cb125 cases with cb100 internals, carb

E

engin3r3d

New Member
Hey guys, new to the forum.

I have a few quick questions about my cb100 build


I'm wanting some more power out of my 100, the streets of boston can be hectic. Fucking massholes hahaha

I have a complete running 72 cb100 motor as of now. I'm not wanting a full race motor, so powroll is out of the question. If I was to get a set of bare cb125 cases on fleabay, will I be able to swap all the components from the 100 into em granted there is nothing FUBAR? I'm wanting to get a cb125 top end as well, so I'm assuming the crank will need to be from the 125 as well due to the pin size. I can source a big valve head as well from tenavintage, I've had good luck with him in the past.

As for the cam, are there any that will swap in to perform close to a race cam? If not tena can source one for me as well.

Now for the carb.. I'm going to source a keihin from fleabay as well, likely a take off from a similar era honda. What size would be ideal on the 125 with bvh and cam upgrades?
22mm? 24mm? Stock 100 is a 20mm if I'm not mistaken.



I've read through a handful of threads on here, but have to conflicting conclusions. Any help will be appreciated!

Thanks
 
It would probably be cheaper to just buy a 175 engine? They arent too expensive.
Then sell your 100 and youre good to go.
 
I think the "175" was a typo.

Why are you swapping cases? Pretty sure they're the same. Just get the 125 top end.
 
Cb125 is a 15mm pin right? Cb100 is 14mm. Can I just bore the connecting rod out 1mm and be alright?

I was assuming I could just get a complete bottom end crank included. If I can source a full 125 motor I guess that'd be prime.
 
Why are you making things more difficult then they need to be?
Just bolt in a used cb125 engine or buy a lifan 150 and be done.
No need to mix match or machine stuff. Nothing honda single is rare or hard to find so making work for the sake of work is dumb...at least to me.
Not to burst your bubble but if the 100 is to slow you will not feel that much of a gain by swapping to a 125. Might be time to sell this bike and buy something larger, again should not be that hard to find a bike suitable to what you need to do.
 
If you really must make a project out of this then check out the oldrice website and forum.
Read some of the old PDF articals on the site.

125 base, cb750 piston(hell I have an NOS one I could sell you), xr200 head and cam, and be call it a day. At that point it's off the shelf used cheap parts and not crazy stressed so it shouldn't break all the time. Plenty of options for carbs too.

How is the bike set up now? Lots of power is lost with a pod filter and cut exhaust.
 
The 750 setup sounds interesting. And far cheaper.

Bone stock motor. Battery elim from oregon,Xl100 exhaust, uni pod filter. That whole rear section of the frame is being left open hence the battery elim and pod.

125 bottom end/jug, and 750 piston? Or can i just swap in a cb125 crank/jug and bore the cases? Any mods have to be done to the piston?
What year 200 head will fit? I'm assuming the older models huh.

I'm an mech engineering student. I'm digging the project. I will check out oldrice as well.

Thanks for the knowledge man.
 
Battery eliminator might be causing some power loss too, never seen one run will for any length of time with no battery...and defiantly not when working lights.

The 750 piston isn't really bolt in as you need to bore the cylinder.
Stop thinking about opening cases, comeplete waste of time unless you are for some reason looking for a large project to "learn" on. In that case get a law mower or something, that way you at least have something to ride.
Do some reading and searching around for the xr head swap, it's fairly well documentd online.

I'm a mechey too, and have run SAE teams and car/bike clubs on campus. Can't tell you how many people bite way more off then they can chew just for sport. Keep it simple and make real improvements.

CC increase on a Honda really doesn't help much though for HP. The bottle neck is the head and the motor breathing. Large carbs on stock heads are a waste...ask anyone that fell for the "pre jetted" BS from DCC....

Uni is one of the better pods and will help, but I'm willing to bet the bike is cold blooded...
 
Should've just done the 12v swap huh. Probably the way I'm going to go now.

I have seen a lot of threads on the 200 head on a cb. 8mm longer stroke, how do I deal with it? Stack base gaskets?

So, 80s xr200 head/cam setup, cb125 or 80s xr200 jug, and then bore the rod out 1mm to fit the 15mm pin. Bore/sleeve the jug. 24mm keihin and call it a day.

Again, thanks for the knowledge.
 
What does the 12v give you other then the option of different light bulbs?
It's not like these bikes didn't work when new...

Head has nothing to do with stroke...well not a lot.

The idea of boring the piston pin is odd to me. Mostly the question of why comes to mind. Just get a 125 engine or at least a complete bottom end. Are you set up to bore the pin yourself? Bet it costs more to bore it then an engine...again we are not talking about rare parts. Also what would you do for a bearing in the rod pin?

This is the web, nothing gets done here...order some parts and physically fit them together and see
 
CB750 piston needs machining to fit and gives you a 145cc motor
Nothing was available 'of the shelf' in 1975~78 so it was a common mod
Get a Lifan OHC motor, it's about double the HP of stock 100 motor, the OHV is about 5~6 extra BHP
 
6v batteries are massive and heavy. I havent been able to source one nearly small enough for my liking. Nowhere to hide it. Going for the minimalist thing, hence the 12v swap idea as of now.


I guess I will be in the market for a cb125 motor or bottom end.


I will never put a Chinese motor in a Japanese bike again. Ever. Been down that road enough times with mx
 
I was trying to do the same thing as you with my 100.

Didn't seem to be worth the hassle to try to pep this thing up.

Now, all I want to do is get the right parts and get it running.
 
Update!

Found a source with 14mm pin'd piston 57mm and up.

The stock cb125 cyl is a 56.5mm bore correct? How far can I bore the stock sleeve?

Here is the new plan: cb125 cyl, bore the cases to accept, whatever size piston I can fit in there safely, 80s xr200 two piece head and cam, appropriate sized carb. Done.

I'm hoping I won't have to deepen the relief pockets on the piston, but ill cross that bridge when I get there.

Read just about every thread on the xr head swap, and have not come to any complete conclusions. Anyone know if the cam chain be the correct length? And how the cam will run in there? Not a big deal but would like to have some idea of what I'm working with.
 
Measure the diagonal between cylinder studs, you need about 4mm liner thickness so (???mm) minus 8mm is max overbore ;)
(been using that method for 30+ yrs ;D )
As for 6v battery, any emergency lighting one will work, somewhere around 4A/hr
 
Yep, it's going to cost some shekels to experiment, but, look at it like this.
I used to teach motorcycle mechanics
The total cost was between $26~$30,000 and you got some technical knowledge. (you could spend a lot more and be a 'professional student' ;) )
For $3~$800 you'll get some experience and bad habits, plus, you may even end up with a cool bike ;D
For example, Brendon has started buying random stuff that may work on his bike, but, a few things are already being shelved as they just didn't work as expected (I'm just supervising, he's doing 95% of the 'heavy lifting')
I have pics of him working on bike in my blog/build (see link below in sig)
You need to just start doing stuff
 
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