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Old 04-20-2015, 04:59 PM   #1
UtahGuido OP
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Small Bore Carburetion

I was just thinking it would be good and worthwhile to have a thread devoted to puzzling out the mysteries, and virtues, of carburetors designed for small bore motors, 4 stroke, and (if we must 2 strokes).

My current application is a Yamaha TTR125 motor with a Mikuni TM20 round slide carby shoehorned into a Scorpa trials frame and chassis, constituting a Scorpa TYS125.

It runs rich. Plugs are black. I'm at 7000 ft. I could: A) give it more air, which would mean doing some sort of mod to the airbox and getting more air into the intake or, B) going to a smaller main jet; I'm at a 100 right now. 17.5 pilot. The TM20 has an adjustable needle and it is currently set as lean as it will go, clip on the top groove.

Once past the jetting I'd like to work at eliminating a tendency to bog out with a quick application of the the throttle. I think an accelerator pump would solve the problem but there don't seem to be any available in bore sixes in the 20-24 mm range.

Small bore carb = better low rpm torque, or so I'm under the impression.

I'm thinking maybe a Mikuni TM24 might be an improvement or, for bling and gonzo, and maybe killer performance: Yoshimura!?

So whatcha got in the small engine carb department and whatcha want to share?
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Old 04-20-2015, 05:57 PM   #2
lamotovita
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Small carbs pretty much work the same way as big carbs. If your bike/carb is well used you might want to replace the needle jet set. They wear and cause rich conditions.
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Old 04-20-2015, 06:11 PM   #3
UtahGuido OP
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The bike is a lightly used 2007, at least before I got it. I can't imagine that things are that worn, but who knows. All my bikes run rich at 7K ft and I'm thinking I just ought to drop the main jet size and see what happens.

Anyone know of small bore carbs with accelerator pumps? They don't seem to be that common in the 20-24 mm range?
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Old 04-22-2015, 10:23 PM   #4
GBAUTO
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You could also lower the float level. That will have the same effect as running leaner jets across the board.
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Old 04-23-2015, 04:45 AM   #5
stevo7706
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The Keihin PWK(flatside) series is the carb of choice in that size. I believe OKO makes a knockoff. Much easier to jet than the Mikuni.
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