Loss Of MPH

Recently bought a boat and took it out a few times and top speed was about 53. It has a pro v yamaha 150. Was pre fishing for a tournament and all was well. Next day took it out and exact same conditions and only ran 45 top speed. Can any one tell me wats going on I use 86 octane should i be using higher or what? Im confused.Its also only runing at 4000 rpms. Its on a 17ft bass boat. Also has a 8 inch jack plate.

[jb123]

most likely has dirty carbs.

How would i clean those?

[jb123]

run it on hose, look for any excess unspent fuel out of exhaust...if so, leave fuel pump hoses intact but pull mount bolts, pump ball looking for fuel to pour out backside of pumps.

you could try changing out plugs.

lastly pull carbs, break down as far as possible, clean all orifices with cleaner and replace all parts in a rebuild kit. i think those have different jets in each carb so keep parts together AND number each carb as they need to go back in same spot.