XL600R Blown Decompression Chamber


I was wondering if anyone remembers on the early xr's and the later xl's the strange decompression chamber that could be accessed on one side of the head via allen bolts and a side panel. Well, I just rebuilt an XL600R, I got it started for the first time, (I didn't have the secondary decompression line which runs from the head down to a lever arm by the kick start) I rode it around the block one time and I was cruising along a low rpms when I saw the paint on decompression chamber melt followed quickly by the aluminum melting their and the engine loosing all its compression. I am completely lost as to what happened. What is this chamber for? What might have caused the this tremendous exhaust pressure to buid up their? THank YOu, I appreciate any incite.

Never heard of that! Maybe it was rnning very lean? Not having the cable to the kick start lever hooked up did not do it because that cable is just for the automatic part of the process. If you have a manual lever on the bars youdont need to hook up that one by the kick starter.

If I remember correctly, there is a fifth valve in the head leading to that chamber. The idea is that for starting the valve is opened and the extra combustion chamber volume lowers the compression ratio and makes starting easier. It's only supposed to be open when kicking and closed the rest of the time. It sounds like the valve is staying/stuck open and hot combustion gasses are getting in there.

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