Holley Carb Help Needed

[Rice]

You're saying that a running electric fuel pump without a return will not overcome the pressure of the float pressing the needle valve into the seat?

Can you elaborate?

That is correct. 5 psi at the head of the needle valve adds up to a very small total force which the float (along with some mechanical advantage) has no trouble sealing.

However, there is a risk! The only time the carb can hold back the fuel is when it is right side up. If you roll the car or even lay it on the side fuel is going to flow. Modern vehicles have a sensor to shut off the pump if this happens. The fix for this issue is to wire the pump through an oil pressure switch which shuts down the pump if oil pressure drops.

 

[GLTHFJ60]

That is correct. 5 psi at the head of the needle valve adds up to a very small total force which the float (along with some mechanical advantage) has no trouble sealing.

However, there is a risk! The only time the carb can hold back the fuel is when it is right side up. If you roll the car or even lay it on the side fuel is going to flow. Modern vehicles have a sensor to shut off the pump if this happens. The fix for this issue is to wire the pump through an oil pressure switch which shuts down the pump if oil pressure drops.

That pressure number must change per carb model to some degree, since the float volume, and therefore buoyancy, is not consistent.

TIL, thanks Rice.

 

[Rice]

That pressure number must change per carb model to some degree, since the float volume, and therefore buoyancy, is not consistent.

TIL, thanks Rice.

All true. Holley recommends 5-7 psi for most of their carbs but that is what the pressure is supposed to read at idle. The flow rate at idle is so small the pressure is effectively "static pressure", so shutting the needle valve completely does not mean pressure then "builds" (provided the correct fuel pump for the application is in there). At WOT the pressure drops. Again, the bigger of the issues by not running a return are that both the pump and the fuel are being heated if there is no flow.

 

[roadstr6]

All this chatter about flow rates, pressure, heating fuel and the suggestion that to safely run an electric fuel pump, it should be wired through the oil pressure switch (great idea which nobody does) tells me all I need to know to solidify my fuel pump choice for an F or 2F tractor motor. Kyosan mechanical pump.

 

[Rice]

All this chatter about flow rates, pressure, heating fuel and the suggestion that to safely run an electric fuel pump, it should be wired through the oil pressure switch (great idea which nobody does) tells me all I need to know to solidify my fuel pump choice for an F or 2F tractor motor. Kyosan mechanical pump.

Totally with you. The only time I would recommend electric would be in the case where no provision for mechanical exist (effectively all new gas engines since they are fuel injected) or if a motor swap didn't allow enough room. Mechanical is simply too reliable and safe to ignore.

 

[JohnVee]

Great points above. While the pump on my 350 is electric, AND wired to a pressure switch, I still have never bought a spare pump. I guess it’s time to decide to buy a spare or convert to manual.