every 3-6 week we have to replace the plug when we take it out its black. And the bike always stinks like heavy gas

 

My 400 was the same when I got it, I just leaned out the idle mixture a little, the heavy gas smell is gone now, but I haven't checked the plug recently. Make sure it's the right heat range.

 

Mine was running a little rich , but it seemed to be only at idle, once underway and using the thottle is runs clean, Check to make sure the enrichener is off, and adjust the idle mixture screw. You need to be heavy with the thottle now and then, to burn off the deposits.
If youre smelling gas, you might want to check the float level, if its high itll cause rich running.

 

IMO, that model isn't prone to plug fouling at all. You don't give a lot of detail in your post. More typically I've had to richen the mixture screw on most Hondas I've seen, although not so much in recent model years. I would go along with Hondabuster's idea that your choke plunger or cable is hanging up.