Gas tank venting, anyone know how this works?

The reps for Moto-D who are the official US supplier for Bonamici in the US are currently looking into the issue. I'll let you guys know how it goes


-Jack

You've probably got this sorted by now but I'll throw in some info that may help you or others that come across this thread. The two close together holes in your tank you have circled in the picture are the tank breather hole (straight into the tank) and the line that usually goes to the EVAP system (if you still have one)? If you don’t then the tube that used to go to the EVAP system will now probably be joined into the tank vent tube somehow ie its dumping straight over the side? In the stock fuel cap the two circled holes are usually connected by a check valve (the red bit in later pictures).

However, regarding the gasket under the cap you said "You can see theres a small metal passage drilled out of the billet aluminum cap part that connects those two for some reason Im guessing". That is correct. It essentially is there to bypass the check valve ie make it redundant so positive and negative tank pressure can flow freely. That’s why there’s a bypass channel and the holes aren’t drilled right though ie the check valve is not used. Some people with stock caps remove bits from the check valve so it doesn't act as a check valve but your racing gasket with the metal passage achieves the same thing. About now you're probably wondering why the check valve is there in the first place? I believe it is there to aid the functionality of the EVAP system but unfortunately in doing so it causes issues with a stock cap especially if the EVAP canister has already been removed. Of course, when you are adding racing parts to your bike the EVAP system becomes less of a priority and correctly breathing tanks do. So, the company who made that racing gasket put the bypass in for you

In your case, because that check valve has been bypassed expanding or sloshing fuel is free to go straight out of the tank, across the bypass channel and down the tube that would normally go to the EVAP canister. But if your EVAP canister has been removed then it may be venting straight over the side? Where does the tube that the fuel is coming out of trace back to?

Then there is the hole that appears to have been not drilled by the factory. The one on the other side of the gasket. This is the fuel drain line. Its purpose if for when you overfill the tank and the fuel gets outside the cap and pools around the cap. I believe the drain line is only for this purpose. So again, the racing people decided you didn’t need that system for some reason? Probably because you would remove the drain pipe because its just extra weight you don’t want on the track. And then you just don’t overfill your bike. And if you do you wipe it up with a rag. Because you are at the track.

Given what I said above this may be somewhat redundant, but I can see a problem with the check valve in your last picture. In the middle of the green rubber bit you can see that little black piece with the knob that is under the green rubber bit. That black bit should be on this side of the green bit. It should be positioned inside the red plastic bit when you pulled the red plastic bit off. That black knob should be inside the middle hole of those five holes and it should lock in place as the knob should lock into the hole. Once in that position the black rubber bit acts as a one-way valve, flapping one way to open but closing when anything tries to go the other way through. Pull the green bit of rubber out and retrieve the black rubber bit. There should (might) be a spring under that black rubber bit but it might be wrecked because there isn't much room under that black rubber bit as it is sitting in the photo. Or it wasn't put in at assembly and the absence of the spring has allowed the black rubber bit to get into the position it is now. Once you have got the black rubber bit out try to reassemble with the black rubber bit sitting where it should be as described. If there is no spring you might end up with the black rubber bit back in the green hole? If there is no spring go see the dealer and tell them. Maybe they will give you a new free one as it may have been left out at the factory?

Hope that helps.

Is the 899 setup like the 1199? On the 1199 two vent hoses connect to the nipples on the front right-hand side of the fuel tank. The forward most hose vents to the small hole located just to the left of the filler neck which can be seen when the fuel cap is opened. Water (rain or wash) that enters this area will be directed to the ground. Likewise, an overfill at the station will allow fuel to drain to the ground. The other hose vents the tank internally. This hose would feed to the charcoal canister if the canister is still in service. Has the charcoal canister/hose been removed/modified? You can blow/suck air through the forward most hose. If you blow through the internal hose, the air will rush back out when you stop blowing.

If it has been adapted for track use, your overflow needs a bottle to catch vapour and overflow

usually fitting and mounted to the offside radiator