A friend purchased a 2001 Honda 400 EX. It came with an aftermarket exhaust, which has been removed and the stock exhaust re-installed. When the machine is started, the Exhaust pipe gets so hot that it glow's red and the engine has a high idol. Is this a jet problem? Thanks....

 

The pipe glowing red is normal under mostly all conditions if it isnt being ridden, getting air.
Take a look at your own pipe at night after sitting idle for a while and you will notice it on yours.
Many ATVs do this,...especially the 400EX.
While a glowing pipe does,..and can indicate a lean condition for the most part, it doesnt always apply to all models being an automatic diagnosis.

If the carb wasnt messed with when the exhaust was swapped, your issue shouldnt be jet related because if it ran fine with the aftermarket exhaust, switching back to the stock exhaust would make the quad rich if anything,...not lean.

Do you know if this quad was jetted with the afermarket pipe,..or that it has been jetted since?
Have you tried adjusting the idle to bring it down??
Could be that you need to go in a 1/4 turn with your air/fuel screw,..or even replace the pilot if it was bumped up in size.

 

I have a FMF system on my 450 and it glows red at night time. But usually going slow in real tight trails. I never notice it any other time. Mine is jetted also of course. I would also have to agree with knowsalot and the idle.

 

Thanks for the help guys..