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400exrules
03-17-2006, 11:47 AM
My 400 is a 99, and i just bought it a year ago and as far as i know it hasnt been rebuilt. It runs great and doesnt burn oil, but after it sits for awhile and i start it up, theirs a few small puffs of black smoke (thin) and then it goes away after a few seconds. And i took the endcap off my exhaust the other day and the inside of it was black.
Im thinkin its time for a rebuild, but i cant right now cuz racin season starts this weekend. Will it be ok to wait till next winter and go with a 406 or 416?
enduro400rider
03-17-2006, 11:54 AM
probably valve seals, oil leaks in when its not running and burns off once you start it up
GPracer2500
03-17-2006, 11:57 AM
Blueish smoke at start-up that goes away after a few moments only to return after another startup (once the quad sits for a while--ususally days) is ususally a sign of valve seals that are letting some oil weep past.

ETA: Enduro beat me to it!
400exrules
03-17-2006, 06:10 PM
its not really blue tho....the inside of the endcap on my hmf gets this black build up around the inside of it, and i wipe it with my finger and its like coal dust
Honda TRX250ex
03-17-2006, 06:31 PM
Originally posted by 400exrules
its not really blue tho....the inside of the endcap on my hmf gets this black build up around the inside of it, and i wipe it with my finger and its like coal dust
Your burning to rich if its black smoke and a few puffs of it when it starts.
Flyin-Low
03-17-2006, 09:15 PM
That "coal" is carbon build-up... Its just from running too rich, dont get yourself in a hurry to tear the engine apart, try running without an airbox lid for a bit to see if it goes away, otherwise change the jetting or just suck it up and not worry about it if it runs fine :cool: ;)
400exrules
03-18-2006, 07:18 AM
ok lol. thats a relief. thanks!
yah i mean, ive never fouled a plug with it, so....i guess its alright the way it is
Camaden
03-18-2006, 07:26 AM
I get the same black film on the inside of my HMF pipe, but the bike runs fine and plug looks pretty good so im not messing with anything I dont think.
GPracer2500
03-18-2006, 11:54 AM
Originally posted by 400exrules
ok lol. thats a relief. thanks!
yah i mean, ive never fouled a plug with it, so....i guess its alright the way it is

It's pretty hard to foul plugs on four strokes--you've gotta be WAY to rich to foul one. Just cause you haven't fouled your plug doesn't mean your even close to correct on your jetting. A thin black film on the inside of your exhaust is pretty normal even with correct jetting. Black puffs of smoke are not normal.

Jet your bike properly and it will run better/faster/longer. You'll be glad you did!