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Discussion Starter · #1 ·
Going from 14-tooth to 15-tooth on the countershaft wasn't enough after adding a BBK and race head. Now I'm adding this 25-tooth rear sprocket (ordered from Kitaco in Japan, took three weeks to ship) to see if I can balance the extra horsepower with less rpm for a better riding experience. "Cruising" along in traffic at 9,000rpm in top gear has some drawbacks because, well, ****.
 

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Discussion Starter · #2 ·
The stock overall drive ratio (crank to rear wheel) in top gear is 2.329:1. With a 15-tooth front sprocket, the drive ratio becomes 2.174 which is better for my purposes but with, engine mods, I have more than enough power to hit redline in top gear. I'd like to lower my gearing further so that I have JUST enough power to hit redline in top gear while lowering revs everywhere. Changing sprockets from 15/30 to 15/25 puts the overall drive ratio at 1.812. Turns out, raising my gearing that much is crippling to my top end. The next logical step would be going back to the front 14-tooth sprocket (1.941). Except that my drive chain would need to be shortened (it's actually too long already with the 15/25 sprockets). So...maybe I'll try a 14/27 next for a 2.096:1 overall drive ratio.
 

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I'd like to lower my gearing further so that I have JUST enough power to hit redline in top gear while lowering revs everywhere.
That's how I like gearing also. Due to the hills in my area, I wouldn't mind going a bit shorter than that even so I don't run out of steam going up hill. Let us know what you end up with and what your GPS top speed is with it. Also, which kit did you go with?
 

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I am running 15 front and plus one for the rear sprocket. With a high compression piston and cam I am topping out at 70mph on the limiter. I may switch back to the stock rear to get an extra MPH perhaps.
 

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Discussion Starter · #5 ·
The kit I went with? You mean the big bore kit and race head? The choices not requiring engine removal (60mm piston and 143cc cylinder) were either Trail Bikes or Kitaco. For cost reasons, I went with the Trail Bikes big bore kit and their V2 race head both of which I've been more than satisfied with. For liability reasons I assume, TB doesn't provide any instructions but YouTube videos showed me what I needed to install the engine kit parts without much trouble.

I would love to put in the Kitaco 5-speed (the Takegawa 5 and 6-speed transmission kits have a top gear that is no taller than stock) but another $410 for that would be seriously pushing how much money I'm willing to put into a 125cc street bike. If Yamaha brings their M-Slag/MT15 to the USA (liquid-cooled with a 6-speed), it would crush my bike despite likely costing less. Would the Yamaha be as fun to ride and own as the Z126 is? I'm not sure.
 

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Correct me if I am wrong, but the MT15 with 17" tires and weighing 300 lbs, isn't that more of a full-size bike with small engine making 16 hp? I mean, my WR250R weighs about the same and makes close to 30 hp stock. Of course, I'm sure the MT15 would be cheaper, but ****. Check out the Beta 125 rr-s. Those are available here, weight about the same as the Z125, make about 15 hp, can go anywhere, and are relatively cheap.
 
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