I remember when I had my 1986 honda 350x wheeler. My experience is that they have lots of torque and a decent amount of horsepower.
I put 1500 dollars in the motor. High compression piston heavy duty valve Springs and the whole nine yards. And on top of all that I had carburetor work.
In my opinion carburetor work is extremely important.
Many people think that a 400ex carburetor is the quick fix for a hot 350x motor. It's not!!! The opening of a 350x carb is plenty big... all you need are a bigger fuel and oxygen jets. That's all u need..
Remember the more fuel you put in the more oxygen and more outage. So buy some large head pipes and a DG silencer.. after you get the jets and needle right with the out take pipes you will be ready to race a stock 400ex and eat it all day.. good luck you 350x enthusiasts. ..