Black smoke at high rpms

[yernkie]

My 400ex just started puffing a little black smoke at high rpms. I'm pretty sure it is burning oil. It is strange, though, because the spark plug looks perfect (no black build up) and it doesn't smoke at all, even when first being started after sitting for a week or two. Thoughts?

[smelly$cat]

To me, black smoke is unburnt fuel. Maybe your carb jetting a is a bit too rich.

[yernkie]

Could be, but why at only high rpm's? Is the main jet the 3/4 to full throttle circuit? It still runs great a pulls hard. I thought oil was black smoke...maybe not.

[katch26]

yes main jet is 3/4 to wot.....blue is usually oil black is fuel

[yernkie]

I'll lower the jetting and see if it helps. It is odd because I haven't changed the jetting and it just started doing it.

[katch26]

it is weird becuase mine only does it when spring hits and Im still jetted for the colder weather.

[yernkie]

I bet I know what is going on...

I mainly ride tight trails around here, so no WOT. I first noticed it a few weeks ago while riding at the dunes, which is mainly WOT. I am going to lean out the main jet this afternoon and see what that does.

Thanks everyone.

[katch26]

remember 3/4 - wot is a throttle position not a speed though

[yernkie]

Originally posted by katch26
remember 3/4 - wot is a throttle position not a speed though

Yes. I am usually WOT just trying to keep up with the faster bikes when we are in the sand :(

[yernkie]

Thanks everyone...it was the main jet. I just bumped it down a few sizes and all is good now.

[katch26]

what were you at before?
now?

[yernkie]

Keihen 175. I took it down to a 170. I have been running the 175 for over a year with no issues. My mods are full Yoshi exhaust, stage 1 cam, no airbox lid, K&N, and modded airbox. My plugs always look just a little on the rich side, but nothing too bad. The last dune trip the guy riding behind me noticed the puff of smoke. I have never noticed it before, but then again I am never looking at my pipe when I am at WOT. When I jsut had it in the garage I saw the puff when I was playing with the throttle in neutal, but after the rejet it is not there.

[hotrodhonda400]

I have the same mods with the exception of a Uni filter and my lid on and i run a 155 jet... am i too lean 170 seems really rich :confused:

[gtt416ex]

Originally posted by hotrodhonda400
I have the same mods with the exception of a Uni filter and my lid on and i run a 155 jet... am i too lean 170 seems really rich :confused:

it all depends on where you're located. Elevation, humidity, temperature all play effect on your jetting.

[TRXRacer1]

Originally posted by katch26
remember 3/4 - wot is a throttle position not a speed though Proper tuning is dependent on PRMs though.

[katch26]

right but he was using wot throttle like full on 5th gear and my point was the bike doesnt have to be topped out to be on the main jet. Rapping out 5th gear in the dunes has the same rpm, carb and throttle position as winding out second on the trails.

[TRXRacer1]

Originally posted by katch26
right but he was using wot throttle like full on 5th gear and my point was the bike doesnt have to be topped out to be on the main jet. Rapping out 5th gear in the dunes has the same rpm, carb and throttle position as winding out second on the trails. True True!;)