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Discussion Starter · #1 ·
Hey all! I am new to the forum and wanted to post my project log up here. I don't normally do this kind of thing, but since the bike I have isn't well documented online I figured I would document my journey here.

I found a 1990 Honda CB125T on craigslist a couple of weeks ago. The guy was selling it for cheap and wanted it gone, and since I was a few drinks in I decided it would be the perfect project bike! When I bought the bike I didn't know anything about it or it's history, all I knew is that the motorcycle *kind of* ran. Looking at the motor the gaskets on it actually look somewhat new and I have a feeling the motor was rebuilt not too long ago.

When I brought the bike home I did some research on it. Turns out this bike was one of about 2000 that were imported into the United States, and they were originally used to teach people how to ride motorcycles. The original bike had a light bar on the front and back which would tell the instructors which gear they were in, if there were going over 20mph, and more. Well, the bike I picked up didn't have the light bars anymore (or any lights at all actually) but that's ok because I plan to make this bike street legal again.

When I brought the bike home I made a list of obvious things that were wrong with it:

1. Engine ran-ish. It had no real power and seemed to bog down if you opened the throttle.
2. Carbs constantly drained if the gas was on.
3. No front brake level or Master Cylinder
4. Bike is hot wired on and has no battery (so the only way I was starting it was by bump starting it)
5. No lights, turn signals, or speedometer/tach.
6. Exhaust is smokey (can't tell color since I only ever ran it at night :cool:)
7. Other Misc items like air boxes barely hanging on, seat ripped, gas tank leaks, etc.

So far I have been able to identify a few direct issues and either fixed or are in the process of fixing:

Carbs leaking -- After spending hours troubleshooting why the diaphragms in the carbs weren't working properly and why they were leaking like crazy it turns out the person before me had hooked up a vacuum line to the fuel tank. This was causing fuel to flow into the diaphragms of the carb, which would then flow down into the bowls and start over flowing. After running a new fuel line with a filter in the middle the carbs don't leak anymore! Yay! This also seems to fix a bit of the power issues, but it still needs to be tuned and tested for more then a couple minutes. I also installed new air filters and attached the air-box properly to the carbs/frame.

Gas tank leaking -- Because the gas tank leaked when I filled it up, I haven't been able to do a whole lot of testing and tuning of the carbs yet. I took the tank off and used wire brushes to get the paint off. After removing the paint I found a bunch of little holes along the bottom of one side. After hours of cleaning the tank I decided the best thing to do was use JB Weld to fill the holes and then once it was cured line the tank with red-kote. I have completed this but I haven't put the tank back on yet since leaving it off gives me easy access to the wiring and I want to fix that up a bit first.

No front brake -- I ordered a used Master Cylinder online, installed it and found out I was missing the second banjo bolt for the brake caliper. I ordered that and received it this week. Then yesterday I found out the brake line is clogged (and I tried and failed to get it unclogged) so I am now waiting for that part to come in. Hopefully there isn't anything wrong with the caliper!

Electrical -- So far I have been able to remove the second wiring harness that was used for the additional indicator lights. I have removed all the components I could from that. I still need to terminate some of the ends to make sure I don't have any shorts. I also ordered a service manual for this bike and have been reading the wiring diagram to add an ignition switch that I ordered online. Once I get the ignition switch hooked up I plan to install a battery, and hook up the turns signals and lights I ordered for the bike. I plan to get this done this week so I can put the tank back on.

I'll post more updates here once I have them. Since I have the service manual for this bike I plan to take the pages out and scan them. Once I do so I will put it online for those who happen to have one of these bikes so you don't have to order it. I went ahead and scanned the wiring harness today and have added it to this public Google drive folder. I will post an update to this thread once I have added the full service manual.

And of course pictures are attached of my work so far.
Land vehicle Vehicle Motorcycle Motor vehicle Car

Auto part Carburetor Automotive engine part Engine Vehicle

Land vehicle Vehicle Motor vehicle Motorcycle Automotive tire

Red Automotive exterior Windshield Material property Auto part

Engine Auto part Motor vehicle Fuel line Vehicle



Let me know what you think!
 

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Welcome aboard. I've got the FSM in the library, it's in English, German, French?, and Italian? or maybe Spanish. 4 languages. This shows as a US model, what country are you from? You can add location to your basic profile which sometimes helps in suggestions.
 

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Discussion Starter · #3 ·
Welcome aboard. I've got the FSM in the library, it's in English, German, French?, and Italian? or maybe Spanish. 4 languages. This shows as a US model, what country are you from? You can add location to your basic profile which sometimes helps in suggestions.
Thanks! I have the FSM in English. Just need to make an electronic copy of it :)

I'll add my location soon!
 

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Hello from the Eastside! Sounds like a great project. I will be watching.
 

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Discussion Starter · #5 ·
Hey! Glad to see someone else who is close by!

So quick update on the project so far. I spent last night installing my ignition switch/key. Managed to trace-out the wiring diagram and hooked it up properly! Surprisingly it worked, but then I spent a good portion of the night troubleshooting why the starter wouldn't turn over when I installed the battery. Turns out when I disconnected the secondary wiring harness I disconnected the neutral indicator.

Once I did that I put everything back together and took it for a quick spin to make sure it still ran, and sure enough it does! Found two problems with it however. It still doesn't have much acceleration unless I slowely ease on the throttle. I'm thinking it needs to be tuned? The second problem is that the left cylinder seems to smoke a huge amount. I am going to do compression tests on that cylinder today but I'm hoping it's just from running too rich. Fingers crossed!
 

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Discussion Starter · #7 · (Edited)
Brake line came in this morning, so I installed that and bled the front brake. Now I have working brakes on both wheels! Yay!

Sad note, compression doesn't exactly look good. So on the left cylinder I get about 60psi, and on the right cylinder I get about 85psi. Now I am less worried about the 85psi, and more worried about the 60psi but I don't actually know what is normal for this style of motorcycle?

When I look in the field service manual under Cylinder head it says cylinder compression should be 142psi. Using a general compression tester made for cars should I be reading that high? Or is about 85-90psi normal reading? Since my compression tester has a semi long hose, and I was doing this while the engine was cold I think 85 sounds about right. But the left side definitely has a problem!
 

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You should be at the book figures. There'll be some discrepancy but it should be close. Are you testing with both plugs out, throttle wide open, choke off and cranked until the gauge stops rising?
 

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Discussion Starter · #9 ·
No, I only did it one plug at a time and throttle closed. Let me go do it properly.

I did do some research and it looks like since I'm using a compression tester with a long hose it won't be exactly at the book numbers since the engine displacement is small. BRB.
 

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Discussion Starter · #10 ·
No, I only did it one plug at a time and throttle closed. Let me go do it properly.

I did do some research and it looks like since I'm using a compression tester with a long hose it won't be exactly at the book numbers since the engine displacement is small. BRB.

Re-did the tests. Just over 60 now on the left, over 95 on the right. I also started the bike in front of my friend and he hears either valve tap or knocking so I'm gonna tear in and see whats up. Will report back later.
 

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Yeah, need to open it up. First though add 1/2 teaspoon or 5cc's of oil to each cylinder and run the compression again. If it jumps then you're looking at rings primarily, no jump means valves
 

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Discussion Starter · #12 ·
Yeah, need to open it up. First though add 1/2 teaspoon or 5cc's of oil to each cylinder and run the compression again. If it jumps then you're looking at rings primarily, no jump means valves
Well I made the mistake of reading this after I started tearing in. I also made the mistake of mis-reading the FSM and thought I could take the head off without removing the engine. So now I'm about to start the process of removing the engine.

What should I be looking for to determine if there is a valve issue if I physically look at the valves/head? Also what should I be looking for in the cylinder?
 

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Discussion Starter · #13 ·
I feel pretty accomplished, the engine is out! Even though I've worked on motors many times before, this is my first time I've ever pulled a motor out of any vehicle.

Also just determined there is an oil leak where the cylinders connects to the bottom of the engine. I might be pulling this thing completely apart!

Land vehicle Vehicle Motor vehicle Motorcycle Automotive tire


Tire Automotive tire Motor vehicle Rim Vehicle
 

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To replace the rings you'll be pulling the cylinder off so that gasket will get replaced and solve the oil leak.
Once the head is off turn it upside down and fill the combustion chamber with a solvent type fluid. If the valves are leaking the fluid will drain out the ports and you'll know which valves are the specific issue. Note that the valves cannot be resurfaced, replacement only.
 

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Discussion Starter · #15 ·
To replace the rings you'll be pulling the cylinder off so that gasket will get replaced and solve the oil leak.
Once the head is off turn it upside down and fill the combustion chamber with a solvent type fluid. If the valves are leaking the fluid will drain out the ports and you'll know which valves are the specific issue. Note that the valves cannot be resurfaced, replacement only.
Well pulled the head off and I believe I found the issue.
Video shows the way it moves (the other one doesn't move that bad). I went ahead and filled the combustion chambers on the head with SeaFoam and I'll see if it leaks. Looks like I have some parts to buy/find.

Thanks for the help so far! If you have any suggestions or tips for the upcoming work, feel free to let me know!
 

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Discussion Starter · #17 · (Edited)
Pretty sloppy, rotate the engine so you can test the other piston the same. Could be a collapsed piston skirt, bad piston pin or severe wear in the cylinder.
Other piston seems fine. I found Cylinder(s) and pistons online so I am going to order them, still need to find a gasket kit for this part of the engine. I should probably get new rings too, but I haven't found them yet for this bike.

Sorry for this bad photo. Auto part Motor vehicle Engine Vehicle Carburetor


Edit: Nevermind about the gaskets! The gasket set I ordered a while ago actually has all the gaskets I need!!!

Edit again: I'm having a bit of trouble figuring out the exact piston rings I need to order. I found Piston with Rings Set on ebay. I also found a parts list here but I'm not familiar with the terminology. What does .25 or .50 over NOS mean?
 

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LDR will chime in, .25 means .25mm oversize, also called first oversize. .50mm oversize is second oversize. Before ordering pistons and rings, make sure the cylinder will clean up at least the second oversize. The cylinder wear needs to be measured.
 

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Yes, wait to get the fresh cylinder and pistons before ordering parts. Once you have them clean them and take them to a machine shop along with the piston and cylinder specs from the FSM. They will measure everything and advise what needs to be done. Maybe only rings or maybe new STD. pistons or oversize.
Call Motorcycle Restorations at 626-429-4464 and give him the piston and ring part numbers, he's a supplier of Japanese engine parts.
The rings and pistons come up as a one year/model for US applications but they fit other models never seen here like
 

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Discussion Starter · #20 ·
Yes, wait to get the fresh cylinder and pistons before ordering parts. Once you have them clean them and take them to a machine shop along with the piston and cylinder specs from the FSM. They will measure everything and advise what needs to be done. Maybe only rings or maybe new STD. pistons or oversize.
Call Motorcycle Restorations at 626-429-4464 and give him the piston and ring part numbers, he's a supplier of Japanese engine parts.
The rings and pistons come up as a one year/model for US applications but they fit other models never seen here like
I haven't taken it to a machine shop yet, but I just pulled the pistons out this morning and it looks like it is the piston rings that are shot on the left side. To reuse this cylinder I will have to take it to a machine shop to get cleaned up for sure. I am going to wait for the new/used parts to come in and see what kind of condition they are in. If they look good I might just install them for now. Won't know until next Friday so this thread is probably on hold until then!
 
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