Hello forum I have a Honda 400ex that I rebuilt the top end on. I adjusted the valves on intake and exhaust to standard. I'm having problem with 30 second crank up and my header pipe is to hot to touch and when I put it in gear and ride it then it die down when I open throttle but when I ease off its fine. What is the problem
Sorry I missed this post when you posted it, but better late than never; right?
What's the problem with the 30 second start? Honda 400's (XR's and TRX's) are a little hard to start. Part of it is learning the starting procedure (each one starts differently and you have to experiment till you find out what works best). Part of the hard starting is caused by the size of the intake ports and partly by the size of the venturi of the carb. It is designed for high RPM performance, thus the velocity of air through the carb is reduced at low speed due to the size (vol-metric area) of the carb and intake tract. Reducing the diameter of the carb will ease starting at the cost of high end performance. Beyond that; use non-ethanol 91 to 93 octane fuel, keep the air filter clean, do not modify the intake or exhaust (even just putting in a K&N filter will increase the difficulty of starting) and experiment with what seems to work best - when you learn the procedure, practice it. You will find the procedure is different when cold vs hot.
Head pipe runs about 1000 to 1200 degrees, so don't touch it! You can touch it immediately after a cold start, but it only takes a few seconds to get too hot to touch.
Dying as you open the throttle is a carb problem. It is usually caused by getting too much air and too little fuel. Common on vehicles where the intake has been modified. Changes to the exhaust normally only make the carburation lean on the top end of the RPM range. In some cases it turns out to be too rich (over jetted) - usually when it's too rich, it will blubber like the choke is on when you back off the throttle for a bit before it cleans out and it will accelerate up to a certain point and anything past that point it simply runs progressively worse until it still runs, but loses power and slows down. Just backing off the throttle slowly it will begin to run better until a point is reached where it starts to accelerate again. But, either too rich or too lean will cause it to die when the throttle is wide open. The carb need to be serviced and corrected to match your setup.