paragon gears P31-L on the port, stalls the port engine

Hey
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I have a 1973 30ft Chris {crusier express} with twin 307's and P31-L paragon gears.* When I put the port engine in reverse at normal idle revs, the engine seems constricted and will*stall. I have changed the fluid and check the levels and all is to code and spec.* This seems to happen more often when the engines*have run for 30 mins or more, but all cooling systems are good and running as they should.* My boat is not run hard and is always looked after.* Why would the engine stall out from reversing the gear on the port?* Is there a problem with the reverse gear? or a bend shaft or bearing? I would like to know if anyone has come across this type of problem with their Chris and what they found out.*
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Thanks and good boating
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Scott

When the engine is idling in neutral, does the prop shaft turn on the trans? If yes, how fast does it turn?

Yes they turn a little. Enough to move the boat around in its slip but seems to have little torque

If they are turning while in neutral, that means the clutch plates are ballooned and are dragging(rebuild time). When you put it in reverse the forward clutch plates are fighting the reverse clutch plates and stalling the engine.