An air pressure regulator has a similar function.  But there's a vent, which an FPR doesn't have.

https://www.instrumentationtoolbox.com/2017/03/how-air-pressure-regulator-works.html

Darn it, Google must have showed me this video because YZFMax posted it in a different thread.  It should be here anyway.  Double odds of seeing it.  Look at the float bowl action.

 

Edited July 2, 20213 yr by Zed Head

"SMARTER EVERYDAY"

That's what my Dad taught me too, learn something new everyday. 

22 hours ago, Mark Maras said:

 @Captain ObviousPlease define "cracking pressure when the valve is closed"

Cracking pressure is a term most associated with check valves. They are be held closed by something (usually a spring), and there is a certain amount of pressure required to overcome the combination of stiction and the force of the spring in order to force fluid through the valve. And even though not necessarily considered a check valve, the construction of the needle valve is identical to a check valve. The only difference is that the compression of the spring (and hence the force supplied) is variable because of the moving float.

Here's some good pics and description of cracking pressure:

https://www.hydraulicspneumatics.com/technologies/hydraulic-valves/article/21884995/engineering-essentials-pressurecontrol-valves

And from that page, here's a neat sketch showing why (even though it's not a "check valve", the concepts cannot be ignored even for the float valve. If you replace the adjustment screw with the float tang, here's the carb float valve:

And by the way, I think we're all saying the same thing here... 

Carbs need fuel in the bowl in order to operate correctly. And that fuel won't get into the bowl by magic. It takes pressure. It might not take much... In fact, in many circumstances, simple gravity is enough. But there must be something that pushes fuel into the bowl.

But for a car like the Z with a tank located below the level of the carbs, you need a pump. And a pump needs specs. So Datsun provided some specs in order to qualify and test the pump(s) used in the cars. "Should be able to provide this volume at this pressure and no higher than this."

It can be somewhat arbitrary as long as there's enough fuel provided at extended full throttle, but not so much as to force the float valve open at idle when you don't need nearly as much fuel.

In theory, principle of operation, when the engine is running the valve is never fully closed, so cracking pressure is never needed.  The bowl supply is always being depleted and the valve never fully closes.

OR, if you consider the transparent carb video demonstration, cracking pressure is needed between each intake cycle if the needle closes fully, depending on how fast the bowl refills, which would depend on pressure/flow rate of the fuel supply.  With more cylinders per carb, the needle has less time to seat fully, and things move toward constant flow through a partially open needle valve.   Always open, never fully closed.

And, stiction would really only be a factor at the pivot point for the floats.  The needle and seat are a conical shaped neelde, and the metal edge of a the orifice.  Not really a lot of stiction there.

The deeper you get in to carb operation the more you understand why they they rarely work well consistently.  There are always days where "something's off" and you get in to tweaking and tuning.

Well to break the rule like most others I guess, mine were tuned and corrected until they are as good as possible. I want to remove all the brand new efi components I put on my '77 and put a spare set of SUs on there. I am a minimalist, less is best, and would drive the 240 anywhere. The efi 280, not so confident. Too many sensors trying to correct problems that are barely there.

I have a friend with an efi John Deere mower that's popping through the air filter. Can not get him to understand it's most likely a sensor gone bad that I can not just "unplug". People are stupid when they get tunnel vision.