I have a Bruce Nicholson built motor that is essentially the same as the subject motor (I believe this one was used by Denny Henderson to win the PRO 250 hydro national championship in 1992). It uses Yamaha dirt bike heads with the exhaust port barrel-type valves. These valves do change the effective port timing thereby enable using more aggressive port timing on the top end. The valves are tied to the pipes so when going into the corners you let the pipes blow back. This pulls the exhaust port "curtain" down to achieve less aggressive port timing and thereby still have decent low-end torque.
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