Yamaha Rotary Valve Motors

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In the late 70's and early 80’s Yamaha entered the International karting field by producing a rotary valve motor to take on the Europeans.

The motors enjoyed some success but did not make the major breakthrough expected. 

Mark Weaverling in the USA posted this summary on a forum with details of an interesting modification

“The KT100A slightly modified made a decent motor that won in the PKA 100cc open series of years gone by in the very capable hands or Rick Gifford.  Take the cylinder head and drill the stud holes for 8mm studs.  Cut the combustion chamber to almost a complete hemisphere with a narrow squish band, say 3-4mm.  Set the cc's to about 8.  Throw away the KT100A cylinder, grab a KT100S (piston port) cylinder, raise  the exhaust ports 2mm and eye brow.  Block off the piston port.  Mill/grind in a TT port with a small lead, about .5mm.  Don't worry about  filling in the piston port, just block it off.  You can also make an adapter plate to change over to a motoplat ignition if you don't want to mount all the original electronics. “

Graeme Sutton has sent some photos of an example of a USA sourced engine now in NZ owned by Phil Davy, It has a common for the USA a KT 100S barrel fitted , twin carb intake and left hand clutch drive,