[Shiny]

Thanks for the explanations. I apologize, but I'm still struggling to understand the pressure at the carb inlet when an electric pump is in series with a stock mechanical pump and there is low fuel demand.. (at idle).

Originally Posted by glhs#116

...On the other side it produces pressure to the carb that it regulates to whatever pressure....

This is the part I'm trying to understand. The term "regulate" is more like "self-limiting" I think. Because the pump is all diaphragms and springs, is the max pressure it can put out imited by the diaphram spring in this image:



But isn't the max (idle) output pressure of the mechanical pump always relative to the inlet pressure?

Similarly, doesn't an electric pump build pressure at its outlet when there is low flow at idle (or dead-headed)? It may be internally regulated or limited but I have to believe it is going to be 5-7 psi, correct?

So if the pressure above atmosphere (which is what a carb cares about) is 5 psi at both the outlet of the electric pump and the inlet of the mechanical pump, how can the pressure at the carb not be the sum of both "dead-head" or low-flow pumps?

I think I am missing something here about how the mechanical pump works because no-one on this thread has mentioned flooding at idle because the pressure got too high.

I think if someone measured idle inlet pressure at the carb with and without a series pump powered, I'd at least see whether the two pressures added or if the mechanical pump somehow delivers the same output pressure regardless of inlet pressure.

Mike

[pfilean]

Quote:
Originally Posted by pfilean View Post
Jay,
the "momentary" switch is only a toggle switch that is spring loaded to return to its original position when pressure is removed from the switch. As long as you hold the switch it powers one circuit and when you release it it returns to its original position to power the other circuit. Completely independent of the ignition switch.

In my case I'm only trying to fill empty (3) carbs after extended down time. The engine is not running so ignition and throttle are not important. Momentary switch is independently powered. I could use the electric pump to try to help the mechanical pump, if necessary, which is seldom to never is.

The reason for the switchover valve is that I thought that the check valve trick might (stress might) add flow resistance through the check valve. Didn't want any unnecessary strain on the mechanical pump. Flow through the switchover is straight through with no interference.

[Jay S]

Originally Posted by Shiny

Thanks for the explanations. I apologize, but I'm still struggling to understand the pressure at the carb inlet when an electric pump is in series with a stock mechanical pump and there is low fuel demand.. (at idle).



This is the part I'm trying to understand. The term "regulate" is more like "self-limiting" I think. Because the pump is all diaphragms and springs, is the max pressure it can put out imited by the diaphram spring in this image:



But isn't the max (idle) output pressure of the mechanical pump always relative to the inlet pressure?

Similarly, doesn't an electric pump build pressure at its outlet when there is low flow at idle (or dead-headed)? It may be internally regulated or limited but I have to believe it is going to be 5-7 psi, correct?

So if the pressure above atmosphere (which is what a carb cares about) is 5 psi at both the outlet of the electric pump and the inlet of the mechanical pump, how can the pressure at the carb not be the sum of both "dead-head" or low-flow pumps?

I think I am missing something here about how the mechanical pump works because no-one on this thread has mentioned flooding at idle because the pressure got too high.

I think if someone measured idle inlet pressure at the carb with and without a series pump powered, I'd at least see whether the two pressures added or if the mechanical pump somehow delivers the same output pressure regardless of inlet pressure.

Mike

A mechanical pump fuel pressure is referenced to the atmosphere. Same as an electric. There is no reference back to the inlet pressure.

I have seen some old low boost turbo set ups where mechanical pumps ported a boost reference too the intake manifold. I think it is usually up on the side of body of the pump. Tom V probably has seen that also. I think I recall seeing that on Hugh Macinnes’s turbo charging book.

It is not usual to see the idle fuel pressure jump up a couple pounds with you turn the electric on though. But only a couple lbs, depends how the electric bypass is set, and how well the bypass on the mechanical pump works.

[Shiny]

Originally Posted by Jay S

A mechanical pump fuel pressure is referenced to the atmosphere. Same as an electric. There is no reference back to the inlet pressure.

I have seen some old low boost turbo set ups where mechanical pumps ported a boost reference too the intake manifold. I think it is usually up on the side of body of the pump. Tom V probably has seen that also. I think I recall seeing that on Hugh Macinnes’s turbo charging book.

It is not usual to see the idle fuel pressure jump up a couple pounds with you turn the electric on though. But only a couple lbs, depends how the electric bypass is set, and how well the bypass on the mechanical pump works.

Ahhhhh!!! Thanks for dealing with my confusion and answering.

I'll have to digest this, as I thought gas was pulled into the pump (or pushed in by an electric pump), the diaphragm started moving, pressure built up, the inlet valve closed, and gas exited through the check valve at the outlet.. so I've got to figure out why if the gas was above atmospheric going into the pump chamber, it wouldn't stay at that pressure.

I understand if the mechanical pump was "off" (pretend the lever wasn't being actuated), an electrical pump could push right through it. If it didn't work this way, an electrical pump couldn't prime a carb.

So you are probably correct about "relative to atmospheric, not inlet pressure", but how that works remains unclear to me.

I still somehow think if you plugged the outlet of the mechanical pump, you'd see the pressure generated by the upstream electrical pump inside the mechanical pump chamber, whether the mechanical pump was operating or not. And this pressure would be above atmospheric, any pressure generated by the mechanical pump would raise it even further. This is where I'm still stuck!!

I probably has to do with the way the mechanical pump pressure is limited but I'm not clear on all this yet....

hmmmmmm...

Mike