Yuminashi 143cc big bore kit (BBK)

[PeteFinVA]

I'm strongly considering A 143cc BBK. There are a few other threads on BBK's but those move too far away from just doing something as simple as a Yuminashi BBK and an injector. On Youtube there's a relatively new video by 'Ozark Garage' that has me just about convinced to install a 143cc kit, a 140cc injector, and nothing else. I'm not looking to increase top speed. I just don't want to slow down to unsafe speeds when have to be on pavement (like a two lane truck route in the mountains).

Can I get some feedback from anyone who has done only a 143 Yuminashi kit and an injector? Is the bike still reliable? Are there heat problems? Any failures? Knocking or pinging with different fuel types?

 

[JPMcGraw]

You should be good with a 143, the injector they suggest, and a $35 efie device to richen up the fuel mixture.

 

[PeteFinVA]

I bought the Yuminashi and installed it this weekend. I went with a 140 injector. Noticeably more low end power but top speed dropped. Best I can figure at this point is that the stock exhaust is the limiter and restricts too much. I figure that at 51mph now I reach the same volume of air that the engine was passing at 54mph before. I’m reluctant to swap the exhaust because (a) I like quiet, (b) I prefer a spark arrestor particularly when the forest is bone dry like it is now, and (c) exhaust thru a catalytic converter smells better than without. Plus, a bunch more money to see if I can get it back up to 54mph just doesn’t seem like a good idea. Any suggestions?

 

[RustyRodder]

There are some other exhausts that still have built in cats but are less restrictive. But they still arent cheap- I have a stock pipe on mine again (Had a minimoto pipe), wasnt a big fan of the noise either.

 

[m in sc]

the stock exhaust is restrictive even for a stock bike. if you wont change it, then you're stuck with that power level IMHO. with my 143 i could hit 62-64. not huge gains but def better. the ecu is the next restriction.

 

[PeteFinVA]

I’ve put a few miles on it. Top speed is about the same (I did hit 56) but I can get to it a little sooner and don’t lose as much speed on hills. Fuel consumption is up (down to maybe 100mpg from 144mpg) so range is quite reduced. I don’t think I would do it again and will probably put the stock 125cc back on to get the range back since even with the power gain it is still too slow for me to feel comfortable hopping on a busy road (like the short stretch of US460 on the MABDR).