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I went through a lot of posts regarding this exact topic, I know how most of them seem to say its worthless and a waste of time but the main point they seem to present is its cheaper to just buy a new bike, I agree its a lot of money burned down just to gain a few extra horses but for a person who is about to go through a complete engine overhaul don't you guys think its interesting and worth giving a shot ? I am in pursuit of this extra few horses as i might have to get my cylinder head and cylinder changed but more on that later. I found a website that lists the parts manual for the CB300r, It looks like it uses about 60% of the exact same (KYJ-901) parts. The few differences I found are in the balancer shaft, Cylinder head, Water temp sensor at the place of our O2 sensor and the Cylinder itself. I am going to keep my cbr250r and i have no plans of buying a new bike so this might be something that i can do. Hope you guys will help me figure things out :) Here's the link to the website N K Motors | N K Motors
 

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don't forget you'll also need from 300 (in addition to parts you listed):

  • crankshaft
  • connecting rod
  • piston
  • throttle-body w/TPS & MAP sensor
  • injector
  • IACV set
  • ECU

A simpler solution may be to swap 300 engine into your frame. I've seen this done in couple hours. Rather than the weeks it'll take you to pull 250 engine, disassemble it to put 300 parts in and put i it all back together
 

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Honda: INNOVA125i(2010); CBR250R(2013)
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Rather than the weeks it'll take you...
Agree
Time is also a function of knowledge, experience, tools, workplace...
I estimate it would take me about four weeks and without stress maybe 8 weeks, and I would add to that another month maybe a month and a half for waiting the parts I ordered online.
 
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I went through a lot of posts regarding this exact topic, I know how most of them seem to say its worthless and a waste of time
Me too.
This is what is sometimes called: "The wisdom of the masses", OR "To shaving on the beard of others* ... If you are young in age or young at heart "our" wise advice will not help you, I too learned some of my best lessons when I went with my heart and against the current.
Good luck to you!!!
 
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I went through a lot of posts regarding this exact topic, I know how most of them seem to say its worthless and a waste of time...
Yes, it can be very valuable learning experience. About 35-yrs ago, I blew up engine on my 1st bike, VF500F. I had particularly difficult semester at school (O-chem), and didn't have time to ride my bike. Left it parked for 6-months and one of carbs clogged up. When I did get around to riding it, lean mixtures in cylinder with clogged carb melted hole in piston and snapped con-rod. All estimates and advise was to swap in new/used/junkyard engine, but I didn't have $2000-2500 to do that.

So I figured I'd tear engine apart and replace just piston and con-rod. While apart, I had cylinder-head shaved to increase compression and port/polish intake & exhaust tracts. Took me couple weekends, I learned A LOT and had better performing bike for only couple hundred bucks instead of thousands! Still have this bike today! :)



At another time, I took it completely apart and painted frame black. Much better looking.
 
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