WhyBike Motorcycle Blog

Mailbag: Motorcycle Backfire

By James - 10/26/2005

I bought 1994 Kymco 150cc motorcycle in Taiwan. It rides well, but when I am going down a hill it backfires. The local repairshop said it is okay because it is an old bike. What shall I do to ride down the hills without scaring all the living creatures around?

- Milan

This summer I was in the same situation, when I used the engine to control my speed on downhills, the bike would backfire continuously. Luckily it was only temporary, since I was on a ride through the Sierra Nevadas. At that altitude, 9300 feet, all cabureted bikes run rich which means that not enough oxygen is mixed with the gasoline to burn it all. When the unburnt gas hits your hot pipes, it ignigtes which causes the backfire. At more reasonable operating altitudes, there are two reasons why your bike may be running rich. Either there is too much gas being introduced to the cylinder or not enough air is being sucked in. Try cleaning the air filter, that is the easiest solution, otherwise check the carburetor, if it is sticky it could restrict the flow of air. If too much gas is flooding the engine, try restricting the amount of gas coming out of the jets. This varies from bike to bike so check with your mechanic or a service manual for the correct way to adjust them.

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