Most, if not all, de-compressors work by slightly opening/lifting one of the exhaust valves via a small cable operated cam or lifter within the cylinder head.
My 86' DR600 had two, one for each exhaust valve. One was automatically operated via the kickstart, the other was operated by a handlebar mounted lever. I ditched the second one because it was not needed.
As with everything else, the cables that operate the de-compressors need to be kept in adjustment. I do mine when I do the valve clearances as the valves & piston need to in the same position to adjust both anyway. I could always tell when mine needed adjusting as it was that little harder getting the piston past TDC.
Defintely worth having, a big single without a de-compressor can be a nightmare of shin shredding kickbacks - I still feel the pain when this happened on my XL500, my foot slipped off when the bike kicked back & the serrated footpeg tore through my jeans & ripped my shin.
Steve
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