Just a tap, or is it?
It's got a tough job on its hands - stopping the flow of petrol. A substance that is flammable, carcinogenic, tough on plastics, a poor lubricant, and likes to swell most rubbers.
The original taps on my BSA were a push/pull design (Ewarts I think), sealed with corks, and worked well when they were new. Then, as the years rolled on, the corks would harden and shrink, and the tap ended up with a constant drop of petrol hanging from it. But the corks were replaceable and, in those days, it was considered normal, basic maintenance. Just a walk in the park compared to the regular decokes needed on your way back from your holidays.
But onwards, there was the brass type with a tapered seat, often chrome-plated. Again, when new it was fantastic, but a year later you could barely turn the thing on because the mating surfaces were so badly scored and dry, through a lack of lubricant, that they began to leak. See a pattern forming?
Then we got modern.
I'm not sure who was first to do it, but eventually plastic was used within an aluminium body. Self-lubricating, and sealed with O-rings, it was easy to turn, seemed to last forever and we thought we'd reached the pinnacle of tap technology. Of course, the O-rings hardened and the plastic insert would wear, or break, and we had another leak on our hands. Good job petrol is so cheap huh?
So why is the tap there anyway?
Apart from the obvious issues of needing to keep the fuel in the tank when removing it, or the carb, it's a back-up defence in case the needle valve within the carb's float chamber is worn and allowing fuel past. If the tap is left on and that valve leaks, you'll end up with fuel all over your garage floor in the morning. Or a crankcase full of fuel on some two-strokes.
This happened on my trusty Can-Am ex-army bike in the UK and, because the garage was adjoined to the house, stunk the whole place out. The next morning it smelt just like a gas leak, and I had to get British Gas out to check for leaks! Just as well I don't smoke.
But back to the tap and it's many varieties - it's also a handy spot to fit a petrol filter as you can see in the pics. This is where all the crud/rust will congregate in the tank as it tries to get down and block your carbs. Worth taking out every now and again to make sure it's nice and clean.
On/Off/Prime/Reserve - what's it all about?
On older bikes it was common to have two taps, which were either on, or off. One was for normal use, the other was a "reserve" in case you ran out of petrol - ingenuity huh?
Eventually manufacturers combined it all into one tap so, for normal use, petrol ran down a small length of tube mounted on top of the tap (all within the tank), and you switched it over to reserve when the level of fuel in the tank got too low to enter the tube. The reserve side of the tap has no tube, and should drain the tank fully - although I have been on the side of the road furiously shaking a bike trying to get fuel from the right side to left. Solace can often be found in religion on these occasions, praying you'll find a petrol station before midnight.
Of course, all this is probably so ridiculously easy that you're wondering why I even sat down in the first place to write it all up (it's 'cause I'm trying to put off fitting a Toyota Aurion water pump).
So with the On/Off/Reserve functions nicely covered, we're brought bang up to date (as in the eighties) with the new-fangled tap system that's fully automated. Yep, no more turning on the tap when you're ready to ride; and when you get home, switch the ignition off and that's it!
Imagine an intelligent tap... that knows when the engine is turning over. No sooner has that engine spun over a few revolutions, but that tap is now feeding fuel to those thirsty carbs. How?
Take a look down there and you'll notice a smaller rubber tube going to the tap, the larger one being the fuel feed pipe. When the engine spins it creates a vacuum, or depression, between the throttle butterfly and the combustion chamber (this is how the servo, or booster, works on your car brakes.) The vacuum pipe is connected between the two and sucks/pulls on a diaphragm mounted on the back of the humble petrol tap. At this point, a valve opens and fuel is allowed to flow. Beautiful. No fiddling about with taps, just apply the choke (bloody hell, remember those) from cold and crack on.
What about the Prime function?
Glad you asked... if you've ever run out of fuel, or removed the carbs for cleaning, there's a simple method of refilling the float bowls - PRIME!
That's all it is, it's what used to be 'ON' on an older styled tap. Just use it until the engine is running, then switch back to ON... clear as mud.
Those four screws on the back of the tap is where it all goes on. A spring-loaded diaphragm holds the tap in a closed position until the small pipe is sucked on. Put your mouth over it, suck and you'll feel it move. If nothing happens, you need to open it up and inspect.
Should fuel run out of the vacuum side?
No, if any fuel come out of the smaller pipe, the diaphragm has gone.
When the tap is on the tank, and no vacuum/suction is applied to the smaller pipe, no fuel should ever run out unless PRI/PRIME is selected.
In fairness, the fuel tap is a pretty reliable device, but all of them wear out eventually. The one pictured is off my GSX and works perfectly, at this point. I'll clean it up, fit a new sealing ring and see how it goes. If luck's on my side, I won't be seeking religion before midnight anytime soon.