the hole on the main body (circled in red) held vacuum.



So, as far as I can tell, there's no leak inside the main body. It's pulling air through like it should, and holds vacuum properly.


Lastly, I repeated the experiment with the power valve, metering block, fuel bowl, etc. in place.



This did not hold vacuum! No matter what, the vacuum simply dropped back to 0. I tried putting in a power valve block off plug to be sure there wasn't just a blown PV. I tried a fresh PV gasket, tightening it down more, and tightening down the fuel bowl bolts a little more than I was comfortable with, and the same thing. I simply could not get it to hold vacuum.

If that's the case, that'd make sense why the power valve is always open. The chamber it's in won't hold a vacuum, so it doesn't pull it open.

Thoughts? Ideas? Next step?

Thanks everyone.

 

It sounds like you have two problems - one being that there is a leak in the PV chamber, and the other being that there is some restriction to vacuum "flow". (I know vacuum doesn't flow, but....) Coupled together you've found the problem of the leaking PV chamber that you might not have found otherwise since w/o a restriction on the vacuum most carbs would open the PV. Yes, you might not have full vacuum in the chamber, but you probably wouldn't know it.

As for what the problem is in the PV chamber, I don't know. A casting flaw? On two of them? Not likely. I think I'd call QF.

 

Looks like some headway on this issue:

https://www.ford-trucks.com/forums/1...l#post16761111

Turns out the restriction on the power valve vacuum port was the issue and they're actually going to look into it further as far as a redesign goes, based on this.

 

Hallelujah! I'm glad to hear it.

 

I posted on the other thread, but I'm not impressed by QuickFuel. Which scares me because my Street Demon comes from the same company.

 

As I said to them when I first wrote them, I understand trying to work out some kinks on a new model The 450 only came out late last year, and the Black Diamond series is new. However, when someone's complaining about an issue and willing to go through the work to help them figure it out, you don't blow them off. I think that's what bothers me more.

The fact that it was actually a port diameter issue explains why I went through two of them with the same issue. And that should have been a serious red flag to them. But, what can I expect from a tech guy (the manager of the tech support team, no less), who told me that a power valve is always open to some degree, and that to lean it out, I should put in smaller PV restrictions?

Granted, that was the tech support team, and not the actual carb designers (who had no knowledge of my issue until now).

As for a carb that's been around a while, like the Street Demon, I imagine that's been well refined by now.

 

anyone know if quick fuel 450cfm metering blocks can work on a Holley 390cfm body. no double pump or feed, just basic carb. thank you