89 150 can't get back on plane

[para_medic00]

I have a 1989 yamaha 150 that starts out fine it runs strong and has no problem when I first open it up speeds are 35-40 mph.After the speed has been reduced to an idol speed and run that way for a while I am unable to get the engine back up on plane, the speed refuses to go above 9-11 mph. I have changed fuel filters and water separator and New plugs ,Im lost at this point any help would be appreciated

[jb123]

you say it wont go above 10 mph, how bout the rpms.
tank vent clogged or pump ball collapsing?
have you tried another prop?
i would try an aux gas tank to isolate the delivery from there.
let us know what happens.

might want to de-carb also