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Join Date: Jan 2011
Location: Smack in the middle of Illinois
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Most generally starting without choke indicates one or both of two things; float level is too high or the idle circuit is jetted too rich.
Bogging at full throttle can be either too rich or too lean provided you are not experiencing some sort of ignition problem. The 400EX has an AC excited ignition - it does not need a battery to run - with what you describe, it's possible the ignition is becoming weak and spark goes away at high RPM or when under load. The fact the engine performs normally at low speed means the CDI unit is good and the pulse generator which signals when spark should occur is working, that leaves the coil, spark plug, plug cap, stator and flywheel.
Usually when a flywheel goes bad, it works better at high speed than at low speed. Coils can go bad in many ways, but the coil is just a low amperage/high voltage transformer and it's pretty durable - it's usually the last thing that goes bad spark plugs fail frequently, plug caps fail more frequently than coils and stators fail at a relatively high rate.
If you determine the problem is electrical and not carburation, install a new stator first.
Since the carb started leaking while setting, attack the carb as the primary source of the problem. Question is why did the carb start leaking while setting? Did the petcock fail due to alcohol damage to the seal? The only other way the carb could start leaking while setting would be if the fuel was not shut off. Not shutting off the fuel will lead to carb damage from water (carried in with alcohol), from Algae (grows in the water), petroleum distillates (what is left after gasoline evaporates) and corrosion caused by oxygen and all the other stuff in the carb float bowl. Sometimes if the carb is over rich, you can shut the fuel supply off and as the level in the float bowl is depleted, it will start running better and runs best just before it is completely out of fuel. If it's lean, you might try putting it on choke to get it to run better. If putting on the choke makes it immediately worse, then it's probably over rich.
If you have had luck with aftermarket carbs, you are part of the minority - I have scrapped new after market carbs in favor of rebuilt OEM carbs when the POS Chinese copy failed to perform properly and the jets were unmarked and had odd threads making rejetting nearly impossible - I have decimal metric drills for rejetting Chinese carbs, but many times I end up junking them and reverting to an OEM even it I have to get it used.