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Home Workshop: Problem Yammie

Firstly, I have to admit that I’m not fully up to speed on the Yamaha QT, as there are so few of them left around they’re not an everyday visitor to the workshop.

The biggest clue you have given is that it runs best on the top groove on the needle. This would indicate that the bike is running too rich and by lowering the needle you are leaning off the mixture, but not quite enough. Given the tasks you’ve already carried out, I would be looking for two or three things, Check the fuel float level height; if too high, it can give a rich mixture. Check the condition of the float needle valve. Thirdly, remove the throttle slide and needle and peer down into the atomiser tube. These are brass items that with vibration and prolonged use, can become oval allowing over fuelling.

The fault could probably be masked by the fitting of a smaller main jet to compensate for wear in the atomiser tube of the carburettor. While not a long term fix, it will give you a good indication of where the fault lies.