Yamaha TRX850 Enthusiasts Forum

Hi everyone, bit of an embarrassing question here.. But I searched for a while and couldn't find an answer so here I am.
I've been riding my first bike, an 01 honda vtr250, for the last 7 years and just upgraded to a TRX (life changing experience). I considered myself a competent motorcyclist for years and can ride the hell out of a small bike but I feel like I'm starting all over again now.

So onto my debut newbie question for the sub:

How the hell does the fuel tap work??
On my VTR250 I had up/reserve, right/off and down/on.
After looking up something unrelated (what fuel to run or something) i saw a discussion about the "primer" position..
I asked my brother and he said to treat it like my vtr, but I said it felt wrong because what the hell was the "PP" for.
I listened to him cause he's older and knows better 9/10 times in these situations.
So the last 3 nights I've owned the bike any time it's been off I've had it in the primer position. The bike is in immaculate condition despite 95k on the clock (story for another thread). Usually starts beautifully hot or cold. Tonight I started her up to run to the shops, idled for a minute or two and then she stalled before i left. Battery and starter seemed fine. I thought it might be low petrol as I was pretty close but even on reserve it took a while to get going (barely used choke as it was about 30c outside).

SO - have I really messed things up with the fuel tap, or am I safe?
And how should I use it in the future?

Sorry for the massive post I just really hope I haven't hurt my baby already

Hi Chompy!

At first, a heartily welcome to this forum from me - have fun here as well as in the real life!

Then regarding the fuel tap and how it works:

It's not a mere mechanical working tap but controlled by means of a "vacuum" (i.e. a pressure below atmospheric pressure).
As a consequence, this tap is (should be) closed when the engine is not running since the engine prduces the "vacuum" for opening the fuel tap and, thus, it has got no "off"-position.

Position ON:
"Vacuum" produced by the engine opens the tap automatically, so to say.

Position "RES":
Same as "ON" but allowing the fuel tank to empty completely.

Position "PRI" (for "Prime"):
Open the fuel tap despite the engine not running, e.g. for draining the fuel tank.

In summary: The tap can (theoretically) be left in the "ON" position all the time.

But this applies only when the membrane in the tap seals tight, otherwise there is a danger that the fuel flows through the carburettors in the engine, what can lead to a serious engine damage! If there is a miraculous increase in the amount of engine oil (visible in the inspection-glass) then DON'T start the engine!
Then it's very much advisable to drain the oil (with the fuel in it) and repair the tap first.
I put a decoupler in the fuel hose after the fuel tap, which I decouple for safety reasons when my TRiXie is supposed to stay in the shed at least for the night.

Hope all is well with your TRX and enjoy it!

Explained greets from Munich!

Don't panic.
The fuel tap is rubbish and as these bikes age it might be something you want to replace.
As already said, the Pri position opens the tap if the engine is not running.
In the On or Res position the tap should be closed if the engine is not running, but the diaphragm may leak due to age and wear.
If, and it is an if, you also have leaks from the float needle then a leaky fuel tap can allow fuel to flow straight through and into the cylinders.
This is bad and can cause massive damage to the engine should you try and start it.
You can buy a 'normal' fuel tap to replace the stock one so that Off actually does what it says and prevent fuel leaking through to the engine.
You can also buy carb kits to replace the needle seals.

Or you could get out into the hills and ride it like you stole it. Any good twisty road, just stick it in third and ride the throttle.

I bought a couple of replacements via AliExpress, about $9 each, so far so good, though I did forget to turn it on a couple of times, ran out of fuel about 40m from my driveway.

Thanks everyone for the advice, I forgot to check back in for a while!
Everything was fine and I haven't had any issues but will probably swap the fuel tap out at some point.

Place an in line fuel filter to help prevent and rubbish getting caught under the float needle and flooding the cylinders with fuel. Can be catastrophic to the motor if not aware.