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I cleaned the piston head, the valves, and did a deep clean on the exhaust port (I guess that's what it's called??). I lapped the valves and hopefully didn't over-do it. I put it all back together with new gaskets and o-rings, set the timing, and hopefully she'll run better than ever... at some point down the line.
As I read what I've recently written, it all sounds so easy. It wasn't. I was constantly searching up stuff on this site, studying YouTube videos, and trying to read between the lines in the shop manual. It was slow going and I had no idea what step was going to be the next step until I was there. It was mentally exercising, physically exhausting, but damn it was fun. At some point I had unscrewed the timing chain tension bolt ALL the way. Not understanding how it worked, I thought I could just screw it back in as the last step of re-assembly. By some crazy chance I just happen to see a little washer just laying there, beyond the magneto. It was from the tension bolt assembly. At that point I knew I was screwed. I was trying to avoid taking more stuff apart, especially after getting things looking somewhat normal, but the whole magneto assembly needed to come out. That required a master class figuring out how to do that with the basic tools I have. I got it out (front axle I think) and started looking for the parts to the tension bolt. I found them all... but one washer and I was convinced it had fallen into the gear box. OMG! I searched with needlenose pliers, magnetic screw drivers, flashlights, picks, etc in the tiny little areas I could see in. The was a small hole that was the perfect size for the washer to fall into, but I found nothing. I knew this would bite me in the ass if I just left it in there, but I really really didn't want to split the case and open up the bottom part of the engine. I re-assembled the tension bolt mechanism with an extra washer and put the magneto back in. I hadn't decided what to ultimately do, but in that process I saw that the missing washer was stuck to one of the magnets on the magneto. Thank god! I had just wasted a couple hours trying to find that washer and several more removing shit, pissed off the whole time, but at least I knew it wasn't in the gear box and I could move on. I pretty lost a full Saturday (or it might have been the whole weekend), but I was so happy to move on.
Next was removing 40 year old grime and rust off the frame and some of the steel parts. It took some wire brushes, a variety of soaps, and ultimately (against my desires) some gasoline. Not great for the environment, but gas and some elbow effort is an amazing grease and grime remover. It was worth it after I saw the nice coat of paint. Going the economic route, I chose to paint these with black appliance epoxy paint. The frame I had wire brushed all the rust off and painted directly. Some of the parts, like foot pegs and kick stand, I primed with self etching and then top coated with the appliance epoxy.
Not a good shot of the frame drying, but you get the idea
Still so much to do, but I'm getting close to starting the process of putting the pieces back together. There is still a lot of parts cleaning as it comes together but I'm excited to be on this side of the hill. Critical point in the process... now, which part goes where??
This is not even near being all the parts, but you get the idea.
Good job of labelling. I find that going back together can go faster than coming apart as you know now how it goes. The worst for me was three-day searches for the next part in the assembly, giving up and ordering a new one, then spotting the mischievous part snickering at be from a shelf where I had lain it to be obvious.
Yes, I must admit, the amount of cleaning has been in a pain in the ass... every time I think I've rounded the corner, I find another part with 45 year-old gunk baked on it and/or a ton of rust. That said, the most rewarding aspect of this project is when these parts start to look new or sometimes better than new. No-Pain-No-Gain applies here, too.
How's the build going? I just made a deal for a complete non runner 75 CB125 and I'll be taking it down a similar path as this over winter with my 16 yr old son. Will be his upgrade bike from his 50cc scooter.
Hey Maritime, sorry for not replying sooner. How’s your build coming along? My progress has been so slow, I never feel like it’s been worth posting. However slow, it’s continued. Mechanically it’s back together except for the front brake cable. I still need to install/wire the lights. I’ve rode it around the block a few times. SO MANY things are fixed/improved, but it still runs like crap at higher RPMs. It’s so nice and rewarding to see the bike assembled and rolling!
So... I’m having trouble with the engine. Starting around 4500RPM it starts running really rough with no power. It idles great and runs/rides well if you keep the rpms low. I assume it’s the carb, because I adjusted the timing recently. But after I exhaust the carb rabbit pit, I’ll recheck the timing.
One thing I discovered is that this carb is a Keihin 660A. Factory spec for a ‘75 CB125 is a 660B. I have no idea the difference however the manual says the SL125 and CT125 used the 660A. A key difference in the spec is the 660A calls for a 98 main jet and 660B has a 105. Should I go with what is spec’d for the motorcycle or what is for the carb?? I’m wondering if this has anything to do with the rough running... also it seems to always be running rich as well (based on the black spark plug).
Any ideas on why it runs rough above 4500RPM? I’m doing (another) thorough clean now and hopefully progress.
And just to be clear... I’m fairly mechanically inclined and I know enough to follow advice and manuals. Beyond that, I don’t know shit.
Hey. I'm at fully rebuilt. Check my thread. Just found out the floats leak and it caused me a bunch of grief. It ran great, let it sit for a bit with fuel in the carb Nd then it ran like crap and fuel pissing all over. Just pulled the trigger on new floats. Hope that sorts it.
Well, I just had some good luck. When cleaning the carb (again) I realized before I rebuilt the engine I had tried a few things to get it to run better. I had switched the main jet to 105 to match CB125 spec and I had adjusted the needle to lean it out. I went back to a 98 jet to match 660A spec and put the needle to center position (3, spec). It ran SO much better. I’m thinking that it ran rough starting at 4500 because the “middle throttle” circuit was too lean. Or maybe it was the cleaning, but it was pretty damn clean before. However, mid throttle (4500-~7000) is still kind of rough. It cleans up a lot 7000+. But at these high RPMs it starts to pop and backfire(?).
I just read your entire build thread. Great job. I’m going to be asking questions!
First off... is your carb a 660A or 660B? My bike has never run well, even before it sat outside for 15+ years. It seems the carb is an issue. Of course I took the air box off and added a pod, so I may not be helping my cause.
Hopefully the new floats will get you back to speed.