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1975 cb125s engine rebuild or....

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I picked up a 1975 cb125s the other day from the original owner. The engine needs gone through before being road worthy, so I figured while I'm in there I could do some "modifications". My question is are there big bore kits for this engine? I also read about a 200cc lifan swap that bolts in with the only modification being to the top mount, and the lifan 200cc engine is a direct bolt in on a crf 230. So that got the wheels turning about a possible 230 swap with cdi and possible big bore kit into the CB. I'm posting this in the dirt section because the off road guys know the ins and outs of these motors better than anyone, any help is greatly appreciated, and suggestions are welcome, Thanks in advance!

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Happen to know these motors quite well. Up to 240cc possible but usually suggest a 175 which is a stroker/big bore combo. About all the head will flow as far as cc's. Run different head and bigger is better.  The lifan would definitely be cheaper and they run pretty good but as far as long term a little iffy in my opinion. Ask me any question you like. 61mm bore for 145cc is cheapest bang for the buck. Add a cam a little headwork a budget 26 carb and it runs vey well.  Don't get carried away as some of the high-end race motors i have built have reached 3000 bucks. Keep it simple and use the right combo of parts and good fast reliable. By the way I have everything but forged piston on shelf. 

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So what would I be looking at budget wise? I can assemble the motor, but if there is machine work involved I'll have to send it out. Is it recommended to change the timing chain and cam gear during the rebuild? Cost wise am I better off picking up a stock 230 for around 900?

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cost wise you are best off with 230. I end up near half that cost just in piston/sleeve and bore for a mere 145cc 11-1. You would end up with that much again to have head ported new valves,springs, performance cam and a carb. Yes it would run very well but 230 is a torque monster so put that in and gear up bike. For street it is hard to beat big cc on small bikes. Yes I can make the little motors scream but costs get high. Fellow roadracer in Australia needed a motor to fit in class. Went from 66 mph to 96 but literally ran out of gears. And this was basically a 98 octane moderate race motor. Of course killer skinny race fairing etc butjust shows a 150cc or so will run.  Get the 230 for 900 if it runs good and in good shape. Real charging system 6 gears and potential for more power later

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Ok, that's the path I'll follow then. I may put the 125 on a shelf for parts or another build. I appreciate all your help!

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Well I may be picking up an 84 200x three wheeler tomorrow, so that engine would work well in my CB. Supposedly runs, but needs a cdi box. We shall see. I believe I'll have to convert it to rear kick, correct? And these engines share some parts with the xr200 right? I've been researching online about compatibility and some of the info out there will make your head hurt! Thanks again guys.

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