A problem with the 3MA is when riding a constant speed, the engine starts to build up fuel.
Du to the construction/location off the reed blocks.
Some part off the fuel is flowing back behind the reed petals and stays there.
Making the mixture richer
Yamaha has tried to solve that problem with the use of crossed one way valve's
Although the 3MA is only build two years, trying to solve the fueling problem.
Changes where made on the crank cases
Here you have the 3M1 crank cases with reeds
(Also look to the location off the valve)
Here you have the 3MA3 crank cases
YAMAHA has changed the reed block with a extra reed at the bottom.
(and the location of the valve has changed to)
Still its a problem. (personally I haven't got a problem with it, its more like a 3ma characteristic)
Perhaps the problem can be made less by adding a wig shaped block under the lower reed stoppers
So the flow will be more upwards and less to the reeds (Yellow drawing)