Hey my 7.5hp mercury thunderbolt had been sitting for a good 20 years in a barn, I replaced spark plugs,impeller and switchbox with a second hand unit, cleaned carb and replaced tote tank. It starts first pull and runs mint but in foward gear in the water it seems to only get 50% rpm. I have a 10ft fyran tinny. My mate has a 6 hp and he's going twice as fast any ideas? Thanks