rotrax
Well-known member
FWIW
Yanmar say it is because fuel accumulating in the cylinders could make restarting difficult !
With all due respect to Yanmar, if the decompressors hold the exhaust valves open, excess fuel will be ejected past the exhaust valves.
Old fashioned British motorbikes-as far back as 1902-were fitted with decompressors acting on the exhaust valve and they had no clutch. They had direct vee belt drive untill chains took over. If the engine was running-the bike was going forward. The decompressor was required for push or pedal starting.
To slow down-the brakes were pretty useless-the decompressor was used. The overun and gas being forced from the cylinder was quite an effective brake.
In later years-20's through 60's expert trials riders would use the decompresor when decending steep hills. It gave better control.
Fuel build up was never a problem.
Operating a decompresor at high speed-as in an emergancy situation-could cause pushrod damage if one became dislocated. I have repaired a motorcycle where this happened, a BSA B33.