74 CB125 scrambler/tracker project

BFT Boyle

New Member
Hello my 2 wheeled friends. Noob forum guy here with a 74 CB125 I'm building up. I started working on the bike last year, but haven't been able to work on it until recently (Broken arm via pencil rolling a go kart. I'm safer on two wheels....).

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So here she is the day I got her. Was given to me for free so I was happy to take her home. Piston was seized up pretty bad. Like to the point where I started trying to cut the barrel in half. I was finally able to blast the SOB with enough heat and liquid wrench to bust it free.

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First thing was getting the barrel, piston, and head replaced. I opted for the Chinese knockoff EBay special.

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Main issue was the new head. The engine mount was in a different location and it was made for a newer cam that used ball/roller bearings instead of just the bearing surface being machined in. The mount I can deal with later, but I ended up buying another head, this time complete with valves, rockers, and a new cam. This cam however is setup for CDI so it didn't have the extended nose for the points system. I machined an extension piece, but I need to get it JB welded in and clock the roll pin that connects to the points rotor. I tried regular wire feed welding it, but the welds kept cracking. Not sure if it's the cam material or what.

Anyways that is where I am at right now. The pictures below will explain the rest.

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Oh yeah, and I'm playing around with a velocity stack design that incorporates an adjustable shutter to control air flow. I love the look of a simple velocity stack, but I know they can be a pain to tune. The idea is by being able to adjust the airflow, I can tune the stack to the carb rather than being limited to tuning the carb only. I downloaded a student version of this AutoDesk software that simulates fluid/airflow so once I figure out the software, I should be able to see if it is a viable design.

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Quick update:

So along with the new head and everything, I bought a new "Fakuni" carb off eBay. Up until a read a few forums, I didn't realize that it was likely a fake Mikuni.... That being said, I decided to rebuild the OEM Keihin that was on the bike. I dropped this into the ultrasonic cleaner bout 20 mins ago. Will see how she cleans up.
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A sonic cleaner is not a substitute for hands on cleaning. you need to disassemble the carb completely. Your adjustable velocity stack is nothing more than a choke. Don't use it.
 
Here is the outcome. Still have some oxidation in there, but the jets and other passages seem clear. A little elbow grease and a new seal kit should do the trick.

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DohcBikes said:
A sonic cleaner is not a substitute for hands on cleaning. you need to disassemble the carb completely. Your adjustable velocity stack is nothing more than a choke. Don't use it.
Yeah, I was expecting a better clean out of it. I will definitely be taking it apart for further cleaning. As for the stack, I had been pondering it, and the shutters do look pretty restrictive. Maybe a telescoping length would be the ideal adjustment.... Eh. Something to fuck around with later.

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You've got one carb with a clip on bowl - it shouldn't take you more than 2 minutes to change a jet. The velocity stack idea is not going to work.
 
hillsy said:
You've got one carb with a clip on bowl - it shouldn't take you more than 2 minutes to change a jet. The velocity stack idea is not going to work.
I actually overlooked the clip on bowl.... I admit I pulled it off and tossed it into the box w/o review. While my ideas have been weighted down, I must say I dig this forum for its straight forward honesty. Cheers and happy St. Patrick's day folks!

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Update: getting the wheels prepped for cleaning and painting(?) Pending on how well the chrome shines up, I may paint these over satin black.... As for the rest of the bike, I got a guy who owes my old man some work and I will be hitting him up. Frame will be OEM black, the tank I'm not sure.... This project is NOT tuned towards an original restoration and I am likely to ditch the current tank color.

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Possible brake upgrade.... I bought this CBR600 a couple of tax refunds ago. Scrapped/ Thrown rod... The hydraulic caliper seems small enough to make an adapter for.
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Finally got the aftermarket cam modified to run the OEM points ignition. I'll be buttoning it up this weekend and get a cold compression test. Service manual says nominal is 170psi so we shall see what I get off the bat. I'm guessing it will be fairly low being a cold kick and the rings have 0 time on them. It will be a good reference point nonetheless.

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Those wheels look like they will clean up, but you should really take them apart and do it right. Impossible to clean 100% all together.

Looks like you can order a complete set of new spokes and nipples for under $50... Just a thought.
 
VonYinzer said:
Those wheels look like they will clean up, but you should really take them apart and do it right. Impossible to clean 100% all together.

Looks like you can order a complete set of new spokes and nipples for under $50... Just a thought.
Thanks for the info Von. Id have to look up a few YouTube videos on how to re-lace the wheels, but I'm down for the challenge.

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Captain's blog:

Miscalculated the balance of a heavy object (x1) 125cc engine, and crashed it into my toe.

I was going to get the engine timed tonight, but forgot that I needed a small retaining ring for a component of the timing chain tensioner. The chain needs to be installed before the mag/rotor goes on.

I'll wait till my day off.
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Coming along.....toe cruncher is a bummer :(

Shame to lose the patina on the stock tank....could you leave as is?
 
following along. these little bikes are quite fun to work on. if you don't have it already here is a link to download a factory manual
http://www.dotheton.com/downloads/Honda%20CB100%20CB125%20CL100%20SL100%20CD125%20SL125%20Workshop.pdf
 
If you can set a piece up in a lathe, you can lace a wheel. It's tedious, but as someone who is comfortable in a machine shop it's a tedious that you're used to.

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VonYinzer said:
If you can set a piece up in a lathe, you can lace a wheel. It's tedious, but as someone who is comfortable in a machine shop it's a tedious that you're used to.

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Hey Von,

Ah ok, I can picture it now. Mount it on the lathe and use a indicator to check for roundness. Tighten/loosen each spoke as needed: correct? I happen to work in a machine shop and have access to all of that. Do you have an idea for nominal roundness? +/- .???

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NoRiders said:
Coming along.....toe cruncher is a bummer :(

Shame to lose the patina on the stock tank....could you leave as is?
I've thought about leaving the patina. Just do a clear coat over.... Painting the tank will be a last step, so I'll get everything together and see how it looks before making a decision.

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I think he was just implying that setting a piece in a lathe and truing a wheel are similar in terms of difficulty and tediousness...
 
Got the missing retaining ring and a washer for the cam tensioner piece. I can get this thing timed now.

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