Big corner missing off each exhaust port for a start. At a rough guess, I'd say a good 20% less area.

There's also a bit of a hike in compression on the later models, plus the use of bigger carbs. Lots of smaller things soon add up, but the porting is substantially different.

Here's a thought. The MC16 is rated at 45hp with no restrictions in ignition like the later models. The MC18's and MC21 are also rated at 45hp, but altering one wire on each model lifts them to at least 60hp (if the motor is in good normal serviceable condition). There certainly ISN'T 15hp in just carbs and standard pipes alone.
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Andy.
NSR-WORLD.COM

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