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Hello new to the forum and new to the Hondas sorry if I posted this in the wrong place but I need help, I recently purchased a 2002 400ex of a guy that could not get it to run. Currently it will only start when throttle is half open and it pops/backfires like crazy I trued full throttle one time and man sounded like a AR15.I was told the piston,rings,timing chain,stator,cdi,coil,spark plug, and rectifier was all new. So i decided to start with the timing. I put the piston on top dead center found this by the lines in the eye glass and confirmed it was at top dead center by putting screw driver in spark hole. The lobs we’re facing down but the right side line on the cam gear was up a little not exactly what I would call lined up with the head so I tried my best to line it up. I loosened the chain tensioner and tried to move the chain one tooth put it on and it made it worse tried to go a tooth the opposite way and still not exactly lined up. But better then what it was. I then adjusted the valves spec .004 intake and .005 exhaust put it together and it started but died, then started and died only way it would stay on is if I held the throttle open 1/2 way thing was backfiring/popping like crazy and will not idle at all. So I decided the timing had to be on and the valves had to be in spec. Only because it fired up, next I opened the carb it had a 150 main 38 pilot and the needle at the third slot, I cleaned the carb real good this I’m sure of I’ve cleaned quite a few carbs nothing is clogged no hoses no jets. Tried it same thing only starts with throttle opened 1/2way or more and the thing is popping like crazy I have some main jets for a kehlihan carb I was thinking that the main jet is way to small so I changed it to a 170 I don’t have a larger pilot so had to keep that in there oh I forgot to say I was messing with air screw to try to get it to idle nothing turned it 3 turns out and to me that’s way to far but did not change anything it’s crazy because it will start right up if you hold the throttle open but pops like crazy I also checked intake boot and air filter boots both are in excellent shape I’m stumped and a little frustrated. Did I miss something? or could this all be because of my pilot jet being stock? almost forgot bike has a pro circuit t4 slip on k&n air filter, air box lid on. Or maybe the pump accelerator in the carb? Any help would be great. sucks because I have never heard it run till I messed with it I thought this would be a easy diagnosis lol guess not
 

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First of all, where are the replacement parts purchased from? If eBay or Amazon they will be China junk and stands a good chance that's your issue.

Secondly, are you 100% the timing is dead on? Have you confirmed by reading the FSM (factory service manual)?

Thirdly, replace all carb jets for stock (genuine replacements), adjust pilot as per FSM and see how that works for you.

The most important thing to do is, ONLY change one thing at a time. Process of elimination.
 
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