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1975 Honda CB125S

8971 Views 70 Replies 10 Participants Last post by  knappyfeet
I purchased this old 125 on offer up for $500. It needs a lot of stuff but it runs, rides great, and best of all it has a title and is currently registered… kind of a rarity for a $500 project on offer up or craigslist. My wife has been wanting to learn to ride on the street for some time .....she knows how to ride dirt bikes and stuff from years of desert trips...... but I figured it would be best to start off on something as small as I could find. As you can see it needs a few things and from just a preliminary inspection I would guess an additional $500 to $700 worth of parts……. So it could potentially be a $1200 motorcycle/project before I even take one rag to wipe the dirt off...but I feel it's worth it.

In my opinion there are 3 pressing issues that need to be addressed before she rides it and one is the front brake. It works but the inner workings are seized and just a big clump of rust. I thought it was a run of the mill front brake but it's some kind of cable squeeze contraption… something I've never seen before. It will stop now but it needs to be addressed. The 2nd thing is the carburetor. The engine sounds tight and not loose but the carburetor is not adjusted correctly or is not the correct one. It'll start…. Sometimes hard… It will idle high and then sometimes normal. It looks like at 1 point they replaced it with a Chinese knock off. I've had hit-or-miss issues with Chinese knock off carburetors so it may be that. It has 6800 miles on the clock and from the tapit covers it looks like the valves were probably never adjusted.

The 3rd thing is electrical. The horn and the headlight works and the tail light is dim. The previous owner told me it had a slow power drain so it may or may not be charging. I haven't taken a multimeter to anything yet so who knows but I'm preparing for the worst.

Good news is parts seem fairly abundant. Some things are dirt cheap and other things are through the roof. I can find a full electrical knock off wiring harness for 40 bucks new and mirrors and gators and little rubber things cheap.

So for now I will address the front brake before anything else so my wife can ride around in circles in a parking lot.. She's small at 5'2" and this bike is extremely small but something tells me she will outgrow it very quick.

It's nice to have a small bike back in the garage…. even if it is a work in progress. I'm sure things will surprise me along the way.

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The little ones are hard to find in decent shape and it looks pretty good. A sorted one for $1200 and some time is a pretty good deal. Old hondas are great to learn to ride, right position, passive engines. I still really enjoy riding them.
I couldn't agree more and that was kinda my thinking....... non intimidating and great to learn on

lol knappy you are so addicted to nice little motorcycles :cool: and your wife is awesome.
Haha!!...... I was thinking if another good deal came along that was slightly bigger than this we could ride around together and then she could graduate to the bigger bike....slowly
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I have a 1975 CL125. Owned for years now. Got it from the original owner with about 900 miles on it. Think over 4k now. Great bike to have around for runs for Chinese food and beer.

-Yes the front brake is a cable operated unit. Not the best stopping power, but it gets the job done. Should be able to strip everything down, clean it up, replace anything that is needed and be done without much hassle.

-That carb does look new, and would bet is aftermarket. Ive never had luck with them. Best to fine an OEM, and rebuild that for the bike. Kits from honda are super cheap. Actually I try and only use OEM parts on mine. Get almost everything direct from a dealer still.
These bikes HATE changes to the intake. The manifolds are stacked with spacers and gaskets, so tend to leak. Make sure there is some kind of filer in the airbox.

-Mine drains the battery too. Unsure if there is a real issue, or that is just part of living the 6V life. I replace the battery every other year to keep things happy.
Bike will not start with a dead battery, and runs like poop with a low charge one.
Thanks for the info...... Yeah I found out it wouldn't start with a low battery either.

I've been hit&miss with Chinese carburetors… especially for offroad. I mean they look the part nicely..... but sometimes maybe the innards aren't done well I don't know.

The front brake is the project for the weekend. I've got a small bucket of evaporust with its name written all over it......specifically for the clump of internal rust
did you get the side covers? my 71 CL100 is missing the battery sidecover, all of which apparently migrated to neptune. the right side covers are usually easier to source.
You know it didn't come with any side covers......but I've been looking on ebay and there seems to be a seller from Thailand that has some.... That's the only source I know.

The same disc and caliper were used on the Honda 78-78 MT-125R production road race bikes. Some guys replaced the calipers with hydraulic ones. Not sure with what. Probably Honda single piston from a CB 350, 400, 550, or 750
I may be looking into something like that and even a Chinese knock off as Classic250 suggested.

I've been looking on and all those little internal things are expensive and more importantly..."unavailable to ship"....which tells me unavailable period. I was expecting trench warfare to get it all this assembly apart but ironically it came apart without much fight.

I've worked miracles in the past with a can of Berrymans, Evaporust,...some phosphoric acid and WD40..... Maybe I'll have the same luck this time.

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Well the front brake has been gone through and it's all put together now. It's all pitted and rusty and just gross but it's not seized anymore… the lever action is working again… and it's stops. It's not ideal but it stops 300% better and that's all that's important right now. The previous owner installed a new cable thinking the seized disk was the problem of a bad cable…. but it wasn't. He told me that it was his daughter's project and she liked to work on older things. She just didn't know.... and just threw parts at it. I don't fault her for that because I would have done the exact same thing if I was repairing something I knew little about. I figured since this wasn't a hydraulic disk brake system that the pitting in the barrel and all the corrosion was no big deal.

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The pads had some good life left in them so I just sanded down the rusted corrosion parts and everything slides better now. Again since there's no fluids involved the pitting and the corrosion to me is no big deal for now.

Yes Classic250....... Post any and all links that you may have. It might help me in the future but more importantly may help somebody else who just stumbles across this thread using Google. Thanks a cool link woodsman..... Probably better than just sending people to for the conversion chart.

Now the next step is to figure out this carburetion. I'm getting tired of trying to get it to start with 20 kicks
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I would encourage you to see if the PO still has the original carb laying around, so that you can do the same.

Thanks for the advice but I asked when I was there.....

In fact I asked for 3 things while I was there….. 1 was to see if they had any side covers…. 2 was that if they had the original carburetor or any other parts like the mirrors or whatever and 3..... any type of an owner's manual or shop manual.

They had this owner's manual and it may have been some help for some things but it wasn't the correct manual. Which reminds me..... if anybody could put this to use let me know and it's yours free… I'll ship it to you media mail.

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Maybe somebody on this forum will have a carb laying around and will send it to you. I looked through my stuff, but don’t. 20 years and a few moves ago, had several. I will ask around and let you know if I come up with something.
Do you live near a M/C salvage yard? I am pretty sure that the carb from a CB/CL/SL/XL 100 is the same.
Be careful when buying side covers. The ones used on 1976(?) or 1977(?) and later 100s and 125s are different than those used in 1975 and earlier.
You can find the shop manual online. Check out DTT.
Thank you.....much appreciated!
Thank you

just out of habit took carb off......clean as can be. No jet size stamped. Everything blew out be honest I was expecting nothing less. I'll probably wait for something to pop up on ebay or something....a used stock carb.

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Also it leaks out the top of the float bowl.....not out the overflow. I'll check the float level just to pass the time...maybe it's flooding ...I don't know.

Ordered a new air filter.....the entire airbox needs a good cleaning and a new filter.

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While waiting on some parts to come in decided to start toying around with the electrical.

I also used my Harbor freight compression tester........ that works great on my cars but not so much on small displacement motorcycle engines, lawn mower engines, etc......especially since it has around 3 feet of hose.

I got the full 80psi (which is probably at least 140psi). Then added a little oil to the cylinder and bumped it up to nearly 85psi after a few kicks.

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After playing around with carb settings and installing the new air filter it pleasantly starts in only about 8-10 kicks....much improvement over the 20 kicks of the past....but still not correct. After a more careful inspection I found it doesn't have the o-ring at the carburetor base...just that old paper gasket. I can't believe I didn't notice that before. It was idling a little higher but I found that the throttle cable was getting hung up underneath the gas tank somewhere. When I re routed it no more hanging problems….. Other than after you rev it the idle will hang high a little and then settle back down…. I'm assuming because of all the air leaks in the intake track.

So I ordered all the gaskets, a new isolator, an o-ring, new fuel line and fuel filter...... together when all this is done it should run as good as I can get it with the Chinese carburetor…. until I source a used one somewhere.

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There's a couple of power draws I've found so is the wiring somewhere for the brake light and the rear tail light assembly all together. I'm sure there will be more everywhere but for now those are the ones. I'm still contemplating whether to purchase some new knock off wire harness or go through one by one. I already purchased a new aftermarket tail light assembly...just to make things easy. The wiring harness is so hard and brittle in some places…. with corrosive connectors that I'm leaning towards an aftermarket knock off. When hooked up to an outside 6 V power source all the lights and the horn with the exception of the headlight and taillight are bright . When the tail light is hooked up to an outside 6 volt source it still remains dim and lac luster….. But the headlight gets really bright connected independently to 6v.

I hate electrical

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I have done nothing on this bike except wait for parts to come in..........but was really impressed with these little side covers. 1.........they came from Indonesia in 5 days and 2........ The quality, fit and finish, and the paint was very nice...... All for $44 shipped and in 5 days. I'm waiting longer for stuff to come in from New Jersey!

The seller had all different colors and what not but I figured white is good enough for now……… just in case I want to refinish later.

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I purchased this Honda 200X that is taking a lot of my time right now from a family member for the same price I paid for this little CB125…. And it isn't running and it isn't titled and it isn't registered and I can barely move it with those flat tires.

Good news is I just got an offer on it for $1600 right now….. as it sits. Tempting.......that would fund some new shocks and springs for the XR1200.

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I love the side covers... though they remind me of bowling pins
I was gonna paint it flat black but now that you mention it it stays white....... I love bowling but am no good at it.
Nice project. Just joined the Forum. I just purchased a 1972 Honda cb100. Needs a lot of TLC. Think I'm in the same boat you were when you first started this post.
If mine ended up looking like yours does now I would feel happy....clean bike you have there.

Just adding up the parts I've purchased so far I'm probably close to $200. Still waiting for a carburetor isolator from Thailand but most is here. I need more but I have some new cables from other projects that looks like it would work on this. A lot of it is nickel and dime stuff but it adds up quick. Not counting registration ....... so far into this $700 and it still looks like it was ran over with a truck and abandoned. The time spent working on it is fun and therapeutic but costs can rise to what you can get one online or locally for in great shape. Still....even clean ones don't have titles so for that I feel fortunate.

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Another looming issue but not a problem now is the gas tank.

It looks like it was lined with POR-15 over a rusty tank. Now I understand some liners may if not tolerate prefer "not a smooth" surface....maybe por-15 is one.... but even if it did I would still have removed all the rust inside of the tank before lining it. I haven't had the tank off yet to see if there was evidence of gas leaking out of it. Aside from the cap not fitting properly on the tank due to the gunked up por-15 on it there's no issues with it leaking or not flowing properly.

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I'm not knocking the previous owner and God-bless her for trying because we all learned somewhere…. But it looks like they tried to fix the carburation and fuel issues then probably threw in the towel when faced with electrical and brake issues and everything else. But they still did the right thing and tried to learn on some old Honda beater.
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My bike won't start. I purchase 2 weeks ago and didn't start when I purchased it (took a chance) good price. Has good compression. I just pulled my tank off. It has a tan colored tank liner that is peeling off (nasty). Carburetor as well need to clean. I have never worked on a motorcycle before so this all new to me. Lots of YouTube videos.

Sounds like a Kreem liner.....gross

Jag posted in this thread that the 125 will not start if the battery is low. Maybe that goes to you as well. You pick the perfect bike to learn on… simple to work on and right there
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Still waiting on a couple carburetor/intake related items to come's been awhile waiting..... So I decided to blow money on stupid stuff I didn't need. I figured "how many Sissy bars come available for a 1975 Honda CB125S anyways?". So I bought it and I'm now wondering how I ever lived without it. It looks so cool on it (in my opinion) 70s. Now it's got me on this 70s customizing kick.

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It's like it's right out of a JC Whitney catalog I pulled my old catalog and looked at what they cost back in the day.. $14.95… I paid way more than that.

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I'd love to get a pad for it and stupid crap like this as well........

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Since I already got the fork boots for it I figured I should order the springs and retire this old set. The rustic look.. looks kind of cool but after nearly a half a century on the bike I think it's time for them to retire.

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Without the carburetor/intake parts installed it still doesn't run right but at least now that I got this sissy bar my priorities seem more focused.
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You should just get some CR smoothbores...

Go all out and make something amazing
I may need a smoothbore if I can't get this carb worked out.

This is all the crap we used to laugh at as a kid ..... Was not cool. Now I wish I could order half this stuff for it. Maybe a little pull back bars… cool looking mirrors.. Where are you JC Whitney!

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Since the gang is all here I went ahead and installed everything. Much easier to start but still from cold at least 5-8 kicks.

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Still have to fine tune everything. It's running a stock 38 pilot jet but for whatever reason it's just to low/lean. Air screw needs to be turned all the way in for a high idle.....everything points to a couple sizes up on the pilot jet. Needle position is set to mid position with a stock 100 main (I think...I can't remember). As of this point those setting seems fine.....but low throttle and starting just seems starving for fuel. When the multiple jet sizes come in I'll hopefully iron all that out.

The rubber intake from the filter to the carb has a very small separation......on both sides. It's no real problem so I just used a little electrical tape and a bigger hose clamp to seal things up. Overall it's a big improvement.......and hopefully a bigger improvement once the fine tuning is complete.

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I know this big clamp looks a little gross.....but when I come across a proper one it will be replaced
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Well I guess there may be another Chinese carburetor in my future.

I finally got it to where it would start much easier…. idle without the hang… and just run better overall. It had a 38 slow jet so I went up 2 sizes to a 42 and it seemed to solve most of the problems.....that and the needle in mid position, about a 2 turns out, and a stock main jet. It seemed to be running a little rich and maybe once warm a tick harder to start so I decided to try a 40 pilot jet. As I unscrewed it only half came out....nuts!

I haven't tried everything yet to remove the broken piece but almost everything. Now I wish I'd just left it alone and lived with the rich running condition. It seems to be smoking a little bit out of the tailpipe…I think I'm burning a little oil. Based on past experience with neglected engines I tend to get the bad rings or leaky valve seals after its ran for a while and rode around. I may be into a top end and stuff.

It took me awhile to get this carb to work as this. Who knows...maybe the next Chinese one will work right out of the box.

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If you are going for a Chinese replacement, then unless you are trying to maintain the period look of the bike, you have a few of choices - a round slide like you already have, a flat slide for a little more performance and a CV carb for better/smoother response...and all for around $30US each.
Thank you.

Now you got me thinking...CV....hmmm🤔
What a difference a Chinese carb from Amazon can make. 1 kick starter now. All I did was put in the 40 pilot but to be honest if I didn't do that I'm sure it would still be a one kick starter. Runs WAY better....just everything is better now.

It looks exactly the same but it isn't. The air setting screw had some ink on it....I didn't touch anything.

It's smoking a little more but I don't care.....I consider this a win.

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