Is this normal?...when i open the fuel cap it hissses and lets a fair amount of pressure out. Ive had the tank off for a paint recently, wondering if something isnt quite right somewhere?
This is normal. My 916 does it all the time. However you imply that it lets a lot out. Mine always gives a good hiss. I also have a billet screw in cap and sometimes I can do the bastard thing up too tight. This is especially bad on cold days with cold hands. I think yours is probably okay.
The 998 like all the others in the 748-9** range has a rubber sealer under the filler cap and on top of the tank recess, at the back. That is the vent rubber and seals against the cap to stop fuel sloshing out but allow the tank to let out excess pressure and to allow air in. If it or the cap vent get blocked then you may get pressure build up beyond the norm. Rust in the recess is another way of finding the vent blocked, If it can't breathe at all it may struggle to deliver fuel.
A ha, its like a rubber nipple on the back left...sticks up about 8mm within the void between filler hole and top of the cap? ill poke a screw driver or simular down and see if its blocked. Yes i would have thought with excess pressure the fuel wount be able to feed into the pump. Havent ridden the bike since ive been fiddling, however dont fancy it breaking down!!
Carefully pull it out and check it has not rotted or the hole into the tank is not blocked. Dirt and rust often block this up and then it affects the tank breathing, they also rot into and around the tank vent flange. You can poke a small drill bit down into the the tank vent hole to check if it's blocked or better still blow air through it. One of the tank vents (there are two) actually vents out through the fuel pump base plate, that might be worth checking while you are at it. The other is a water drain that vents out through the pump base plate. You will always get some hissing from the tank due to vapour build up but if you are clearly getting some pressure build up you might have a venting problem.
