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Mikuni TM27

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A 27mm carb is not too large for a 125 fourstroke. There are some things to keep in mind.

First, until you open up the intake and exhaust, the gain from a larger carb will be less noticeable. The reason to go to a larger carb is to reduce the drag of a smaller carb bore. However, until you reduce the drag from the intake and exhaust, the stock carb may not be the most restrictive part of the system. Basically, you should upgrade the part of the flow structure with the most drag first, the next most drag second, etc. Usually that means really opening up the airbox and going to an aftermarket air filter. Next, the entire exhaust should be replaced with something with bigger manifold piping and less restrictive muffler. After doing that a carb upgrade begins to make sense.

Second, going up in carb size can significantly increase the responsiveness of the engine, but it may come at the expense of ease of jetting. My experience is that larger carbs tend to be more sensitive to altitude and weather changes than the smaller stock carbs. That is probably due to reduced vacuum in the carb. The bigger bore reduces the carb's drag (which is what we want) but also tends reduce the vacuum in the new carb. That tends to make it more sensitive to the factors that affect jetting.

The DR-Z125 uses the same carb stock that the TTR125 does. I upgraded my intake and exhaust and upgraded to a Mikuni VM-26 carb. Wow! However, it is much tougher to get the jetting right.

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Great info,, this has been gone over here before. the 27 is not ideal the 26 vm is ideal. the flat slide carb don't atomize on this bike as well so it's been a no-no here.

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