cb125 performance mods. Got any?

Redliner said:
These bikes show up DAILY on CL for $100-200 complete. Just this year I bought a non-runner in that range.
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Daily my ass.
Everyone around here wants about a grand for a 125 single.

Anyway,
Mine ran good with a mikuni carb and a unifilter.
You're better off to just enjoy it as is IMO since you could do 2000 dollars worth of work and still end up with 13 HP.
 
Look. I get it. My cb125 will never be fast. Its just that I've always exercised the idea that when you add more air and fuel to a combustion engine, it creates power. But for some reason this rule seems to be exempt with the cb125s. I have a 400cc single Suzuki that I installed a k&n filter, Yoshi exhaust and jetted the carb and guess what? It's faster. The same rules don't apply to a125cc? Whatever, it's cool. I love my 125. If anything, it's classy.
 

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Nobody is putting down the bike. We just have no idea what your end goal is. Looking at the pic FINALLY, I think it'd be a shame to chop and mod it too much.

Improve air filter over the paper one which by now is crap anyway. Change the sprockets to gear up which will really only increase top speed and drop horse. Those are the easy bolt-ons.
 
CB125 and their Chinese relatives are pretty tough motors and there are tons of performance parts available form big bore kits to 4 valve heads and so on. Or get a whole replacement hot motor for a couple of grand... In other words teh range is huge so take your time to find out what parts are available and what fits your motor.

Back in the day people modified CBN750 pistons and did all manner of crazy stuff. Then Powroll came along and had big bore kits and hot cams and stroker cranks and all other madness.

Best to shop virtually until you work out what you want to do and what fits the budget and what makes sense. The last one is the hardest to face and teh one I struggle with. Just because you could doesn't mean you should. ;-).
 
agallese0424 said:
Look. I get it. My cb125 will never be fast. Its just that I've always exercised the idea that when you add more air and fuel to a combustion engine, it creates power. But for some reason this rule seems to be exempt with the cb125s. I have a 400cc single Suzuki that I installed a k&n filter, Yoshi exhaust and jetted the carb and guess what? It's faster. The same rules don't apply to a125cc? Whatever, it's cool. I love my 125. If anything, it's classy.
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Don't get your panties in a bunch.

It is classy. That's the point.
The factory has teased out most of the HP already, like mentioned above they are pushing 100hp/l.
Sure tsos an exhaust and a filter and jet the carb, it will look better and sound better but won't be appreciably faster.

All I'm saying is you have to spend big money to get any real gains, and in the end a "big" gain is a couple hp that you may or may not even notice.
 
It's certainly not impossible to make a little extra power out of the little singles, but I think the point most are trying to make is, how much are you willing to spend to rob an extra 1/2 hp? Honda did a surprisingly good job of designing the intake (both the airbox and the intake tract) and the exhaust, which was engineered to get the most power across a wide powerband.

If you want to bore it out, get some headwork done, stroke the crank, etc just to do it, I get it. But you're gonna be paying good money for very small gains. Good luck with your project!
 
So here she is. I'm happy. Maybe some cosmetic repairs, but its summer and I wanna ride. New tires, bars, trumpet style muffler and blasted/painted rear shocks.
 

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Sounds great. No baffling. Only a spark arrestor. Kind of a sharp, raspy thump. It came with couplers to step the headpipe up to the correct size. Some welding was required around the pipe to seal up gaps due to it not being perfectly round. And the clearance is tight for the kick start. But overall it was cake.
 
I like the idea of finding an XR/XL200 motor or complete donor bike(I did for cheap$)and swapping everything over into my 81' CB125S frame.My donor bike is an 83' XL200R and they would top out at 80 mph;I think when I'm finally done I should be able to bury the speedo and take short interstate 'hops'.I'd much rather stay off the interstates anyway because the old roads are more fun.I realize I'd have more into the bike than it's worth but if the price of gas goes up again I'll be able to motor around for cheap and basically go anywhere I need to w/o 'having' to stay off the interstates at all.I wouldn't want to spend much time on them anyway.The CB125S is a great all around Basic transportation bike,if you aren't in any hurry... I'd rather take it easy and 'Enjoy the Ride' ;)
 
1969/70 CB750 piston gives a 145cc bike with minimal work. I think K0 was the last year of 'thick top' pistons, later ones were lighter but thinner
Piston needs machining, can't remember spec. I did quite a few of them around 1980
 
check out "old rice"
the XL board has a tone of old PDFs about oldschool tricks.
Lots of info on the CB750 piston swap
 
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