Understanding petcock

[LiranT]

Hi everyone,

I'm trying to understand how the PRI (Prime) position on the tank-mounted petcock bypasses the diaphragm, especially considering that it uses the reserve hose.

Here's what I know:

Tank-Mounted Petcock:
ON and RES Positions: The diaphragm controls fuel flow based on engine vacuum.
PRI (Prime) Position: Allows unrestricted fuel flow, bypassing the diaphragm.

Frame-Mounted Petcock:
Directs fuel from the main or reserve lines to the carburetors but doesn't interact with the diaphragm.

My Question:
How does the PRI position on the tank petcock bypass the diaphragm, given that it uses the reserve hose? Since the frame-mounted petcock doesn't directly interact with the diaphragm, how does PRI function effectively bypass the diaphragm?

Thanks for any explanations!

[herennow]

What year are you talking about?

In my 2003 the tank petcock seals all flow from the tank when closed. Lets it flow when opened. Period.

The frame petcock does the rest.

[LiranT]

I'm talking about 1993 model.

So in your model the diaphragm is in the frame petcock and the valve directly interact with it. That's make sense.

[Armandorf]

to avoid misunderstanding,you wrote it backwards:


FRAME-Mounted Petcock:
2 HOSES FROM TANK, 1 FOR VACCUM FROM CARBS, 1 MAIN OUTLET TO CARBS

ON (and RES??)Positions: The diaphragm controls fuel flow based on engine vacuum.
PRI (Prime) Position: Allows unrestricted fuel flow, bypassing the diaphragm. CAN FLOOD CARBS, SHUT OFF IS DEPENDANT OF SEAT FUEL VALVE ON CARBS



TANK-Mounted Petcock:
2 HOSES , PRI AND RES, AT TWO  DIFFERENT HEIGHTs IN THE FUEL SCREEN, RES=LOWER, PRI= HIGHER

Directs fuel from the main or reserve lines to the carburetors,  in ON position is dependant of the carb (inlet menifold) vaccum sucking the diaphragm.

PRI connects RES(from tank) to MAIN OUT (to carbs)directly, no restriction

[LiranT]

So the diaphragm is on the frame petcock and not on the tank petcock?

Now it make sense...

I saw a YouTube video but it was probably a different type of petcock where the diaphragm was on the tank.