2002 Honda 400ex Running bad poping and backfiring?
Ok so I had my 400ex taken to a mechanic and he cleaned and supposedly tuned the carb. The bike is stock except for the pro circut slip on exhaust. It ran kinda ok before I took it to go get tuned but the bike was a really slow on throttle response. It would bog on first a little and it would go away. Well any ways… So he supposedly tuned it and everything. When I tried out the bike it was running weird it sounded much louder from the engine. It also poped a lot! When I was in first gear it would pop a lot! It wouldn't really respond and sounds like it wants to die. Then once I get to 2 or 3rd gear it stops popping and runs normal. I'm guessing the bike is jetted incorrectly. It only pops when I give it throttle not when I let it go. When I snap the throttle it revs fine but when I hold it pops and it sounds horrible. And it doesn't blow out any smoke at all. When I hold the throttle is pops a lot and won't stop unless I stop reving the engine. And I took my lid of the air box and it was worse. What size jets would you recommend for my bike? I also want to run no air lid. And also what pilot jet size I heard the 42 pilot jet gets rid of bogs. What size main and pilot jets should I use and what position should I put the clip on for the needle and mixture screw setting. And my elevation where I live is about 3,000 to 5,000 feet of elevation.
You are right it is jetted incorrectly for you modification - it may run like scalded cat at High RPM but doing damage to the valve seats due to excessively lean combustion.
If there are no available jet kits for this model (check the dynojet.com site). Then you will have to go trial and error method until you get it right and this is hard to do and takes many attempts to even get close. Both Idle and Main jet will need attention as the idle circuit is in play through the full throttle range. (to some extent).
OR - reinstall the original exhaust, air box cover and stock air filter - a properly running machine is better than a wee-bit faster machine.
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