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XR280 Mikuni 32mm Jetting

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Just bought my son an 01 with 280 kit, 32mm Flat Slide and FMF Pipe. Air box lid is gone with new clean Twin Air Filter. Bike has a little puff of black smoke when you rap it after it is warm. Recently pulled the plug out (which stripped UGH) and it is powdery black. I have the head off for the repair and it has tons of carbon build up.Bike will start without the choke and the air screw (2 stroke carb) will kill the engine beyond 3/4 way out. Bike runs pretty good with no real bad flat spots except a little up top. I rode behind my son a lot on our last ride and did not notice it running so rich that I was choking. He is a new rider and spends most his time in second gear in the woods. Carb came from XRs only and it has a 195/35 with 6DH3 Needle with clip in the middle position. I know I need to do a throttle chop to check the main but I am confused about the richness? The notes on the Carb say it is jetted for 3200' and I ride 700' to 1000' would should be a lean condition. When I say evidence of richness there was black powder just sitting at bottom of header exhaust where it meets the head. Recommendations? Could the choke be leaking and making it run rich without actually killing the motor? Position of the air screw leads me to believe it needs a larger pilot but that would be richer but it also starts without the choke? Thanks in advance everyone!

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I would try 30/200 first and then go up and down with the needle. After.

I did the 30 and the bike had a terrible bog. Put a 40 in and the air screw is 1.5 turns out now, been raining like mad so I haven't checked the main yet.

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The pilot is only an idle jet. the bog is coming from your needle I bet. My 34MM is very picky about needle height. Try going one step richer on the needle. Also since Mikuni has dozens of needles your needle may not be happy either step.. as mine is right now. If you can start your bike cold with no choke your pilot is too rich.

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Ok, I thought the pilot affected to just beyond 1/4 throttle? Did not have the bog with the 35 in it. This carb came from XR's only and has the 6DH3. I could almost get rid of the bog with the air screw turned all the way in. I'm not dis-agreeing about the no choke issue.and it being too rich. The bike sometimes pees on the side stand so I adjusted the tangs about 1-2mm. I moved the clip up one from middle and I have the 40 pilot in with air screw out about 1.25 turns and the 200 in it. Oddly the bike has never run just terrible, but the dang carbon build up tells me it could run better. Still pouring here so I am waiting to do a high speed run.

Edited by kingsxjt

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My 6DH3 is too lean just off idle. When I run second clip from the bottom it bogs when I hit the throttle, when I run bottom clip it has no bog off idle but runs badly rich all the way up after that until WOT. I havn't taken the time to go through the mikuni needle catalog but I am going to change the needle when I get a chance. TM34 with XR's only adapter.

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BTR, which carb is that needle from... Kiehin?? I'v gone back to center on my needle and it runs so sweet from 1/4 to full throttle but terribly lean on 1/8 throttle barely runs. Pilot jet is a little rich but that dead spot! Oh it hurts. Pops real bad when I let off at low thottle settings too.

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OK so after doing more research on this subject, I understand the problem better. The standard TM series Carburetor only has one slide available.. with a 4.0mm cutout. This dumps in a large amount of air right at the beginning of the throttle, and largely controls mixture in the 1/8 throttle range. This is why so many people have the bog off the bottom I am guessing... you have to enrichen the pilot circuit and the lower needle circuit enough to overlap the mixture in the 1/8 throttle range.

 

Didn't someone already say this was supposed to be a two stroke carb?   :banghead:

Edited by Firestone

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Ok I'm a dummy... I had the air screw out too much. Simple things sometimes get overlooked.

Current numbers:

200 main jet

30 pilot

Pilot mixture 1 1/8 turn out

Needle position Center

Stock motor

Stock exhaust baffle removed

Stock intake with snorkel removed.

65 degrees

34mm TM34

XR's only adapter

6DH3 needle slightly modified (modification not needed or recommended)

It runs well enough for me with a worn slide. 1000' MSL. Pulls strong and smooth all the way up. I may go back to a 195 main though because I can smell soot on me after extended high speed runs.

Edited by Firestone

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