I have an 05 Honda 400 ex that was running good until i got outrun by a buddy a couple weekends ago and my dad decided he would help me out. First he ordered an HMF competition series slip on exhaust and K&N air filter. Ran good with no problems. then he ordered an HMF jetting kit,and put the 170 jet in and changed the 138 jet to a 142. Cranks, idles, and rips up good just sitting there, but when he rode it it started bangging out bad at about half throttle. Where do we go from here? He is not a mechanic so please put your suggestions in simple terms! Any help would be greatly appreciated.

 

what does "Banging Out" mean?

 

Probably means backfire, pull the plug and see if it is pale tan to white ,or black, if pale, go up on the main jet, or lower the clip on the needle until it smoothes out. If it is dark or black, go down on the main jet, or raise the clip on the needle until it runs good all through the rpm range. Sounds like he just made too big a change on the jetting at once, and check to see if the choke is on, that yellow lever in front of the carb, my son left his on once, and it did the same thing, backfired like crazy trying to rev it up. There should be jetting instructions with the jet kit, did he start at their recommended base settings? if you use and HMF kit with the HMF exhaust, it should be pretty close, they will tell you what changes to make with an aftermarket intake filter as well. Are you running with or without the airbox lid? all these things will make a big difference on how it runs. the 138 jet , I'm assuming thats the pilot jet, its always a good idea to raise that up to the 42, Hondas run lean from the factory, it starts a lot better with the bigger pilot, especially with mods to intake and exhaust. Keep us posted what changes you make and the results, we'll get ya running good in no time !!!

 

Well, you can bet your lean. Make sure didnt' bust a carb boot. Make sure carb is set good in the boot.

You said up'd to a 170 main. Is you box lid on or off? If it's off try putting it on and see how she runs. I was running a 190 with the lid off.

 

Thanks for the replies. He did use the hmf jet kit and the instructions said 165/170. So since he added the k&n air filter , he put the 170 in. And he did change the pilot up to the 42. i am runnin with the airbox lid ON . I live in SE Texas , so im probably about 80ft above sea level. We will check plug out tonite. What clip on the needle valve are you talkin about? I know what a needle valve is but guess i dont know where the clip is located. Do you have to go back into the carb to change that setting or can you do that while it is on the bike? We will check the boot for holes tonite. I know what you mean about the choke. We did that also and it did backfire. But when he was riding it and got to about half throttle , it sounded more like spit and spuder. Like it was running out of fuel or Maybe flooding out? I dont know! Like i said we will check sparkplug out tonite. THANKS AGAIN FOR ALL THE HELP!

 

I believe the '05 and newer 400EX does not have an adjustable needle. I know my wife's '06 doesn't.
When I added the HMF slip-on to her 400EX, and a Uni filter I rejeted to the 170 main and 42 pilot. Instead of completely removing the airbox lid I first removed the lid and put the two rear lid snaps turned into the box and re-installed the lid. The two rear snaps held the back of the lid up by approximately 1/2". The quad runs great and it greatly improved throttle response.
I have a feeling you are running too rich with the lid on.

 

Ok. Thanks. That is a quick and easy check. I will try that as soon as I get home..

 

Thanks for the help..I checked my plug and it was black. I didn't have another new one. I went ahead and took lid of my air box and it sunk a whole lot better. Still had a little miss every now and then. I'm gettin a new plug today so maybe that will get the little miss out of it. It is real dry and dust where I ride. Is the open air box with the k&n filter goin to shotgun the life of my bike? Or should I drop back on my jet and put the cover back on it? Thanks for all the help and any other suggestions on the little miss or air box lid would be appreciated .

 

Ran a whole lot better. Not (sunk)

 

K&N filters are known for letting dust past.
I run a Uni filter, it keeps the dust out better than a K&N.
Get an Outerwears cover for your filter and keep your filter cleaned and treated on a regular and frequent schedule and you'll be fine.