i have a 1989 yamaha moto 4 350 not have the power it should builds up alot of pressure in crank case ha wrong air filter where should i start

 

The breathers are open pipes into the airbox so, if they are not blocked off, there won't be any "build up" of pressure. Because it is a single cylinder engine, the breather has to puff out 350cc of air on the down stroke, and suck in 350cc on the up stroke, so will be puffing a lot of air in and out each rev, this is normal.

Set the valves, and if this doesn't increase performance, do a compression test, should be over 90 psi. If that is OK, you may need to re-set the carb jets to suit the non standard air filter, or get an original filter set up.