1999 25hp Johnson misfire at higher revs

[valvebounce]

Johnson J25RLEEB Ser No-G04668104.
Not had the motor on the boat yet.
The motor starts easily,and will tick over,but If I increase the revs and hold it there,the engine exhaust gets smokey and misfires to the point where it would cut out if I didn't come off the revs.
The plugs are new.When it's ticking over the two stroke smoke is normal.
When it misfires it is quite violent at high revs.
I am running it in a well filled tub of water.The cooling water jet is quite strong,and the exhaust cooling water outlet is working well.
I suspect it is an ignition fault,anyone got any ideas?

 

[flyingscott]

Put it on the boat and see what happens. If you are doing this in neutral stop it before you wreck something.

 

[valvebounce]

Put it on the boat and see what happens. If you are doing this in neutral stop it before you wreck something.

Unfortunately I am 75 miles from the sea.
I keep the revs less than midrange,which is the best I can do.

 

[flyingscott]

You run the risk of wrecking the motor doing that. It will misfire when you rev it like that absolutely normal and not good for it. Go find some water quick because if you keep doing what you are doing, you won't have to worry about driving to the ocean.