Quote:
Originally Posted by ol'George
I'm thinking that most needle valves can withstand #5 pressure.
but it depends on the floats displacement as well as the mechanical advantage between the float pivot and the needle in the seat.
Obviously a small float will leak at a lower pressure than if it was bigger.
I find a mechanical fuel pump does the job for a lot of years before needing replacement.
That said, there are valid reasons where someone might find an electric pump
fits the need better than a mechanical system.
But with the little dribble of fuel most lawn/garden engines require, a gravity flow will work just fine, if the tank is above the carburetor 
Now there are those that like to have braggin rights saying they have 120 gallons per minute of flow and #15 pressure on their hotrod motor,
that is bored/stroked/ported/polished/balanced yada yada yada.
Well you get the picture.  |
You're right, 5psi is more than enough for the float needle valve. The pump I chose had a max output of 5psi, and with an inline regulator, the sweet spot going into the carb was about 3psi.
I was doing some heavy mowing, with grass sometimes as high as the tractor's hood. The blades might stall and the belt might smoke, but that engine was a beast and never missed a beat.
A vacuum diaphragm pump sometimes looses some pressure at low engine speeds, and I was getting some lean surge when idling after tearin' it up out there. The electric pump delivers a constant pressure at all engine speeds, si no more lean surge at idle.
I didn't show it in the schematic, but I wired the pump through a switch, such that it could be switched on or off, independently of the ignition switch.
This would allow me to turn the pump off, for example, while parking the tractor after a run. The engine would then suck the carb dry, avoiding ethanol or fuel gumming problems.
Then I could switch the pump on a few seconds before starting, to let the bowl fill and make a quick lightoff when the key was turned.
But at the end of the day, it's like you said, kinda just braggin' rights. For me it was more of a 'Gee can I make this work?' thing. And yeah it did. But the bottom line is, when ya consider the expense and time and labor, if yer existing vacuum diaphragm is workin' ok, just stick with it. It'll be fine.