need help w/yamaha 140 2003

I have just bought a boat 18 feet with Yamaha 140 outboard 2 strock 2003 it wasn't used for a long time by the old owner with the old mixed gas so we added some fresh mixed gas since that fuel gage wasn't working ,i took it for test run it was firing some time . but ran ok ,the second time I filed it full and i went to trip to an island it was running ok in the way going but the way back start firing alot that i couldn't go faster than 5mph other wise it will start firing and shaking every thing looked ok except the firing shaking i mean the temp it was ok , if i take the control sick to zero and slowly forward it will run ok for few second then start to fire i opened the carburetor and clean them with cleaner and changed the spark plugs it kept the same firing . please I need help if somebody could help

 

please im still waiting

 

try fuel supply a simple vacuum gauge will tell a lot
anything over three inches of mercury is not good
(sorry misplaced conversion chart don't recall how many centimeters that is)

 

I didnt get you

what do you mean by the fuel supply ?

 

If the motor hasn't been run for a long time then you need to remove and disassemble the carburetors and clean them. I can promise you that it will not run right until you do and if you don't you will most likely mess up a cylinder from running too lean on gas if you haven't already done so. Start with a compression check. All cylinders should be about the same.

 

thank you guys for your help we found the problem it was really silly it was the the over heating whistle the one located behind the control it wasn't working so the cdi wont let the engine run to full speed in case there over heating in the engine .