New 400 ex owner - Honda ATV Forum
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post #1 of 10 (permalink) Old 02-20-2019, 01:45 PM Thread Starter
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New 400 ex owner

Hey guys, new to site, I recently bought a 2007 400ex it did not run rebuilt motor piston rings, timing chain and tensioner, new valves, cleaned carb, it started and runs, but runs awful backfires bogs, it idles ok but once you give it gas it runs bad. It has a fmf power core 4 slip on so I looked at jets it was never re jetted so I've gone through from 150 to 158 main and 42 pilot still having issues I've made multiple adjustments to fuel screw I'm at a lost. And the plug comes out very black. Would the packing in the exhaust do this? Any ideas would be great thanks.
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post #2 of 10 (permalink) Old 02-20-2019, 04:43 PM
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Welcome to the forums. I didn't see where you mentioned cleaning the Air filter? either way make sure that's done, next it sounds like your problem is still a dirty carb, mainly the low circuit i.e. the fuel screw. I would screw it all the way in until it's lightly seated and back it out 2 1/2 turns. It's quite possible that you haven't gotten it clean enough too? that idle circuit orifice is extremely tiny. Your main should be somewhere in the 160 range too possibly even a 170? Good luck and keep us posted.
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post #3 of 10 (permalink) Old 02-20-2019, 04:47 PM Thread Starter
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Thanks for the reply I will definitely give those suggestions a try
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post #4 of 10 (permalink) Old 02-20-2019, 05:35 PM
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One thing I’d suggest would be to start with stock jets and change them one at a time, one size at a time.

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post #5 of 10 (permalink) Old 02-20-2019, 06:05 PM
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if im not mistaken black plug indicates rich mixture?

05 trx 400 ex
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post #6 of 10 (permalink) Old 02-25-2019, 06:09 PM
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Quote:
Originally Posted by jeraldberald View Post
if im not mistaken black plug indicates rich mixture?
This is true. If the idle circuit is clogged then there is an air restriction thus creating a rich state.

"MOOSE WAGON "
00 TRX400EX aka "MOOSE ROCKET"
91 TRX250X aka "MOOSE WAGON" RETIRED
96 Polaris 400L sport (Mrs. Moose Ride)
07 Suzuki LTZ250 (Sons)
96 Polaris 250 Trail Blazer (daughters)
06 TRX450ER aka === Under construction

Progress and Help
PM FOR MANUAL HELP
JETTING
Service_Manual_Downloads
2000_400EX_Build
400EX_Pivot_Bolt_
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post #7 of 10 (permalink) Old 03-02-2019, 05:00 PM
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post #8 of 10 (permalink) Old 03-06-2019, 09:21 AM
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Hi and Welcome to the honda atv forums!!
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post #9 of 10 (permalink) Old 05-16-2019, 06:31 PM Thread Starter
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Hey guys I'm hoping someone can help or give me some ideas. I have a 2007 trx 400ex it idles fine and revs ok in neutral, but once in gear and and give it gas runs awful. Her is everything I've done. New piston and rings. Timing chain and tensioner,gaskets for everything, valves and rods.cleaned carb tried different jets and pilot jets, tried new carb, new air filter, new ignition coil wire plug, getting spark, sent it to a guy he went through everything as well getting good compression. He is also stumped just ordered a cdi box gonna try that. Any ideas would be greatly appreciated thanks
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post #10 of 10 (permalink) Old 05-16-2019, 06:59 PM
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Can you explain what it’s doing in gear? Choke on? Muffler plugged? Air filter plugged?


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