I have experienced this on several occassions and with 2 different rs125s. Usually, but not always, it occurs if the bike has not been used (even after a week or so). The problem occurs with the fuel that was sat at the alumininium fuel tap - seems to form into a small solid grain for some reason and this can fully or partially restict fuel supply. The fuel galleries are very small here and the problem can show its self even if you change the fuel tap to the reserve position.
As a last resort you can empty the fuel from your tank and strip and clean the fuel tap (a pain in the backside) - however I have developed a neat trick so that a stip down is not required.
Apart from the fuel hose going from the fuel tap to the carb there are 2 other hoses comming from the fuel tank. One is the overflow the other is the vent. You will find the ends of them usually sticking out down either the left or right side fairing (depends who has routed them). You need to find the open ends of these hoses and identify which is the vent. The easiest way to do this is to blow into each, one at a time, firstly with the fuel filler cap closed and then with it open. The vent is the one which gets a lot easier to blow when the cap is open.
Great! Now you have found the right hose. Close the filler cap fully, close the fuel tap. Blow (with your mouth only) as hard as you can until you feel the pressure begin to build up in the tank - now open the fuel tap whils the pressure is held in the tank. Stop blowing, wait a few seconds and start your bike. The blockage hopefully should have cleared. The blockage occurs at the inlet to the tap porting so once it has cleared then both main and reserve will work again.
It has worked for me each time so I hope it works for you.
Good Luck!!
Keith.
Keith Brown
July 2008
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