Merc II transmission TRS drive
 

I have a 24X7 offshore clone, big block with TRS and the merc II trans. The trans had the mechanical shift conversion done when I bought the boat. It shifts forward and reverse fine when the motor and trans are cold (just started up) and while the motor is running at idle. After running the boat at speed 50-65mph the trans shifts to forward and reverse are very slow, sometimes 10-15 seconds. Any ideas? The trans cooler seems to work fine (water comes out the discharge hose). Any input much appreciated.

You need to install a pressure gauge in the valve body and check the psi sounds like when the oil gets hot you might have too low psi to lock the clutches up quickly,the merc book has a spec if the psi low your pump might be going bad or your bleeding off psi somewhere