Vintage Dirt Bike Q & A

Thanks for the info. If the bike is sucking air I know exactly where it is coming from. Which leads me to my next question. When I bought the bike it came with a tube, that looks like fuel line, running from the intake manifold to a metal canister mounted under the gas tank. What is it? I had no clue what it was so I took it off, and evidently did not seel the hole, where it was connected to the intake manifold,very well. Thanks again Jon T

It appears someone fitted a "boost bottle". These were devices intended to give you more low-end snap when you opened the throttle. (There will be someone smarter than I who can explain how...but it is all to do with the pressure waves in the inlet tract and the reed valve, right?) Anyhow they came stock on YZs of the time - I have one on a YZ250H 1981 - and I think you could buy after-market ones too. You may need to buy a replacement intake manifold if you want to put it back to stock.