787 pump/needle/seat technical question

[Hokiebob]

Can someone help me understand how the fuel pump/needle & seat work in the 787. The pump moves the fuel and unseats the needle at some pressure (around 35psi or so) then goes into the jets. What is the reinforced diaphram for that contacts the end of the needle arm? What happens if the pressure of unseating is off or if the needle arm is above/below the adjacent areas. I understand needle/seats on float/bowl systems but I can't understand how it works, and what is the symptoms if something is amiss with it.

Thanks for any insight. You guys clearly know your stuff and I appreciate your time.

 

[Hokiebob]

bump.............any help guys?? Surely someone can explain how it works. If this isn't the right place to post this, my apologies--please direct me to the correct forum.

thanks

 

[timmyboy76]

Needle valves open and close several times over a 10 second period while riding the pwc. The big black diaphrams are called regulator diaphrams for a reason.
When the jetblock is full of fuel inside the carbureators, the pressure balance is weighted towards that side of the diaphram and the valves are closed, not allowing fuel to pass. As the motor uses the fuel in the jetblock portion of the carbs, the regulator diaphrams are pushed inward because the pressure differences change and the atmospheric pressure pushes them inward opening the needle valves to replace spent fuel that is being used to keep the motor running.

The needle valvs work as a completely closed valve, or a completely opened valve.
The motor will start because there is fuel present inside the fuel block portion of the carb body as you crank the motor and the fuel pump fills the carbs with fuel. Once full, the regulator diaphram releases the preesure on the needle valve arm and the valve closes until the fuel is used up and it reopens the needle valves refilling the fuel block. This repeats many times per minute when the engine is running.
..also, check out..http://www.factorypipe.com/t_carbtuning.php

 

[Hokiebob]

Thanks TB. It makes sense--I also found a sticky by Snipe about it somewhere else and he indicated that a lower pop pressure would give more mid-range if you had a flat spot there--is this because the cavity would fill a bit sooner than if it had to overcome a higher needle pressure?