Nicknamed the ‘CYCLONE’ the TM400R was the first Japanese production motocrosser to be mass-produced. The huge 82.5mm bore piston turned out 40 bhp at 6,500 rpm and along with the fan-finned cylinder head it had a compression ratio of 7.3 : 1.
The crankshaft halves were not full circle type as fitted to most two-stroke engines and made the engine rev more; one answer was to fit aftermarket flywheel weights made by Competition Dynamics in the USA. The clutch was based on the T500 road bike with seven fibre drive plates and seven metal driven plates it was operated by a rack and pinion type mechanism. A five-speed gearbox provided transmission. Carburation was provided by a Mikuni 34mm carburettor featuring a fuel trap around the main jet, developed to keep the float level more constant by keeping the fuel in the trap area. Air filtration was by a large capacity, dry paper element housed in an airbox under the seat.