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Discussion Starter · #1 ·
Just picked up a 2000 400ex all stock last week and have done all the maintenance work on it. Been driving it around the house and after I stopped it, came back out to ride again, it won't start. Took the plug out, its black as can be. So I clean it up, put it back in, starts no problem. Drove it around for another 10 min and decided to check the plug again. Starting to get all black again. When its running, it runs great, idles no problem. But why the black plugs. I shouldn't need to rejet it since its all stock. What should I look for.
 

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I would look to see what jets are in it. The quad may have been piped and jetted and they may have taken it off when they sold it. You should a 148 main and a 40 or possibly a 42 pilot. Also does it have any aftermarket air filters on it? And is the air box lid still on it and not drilled or anything at all like that? From what you discribed your running rich. Its not a huge deal though rich kills plugs and burns fuel and lean overheats and burns up motors. Does it smoke any when you start it or when your riding it?
 

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Discussion Starter · #3 ·
He said its never been piped or anything, always stock. Airbox doesn't have any holes in it and its the stock air filter.
 

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Discussion Starter · #4 ·
Also, there is no smoke during start up or running it
 

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Is it a friend or just a random person? Cuz i dunno if i would trust it until i was looking directly at the jet. If its not a jetting issue i dunno what would be causing your problem. If your plug is black your rich. You want it to be a nice dark brown color. I would put a new plug in it and do a high speed shut off pass ( take it down the road get it wound all the way out and then shut it off and coast in to your tools. Pull the plug and tell me what you see. If its black your rich if its brown your good of it looks new (white) your lean. You really do not wanna be lean. Rich is ok your just not getting full power and your gunna fowl spark plugs and burn alot more fuel.
 

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The choke is not on is it?
 

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Haha didnt even think of that lol.
 

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Discussion Starter · #8 ·
Choke isn't on, i'll run it tomorrow and see what it does and also check the jet sizes.
 

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I would say the main jet is too large from the sound of your description. Also the pilot would be a 38. Mud99 could tell you for sure he has that year model. but any 400ex carb before 04 or 05 I think was a 35.5 mm bore after that was a 38mm bore.
 

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I would look to see what jets are in it. The quad may have been piped and jetted and they may have taken it off when they sold it. You should a 148 main and a 40 or possibly a 42 pilot. Also does it have any aftermarket air filters on it? And is the air box lid still on it and not drilled or anything at all like that? From what you discribed your running rich. Its not a huge deal though rich kills plugs and burns fuel and lean overheats and burns up motors. Does it smoke any when you start it or when your riding it?
38 is the stock slow jet 40 and 42 is what you would step up to after a slip on and a new after market filter.
 

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if your needle and needle jet are worn it will cause it to do that. when they are worn, it will pull gas through the main jet even at idle and will foul plug after plug until they are replaced. if this is your problem, spend the extra money and get factory honda replacements, you will be glad you did.
 

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Discussion Starter · #13 ·
Well I did what slammedranger suggested, checked the jets and they are the correct size. Cleaned the carb out. There was a little bit of stuff in the accelerator pump. Did a run then pulled the plug. It was black around the threads but at least the tip wasn't black this time. Kinda hard to see in this pic. Checked the compression and its 120. Well I'm going to ride it this weekend on the trails and see what happens. Might as well get the carb kit and redo that. Thanks for your help.
 

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The tip is a nice brown color. I would say you are in the correct area. Good job.
 

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Yeah, looks pretty close.
 

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120 is a bit low on compression.You might have a rebuild in the near future.
 

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120 is a bit low on compression.You might have a rebuild in the near future.
This is what my 1999-2002 400EX repair manual says for the compression. The reason that it's supposed to be so low is that it has a decompressor assembly built into the cam.

 

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could have some worn oil rings maybe ?..that will make the compresson low.not to mention black spark plugs....i would just redo the carb for now..see how that does.
 

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His compression is good, they have an auto decompression thingy on the cam...
 

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see..this is why i run stock..i don't have to even guess why this is that ? or why is this ?....takes all the guess work out..no after market muffler..no after market air filter..no after market jets...eliminate all that..and if i have a problem..i know where and what is causing the problem..give me stock..or give me a dead old school honda !..lol.
 
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