How does the power valve work

BrianCT

BrianCT

Well-known member
Simple Question I know but I have few reference materials
for the later flatheads. Does the gas flow down the stem into the body
when vacuum is low?
Is there any other test besides the overnight drip test?
Thanks
 
If your engine is running, you can try to screw the idle mixture screws all the way in. If the engine shows no difference in idling, chances are the PV is kaput.
 
Here's how I think they work. The engine starts and the vacuum pulls on the diaphragm and shuts the valve. As the vacuum drops (when pulling hard, or accelerating), pull on the diaphragm diminishes and the valve seat opens this allows the carb to draw extra fuel through the passages in the valve body. No fuel passes through to the chamber below, this is (supposed to be) a vacuum chamber for the engine vacuum to act on the diaphragm of the power valve.

Note the vacuum pulls the valve shut and opens it as it falls. If you have a hot cam you will have less vacuum at idle. To stop the valve opening prematurely you fit a lower number power valve. That means it takes less pull from the vacuum to keep it shut. Only when the vacuum falls below a lower value than with a stock valve, will it open.

I changed the valves on my roadster last night (twin 94's) and the chamber on the front one was bone dry. The vac chamber on the rear one was wringing wet with fuel. From that I presume either the diaphragm was shot or the valve wasn't seating properly.

Anyway, it did drive better afterwards so I think the fuel leaking past the rear power valve was causing a problem.

Changing the cam from a Isky Max1 to a Schneider 270F reduced the vacuum at idle hence the need to swap out the valves.

Mart.
 
Mart said:
Changing the cam from a Isky Max1 to a Schneider 270F reduced the vacuum at idle hence the need to swap out the valves.

Mart.
Click to expand...

I don't want to hi-jack this thread with another subject, but maybe in another post please tell us about the 270F. Size of engine, Idle, vacuum at idle, where it makes power, and your impression.

Thanks, Gene
 
Here's a good picture from a Ford manual. You can see the seat on the stem of the PV.
 

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