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every little thing adds up, some tuners do restrict power big time.
look at mopeds today, most have a restricted exhaust, once removed they fly.
if the exhaust cannot get out the reverse flow causes huge power losses,
bigger carbs means more fuel, more fuel = more bang, but the big bang also needs to escape...
if you remove powerhead atleast you can see if any other restrictions are there like in the adaptor itself, plus clean it out of any salt or scale build up.
the powerhead is easy to remove and gasket is only cheap.
clean up the adaptor but dont go made in the exhaust area, smooth it out as much as possible to give nice flow.
instead of going to normal 50hp stuff, why not look into what the 50hp has done to it when in race trim ?
parts will probably cost the same or even be a touch cheaper to buy.
some of those 50hp Tohatsu / Nissan motors are bloody fast.
just a couple of thoughts
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As far as putting larger than stock carbs, cutting heads and port timing changes. I am still trying to retain some tractability with the motor and fuel as it is on a fishing boat. I will probably leave the rev limiter on the motor to protect it for now. I build racing 2 cycle snowmobile engines so I am familiar with mods to 2 cycle motors. I am going to bore out the the 40 hp reed plates to fit the 50 hp carbs. I have noticed that the intake port on the reed plate is not centered over the reed blocks. Since I have to enlarge the intake port ~0.150" for the 50hp carbs I can angle the intake port to end up at the center of the reed block. The only problem is the intake port will be angled slightly in relation to the carb bore centerline. Would it be better to leave the original angle of the intake port and port the reed block to allow flow to the whole reed block or have a slight "kink" on the intake in relation to the carb and a centered port...
thanks,
Mark