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Discussion Starter #1
My shiny new carburetor showed up today (stock 1992 version). I opened it up just to see what's in it... the main jet is a 114. I didn't check the pilot; I figured somebody knows. I went ahead and shimmed the needle since I've read that that's a popular mod. Haven't done anything else other than replace the annoying philips screws with socket-head ones... bowl, top, and the two little ones that hold the needle assembly together. All stainless. :)

I seem to recall reading that the recommended change is one step up on the main and pilot jets. This is on an otherwise stock bike with nothing but a UNI air filter. I figured I would make those changes before installing it, since it runs passably okay with the original carb right now. Once the new one is on, I'll do a rebuild on the old one and save as a spare.

And I'm at sea level too fwiw.

BTW -- what brand is this thing? I don't see Mikuni or Keihin anywhere on the carb body. Is it something else? Wondering where I would even find jets for it.

Thanks!

Rob
 

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Discussion Starter #3
Augh. I KNEW somebody would ask that.

Because NO stock carb setup ever runs right from the factory. EVER. All these bikes are set to run dangerously lean for emissions reasons. OEMs know you're going to likely rejet it.

For example... the AF screw on this is only set to 1.5 turns out. Shouldn't it be 2.5 to 3?

Maybe I'll just go ahead and weld a bung to the head pipe so I can put my wideband O2 sensor on it and just tune it that way like I did on my KLX and DR.

Rob
 

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Discussion Starter #5
Nah it's okay. I just ordered some extra exhaust bungs. Soon as they arrive, I will weld one on the old steel headpipe (since I can't weld on stainless) and then use that to actually see what the jetting needs to be.

Rob
 

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O2 sensor? Way too exact and scientific for our primitive beasts. I am totally impressed
 

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Discussion Starter #7
Haha thanks. I bought the thing back in 2009 to tune a sand quad (Polaris Outlaw 525) since it's fast for a quad and I wanted the best performance for a given elevation. Then the thing ended up in a plastic tub for a number of years until I got my DR and KLX, then I welded bungs onto their head pipes and used it to fine-tune those bikes, especially the KLX since I did it stock and then again with the 351 big bore, and then again when I put the pumper carb on it.

So now with the little TW, I'll figure out exactly what the proper jetting is for my bike at this elevation (sea level). For dual sports I prefer to tune at sea level, since you never know what elevation you'll be at. For a sand quad, sometimes a trip may have you at sea level (e.g. Oregon Dunes, Glamis), other times you could be at 4000-5000' (Sand Mountain Nevada, Little Sahara Utah, St. Anthony Idaho).

And fwiw, the difference in jetting between sea level and 5000' is usually only one main jet size and 1/2 to 1 step on the needle.

Rob
 

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Before you install the new carb be sure to pull the brass plug covering the Pilot screw just forward of the float bowl. I would turn the pilot screw to the bottom and then back it out 2-2 1/4 turns to start with and adjust it from there after the bike is at operating temp.

GaryL
 

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Discussion Starter #9
New carb is in and it's running great!

Yesterday I welded the O2 bung onto the old steel head pipe. Today I put that back on the bike (in place of my prettier stainless steel one, temporarily), then hooked up the Wideband O2 sensor and went for a ride with the original carburetor. Results? WAY fat. Part-throttle and wide-open I was seeing Lambda values of around 0.7 to 0.75 (with 1.0 being perfect and anything over 1.0 being lean). Yet the bike seemed to run fine.

So old carb came out and new carb went in. I jetted it using ProCycle's jet kit... I started as their paper suggested, using their 110 jet, the 42 pilot and three shims under the needle. I also set the AF screw to 2-1/2 turns out.

Went for a ride... Not as fat as before, but still fat.. around 0.8.

Rejetted... dropped down to the 107.5 jet and removed one shim. That leaned it out a little more, now around 0.85 to 0.9. Close, but not good enough since I'm at sea level.

One more time... put in the 105 jet (the smallest one included in the kit) and removed another shim. Now down to one shim plus the factory shim thing with the little notch on it. Now I'm seeing 0.9 at part throttle and 0.95 at WOT. I'd call that as close to perfect as I'm gonna get, and given the 60-ish deg temps, that leaves room for riding on a cold day. I could probably have pulled that last shim from the needle, but frankly, getting to the needle in this carb is a pain in the ass, so I decided it was fine.

I'm guessing that due to age, the "114" main jet in the original carb had worn out and had quite a bit larger hole than it should have. I'm going to disassemble it, soak it, then do a full rebuild on it... then probably sell it.

Rob
 

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New carb is in and it's running great!...

....One more time... put in the 105 jet (the smallest one included in the kit) and removed another shim. Now down to one shim plus the factory shim thing with the little notch on it. Now I'm seeing 0.9 at part throttle and 0.95 at WOT......Rob
Rob, are those results consistent at all throttle openings?

Are your using a Bosch sensor?

And which A/F meter are you using?

....and... do you have a way to compare the ProCycle jets to the stock 114? The last time I asked ProCycle I never received an answer.

Thanks.

jb
 
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