Thread: 25 hp johnson runs 1/2 throttle,than makes strange sound if you go full throttle

Does it start easy?
Does it idle good?
Does it seem 'normal' till above half throttle?
Did you do anything to the carb before this problem?

starts easy ,idles good ,normal until 1/2 throttle no carb. work until after problem started.

Will it run good for a few seconds if you go wide open throttle from idle?
What year model?
Are you running an old gray fuel line?

How many carbs. I've got a late 80's Yamaha 30 with 3 carbs on my duckboat. Same thing happened to it. I done all of the above and no fix. Compression check good.Tore carbs down again and seen no visible trash but...cleaned thoroughly anyway and when I re-installed on motor problem went away. Could it be possible float sticking in just one carb???????

Mine would start shaking bad (tiller steer) when you tried to rev it up. Really don't know what fixed it but it has done it once more since and cleaning all 3 carbs solved the problem that time also. As you know, fuel aint what it used to be...

1999 model, black line

Get back into the carb. The main jet is hidden in the bowl. It's behind the brass fitting with the small fuel line that feeds the enrichener. Find it and confirm it is clear or partially clogged. There is also a small round gasket in the center. It should be replaced. What you are describing is consistent with the motor leaning out as the throttle shudder begins to open up and bring the main jet on line. Either the main is partially clogged or the gasket has failed.

cleaned jet flushed gas thru with fuel bulb .no better tried a new power pak ,per mechanic ,didn't help. could I have a wire shorting out when throttle gets to certain point?

Is it a 3 cylinder?? Cant remember what year I have 98 or 99 25hp Johnson that is a 3 cylinder and I had to replace power pack a few years ago.