So I decided to re-mount my fuel pump and wanted to check whether it was working.
I took it apart and to me it definitely looks it also has a vacuum plunger that closes off the fuel flow when the engine stops running.

However, my pump is flowing crazy amounts of fuel with the engine off.
To add to my confusion, there's no vacuum line to open up the diaphragm. Is it operated by the same vacuum as the actual pump?
How?

I opened it up put my finger on the blue circle (where the plunger normally sits):



I pressed my thumb into it hard (which should close it perfectly) but when I put the tank on PRI, fuel is rushing like crazy because of 20L of fuel pressing on top of it.
How in the world is that rubber plunger cap supposed to stop it?

it is not supposed to stop it....


when the fuel level in tank is above that of the carb, the fuel flows by gravity to the carb. lets you get home even if pump doesnt work, just fill the tank...



only when the fuel level in tank drops bellow the carb, the pump helps it go up.


if you want to know if your pump works, connect a long tube to the exit, keep the outlet above the tank level (otherwise it will flow freely as you seen) spin the engine and see if it pumps fuel up (into some bottle of course )

Ok but then why does it have this triangle part? (where I drew the circle, basically).
If it was just a pump, why have the diaphragm there that blocks the hole? I don't get it.
Are we supposed to turn off both fuel petcocks after riding each time? In case of float valve leaking..

if you have petcocks with PRI position then they are vacuum operated by the running engine, so no need to turn off. they are shut whenever the motor is not running

in fact, a petcock with PRI doesnt even have an "off"....



With motor shut down, in normal and reserve positions there is no flow because there are no vacuum pulses from the motor.

Sorry - my petcock doesn't have PRI. I meant to say RES. I have RES/ON/OFF
So yeah, at the moment, even with the pump installed, fuel is always flowing to my carbs, even with engine off. At least - when the tank is >25% full.

Can you explain to me the purpose of the triangle diaphragm bit ?
There's a rubber cap with diaphragm on the blue circle hole, which is pushed closed by a spring.
It definitely looks like it's supposed to close off the hole.

if you have decent float needles, leaving the tap on on/res is no biggie, nothing happens


if your carb floods, then close the taps.... for us old timer on british bikes that was normal life, shut off after every ride


thats actually is the element that pumps, its task is not to close the hole.

Well, there's 2 petcocks... quite a hassle to turn them off.

Anyway


I'm not talking about the 6 star diaphragm. But the triangle thingy on the left.
Surely the big 6 star pieces are for pumping, not the small triangle one.

in double petcock bikes, i always use one tap most of the time, open the second only when the first started to run dry, switching both to res.


I dont have in my hands a tenere pump like yours to analyze, but worked on other ones for quads and karts. In those, the fuel always can flow through by gravity. cant help you much more than that, sorry.

No problem.

So I opened up my spare one to take a closer look and figured some things out.

1) As you would expect from the arrows on the body, there's a hole directly from the inlet pipe towards the big pumping part, skipping the triangle diaphragm entirely. So yeah, looks like you're right and it's not there to stop the incoming flow. The valve in the pump also opens by pressure and allows flow. At least on this particular pump.



2) The fuel wasn't leaking past my finger or the plunger from the triangle. The fuel was making a full pass through the entire pump, and then returning via the bypass hole into the triangle. On my 1st picture with the blue circle, you can see it on the left of the circle.

3) So I'm thinking the triangle bit is some sort of bypass. I'm not yet sure what way though.
It might be, that with a full tank, the fuel pressure pushes open the plunger and bypasses the pump. Although I'm not entirely sure what the benefit here is?

Or the other way around, if the pressure on the outlet pipe is too big (full carb), it goes via the bypass and back into the pump when the plunger opens.
Again, not sure why because there plenty pulse-pumps that don't have this mechanism and they cope just fine.

Might get a syringer later and do some more testing.....