The most likely cause of you problem, especially if it�s happening under load, is that the "dogs" on 2nd gear have rounded off. These dogs are on the side of the gear and engage in corresponding "slots" on the side of the adjacent gear. The engagement or locking together of these gears happens when the selector fork slides 2nd gear over and tight up to the next adjacent gear, causing the pegs on 2nd to engage in the slots on next gear, resulting in both gears turning at the same speed.

When this happens, you are in second gear. However, if you've rounded off the pegs, the torque of the motor in second gear can actually cause the rounded off dogs (and slots) to spread apart (move away from each other) and the only thing resisting that spreading is the selector fork, which ends up trying to hold the gears together. The selector fork is only designed to slide gears over when a gear change is made, not to hold the gears together or resist the spreading force caused by rounded off dogs. As a result of worn dogs, undue stress is put on the selector fork and consequently, the selector fork eventually bends. As this bending (and wear on the dogs/slots) increases over time the �jumping out of gear�, usually back one gear (ie: 2nd to neutral) will get worse. It often starts intermittently (like yours is) because sometimes the dogs engage fully and at other times they don�t.

Unfortunately the fix is to open up the motor and replace the shift fork and the two adjacent affected gears.

You may find you can get round the problem by taking more time changing up from 1st to 2nd, being gentle but positive with the selection and laying off hard throttle for a second or two, allowing the dogs to settle/accommodate themselves in their corresponding slots. It�s the (sudden) torque which causes the spreading, so build throttle after making the change. Not ideal I know, but you will get used to �feeling� the dogs engage and it�ll get you by until you can strip and investigate.

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