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Discussion Starter #1
Hello new to the trx forum and new to the Hondas. I recently purchased a 2002 400ex of a guy that could not get it to run. 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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Misfiring is caused by the spark (lack of, strength, time) - the 400EX does need a battery to run - it is an AC excited ignition - the battery is just there for the lights and starter motor.

Cam timing and valve lift have little to do with misfiring - in your list of 'claimed' new parts only the stator, CDI unit, coil and spark plug have anything to do with the problem you are experiencing. On part that was not mentioned was the spark plug cap. I have seen the plug cap fail and it is seldom suspect by novice and seasoned mechanics alike. Now, the spark plug is not new! As soon as an engine is started, run and shut off the spark plug is now a used plug.

The fuel air mixture could cause misfiring if it's over rich and fouling the plug. Having to hold the throttle open is a method of leaning the mixture (it's how you start a flooded engine). If the fuel level in the float bowl is too high, the engine can run over rich regardless of the jet sizes. The pilot jet is what the engine idles on. The farther out you turn the pilot screw, the richer the mixture becomes. The POS K&N filter will definitely lean the mixture at high RPM and the pipe will have a minor effect on jetting at higher throttle openings, but the idle is only slightly affected. I would be satisfied with 38 and 150 to 160 jetting or maybe bump the pilot jet up to a 40 and set the pilot screw at 1/2 to one turn out. From your description, Is there any possibility the pilot jet was drilled out? I think I would try a new 38 pilot jet and connect a timing light to the spark plug just to verify spark - if the spark is intermittent and the timing light quits flashing and when it does flash it backfires, that just confirms an ignition problem.

The engine will run with the cam one tooth either way from correct - if the cam is off by 1/2 tooth, it indicates the chain is worn and needs replaced. If the cam is one tooth off either way, the effect is; one way it is hard to start, doesn't idle well, but runs like a raped ape at high RPM - off the other way it starts easier, idles exceeding well, but the power and high RPM performance suffer - you are looking for that happy medium.

Get a few new spark plugs, test the spark plug cap, test the wiring connections in the wiring harness and eliminate the kill circuit as a suspect (disconnect the black wire with the white stripe at the CDI box) if it runs with the b/w wire disconnected, start the engine, disconnect the key switch and handlebar kill switch, run it with the b/w wire connected and it is running right then plug in each kill switch one at a time to see if it affects the spark - if it does, replace the offending switch - also check all ground connections including motor mount bolts and see where that get you.

Concentrate on a spark issue before worrying about a fuel issue - got to get it idling before worrying about high speed running.

Almost forgot - check the oil to verify it is not half gas - if the machine is parked overnight with the fuel turned on, the oil can become contaminated with gas and it will run bad - gets worse as it warms due to the heat evaporating gas out of the crankcase which vents into the air box.
 

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Discussion Starter #3
Thanks so much here to help I truly do appreciate your time I will start my process of eliminations tomorrow night had a long work day I can say the stater is new it’s one of the cleanest things on the bike along with the coil now the cdi looks like it could have been a eBay product but again u can tell it has been changed it has part # written on it lol tomorrow I have a short day and I’m off the next so I will be up all night in the garage I will respond back with all updates again thanks
 

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Thanks so much here to help I truly do appreciate your time I will start my process of eliminations tomorrow night had a long work day I can say the stater is new it’s one of the cleanest things on the bike along with the coil now the cdi looks like it could have been a eBay product but again u can tell it has been changed it has part # written on it lol tomorrow I have a short day and I’m off the next so I will be up all night in the garage I will respond back with all updates again thanks
Osiris,

Did you ever get you problem figured out? I have one doing the same exact thing and am about to start eliminating the kill switches like Here 2 Help mentioned. Thanks
 
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