1979 Honda CB 125 Cafe Racer

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WestChester_CafeRacer

New Member
Hello,

Welcome to my thread about my recently purchased 1979 Honda CB 125 Cafe Racer! I know there will be many topics and issues arising in this thread that have already been mentioned in others, but I'm making this thread to book-keep my progress on tuning and fixing issues that arise with my bike. With all this being said, I really appreciate helpful comments and advice, even if its just a link to another thread with a similar topic.

Well here goes....

I bought a 1979 Honda CB 125 motorcycle that the previous owner had cafe'd. It has great compression, the electronics are all in order, the cosmetics are all good, and the bike is pretty slick looking! The previous owner got the bike in a trade deal so I managed to get it pretty cheap. I've attached an image of the bike, hopefully it shows up!

So the bike fires right up after one or two kicks and will run but not idle. The engine also gets a little choppy at higher RPMs...yet I can ride the bike up into 4th and 5th gear. So I've been doing my research and these are the steps I am going to take to remedy these issues:

1). Drain the gas tank as I have no idea when the last time fresh gas was in this thing (could be the source of the engine chopping out at higher rpms)
2). Put fresh gas in the engine and play around with the Idle Screw and Needle.
3). Clean the Carb, Soak, Scrub, Re-assemble.
4). Check the throttle cable (Currently the rubber throttle cable cover which goes into the carb sometimes gets stuck on top of the carb pulling the throttle cable up and opening up the throttle, very unsafe at the moment, nothing a hose clamp can't fix for the time being)
5). Tune the Carb (Jets, Floats, etc)
6). Buy a new carb

Let me know if you have any advice, I will be updating this thread periodically with the work I've completed and the successes or failures I am finding along the way. Thanks for reading!
 

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Check slow jet, and main jet. They migh be clocked. At that point I would'nt even bother changing the fuel.

I mean change the fuel AND clean the carb.
 
That's some exhaust can you have there.
Are there any air filters on those carbs? I think you need something on there before tuning.
 
I'm actually going to end up replacing the carburetor.....after looking at the carb thats on there (a cheap Japanese knock off...not a Mikuni) I am thinking it will solve a fair amount of problems. The carb I'm getting is a Mikuni 22MM (VM22-133) Carb from AGK. I'm going to use an intake manifold off an old 80' CB 125 and pop a pod filter on the end of the carb....

I will reply with the name of the carb that is currently on the bike....I can't remember it off the top of my head.

alzcbz said:
That's some exhaust can you have there.
Are there any air filters on those carbs? I think you need something on there before tuning.
Click to expand...

I know! The guy who had it before me threw a carbon fiber exhaust on the thing....a little big, but it does the trick I guess. It sounds great, thats for sure.

joevirus563 said:
Check slow jet, and main jet. They migh be clocked. At that point I would'nt even bother changing the fuel.

I mean change the fuel AND clean the carb.
Click to expand...

I am going to get a few different sizes of the pilot and main jets to see if I can get the right combination for the best performance.
 
You MUST have a filter on that bike for it to run even remotely well. Any kind will do better than an open carb, but the stock airbox is very well engineered and you are unlikely to find a better alternative.

That exhaust really is something, looks like it's off a CBR or something? Might be another thing to tackle, the stock exhaust was also quite good on these bikes.
 
coyote13 said:
You MUST have a filter on that bike for it to run even remotely well. Any kind will do better than an open carb, but the stock airbox is very well engineered and you are unlikely to find a better alternative.

That exhaust really is something, looks like it's off a CBR or something? Might be another thing to tackle, the stock exhaust was also quite good on these bikes.
Click to expand...

I was thinking just a regular pod filter might work alright? Yeah, its a carbon fiber exhaust, not sure where the previous owner got it from. Might consider changing it out for another.

So an update on the progress today. The carburetor I have on the bike is called a Minhua (?). Its a Japanese make. I took it apart and found that the pilot jet was clogged so I cleaned it, blew it out with compressed air, and that seemed to fix the choppiness at high RPMs.

A few things I noticed though, still can't seem to get it to Idle, it "idles" with half-choke and runs fairly well. I'm going to play around with the "Idle needle" some more and a few other things. I also noticed that there was a fair amount of smoke coming out of the exhaust while I was running, not so much when I was in neutral at low rpms. Many things could be causing this, but I'm open to advice for how to go about diagnosing the issue.

Thanks!
 
Smoke sounds like your ring blew or running super, super rich. Choppiness at idle is because you have no filter, the stock one (and some aftermarket) act as a Venturi to help the engine ingest air. Get one on there THEN start trying to tune it.

Edit: black smoke is gas (rich) blue smoke is oil
 
coyote13 said:
Smoke sounds like your ring blew or running super, super rich. Choppiness at idle is because you have no filter, the stock one (and some aftermarket) act as a Venturi to help the engine ingest air. Get one on there THEN start trying to tune it.

Edit: black smoke is gas (rich) blue smoke is oil
Click to expand...

I think it could be the ring. The smoke is blue-ish and the oil seems to be burning off as the oil level seemed to go down pretty quick after running it for a bit. I'm getting an Air filter today so I will mess with it more. I will keep updating this thread as things progress.
 
I want to get some opinions on a few things I've noticed on my bike:

1). The blue smoke coming from the bike is definitely oil burning. This could be various things but I've narrowed it down to the Valve Stem Seals being worn and/or the piston rings being broken. I took it to my local motorcycle mechanic and when I spoke to him about replacing the valve stem seals and doing the piston rings he said, "Its going to need to be the entire top end, there is no point in doing both of those fixes alone." With this being said, he quoted me like $500 to do the top end rebuild (parts and labor).....I'm just a broke college student and I got this bike for $500....so I'm going to pass....

2). How hard is it to rebuild the top-end on your own? What kind of figures am I looking at price wise? If I am just cruising around town on the bike, should I really be worried about burning off a little oil?

3). Also, the exhaust on my bike is HUGE, its a FMF exhaust. I think I might replace it with something smaller. Any recommendations for mufflers or exhaust setups?

Thanks for your opinions, I really appreciate all the advice I've gotten off of this forum!
 
Can't really help you with the top end rebuild as I haven't done one, but I can't imagine it would be too difficult to do. The hardest part (I believe) is lapping the valves in, and from what I understand is not very difficult at all.

Depending on where you're located, you ought to be able to find a stock CB/CL125 pipe very easily. Might set you back $100 or so, but you can sell that FMF to recoup some of the cost.

Or you can find a small ID (I think 1 1/8") muffler to work with the header that's already on there. There are a few mfg's for smaller displacement dirt bikes and quads that make them.
 
Consider replacing the engine. No idea what the market is like where you live, but over here a CG125 engine or one of the chinese clones should be far less than $500, and should bolt straight in, as the CG is just an improved reliability version of the CB. Seen them for about £100 on ebay, that is $180 I think.

Top end rebuild is fairly easy if you are mechanically minded, I used to rebuild my 2 strokes as a clueless kid. 4 stroke is a bit more complex, but as long as you can follow the instructions in the service manual you can probably do it too. I would suggest just doing the barrel/ piston and rings first, it seems to me that is the part most likely to be damaged by the grit from running with no filter.

Please note I am not a mechanic, just another guy trying to run a motorbike on not enough cash.
 
If you go the route of replacing the engine, then you might want to look in to XR engines. I've heard the XR 175, 185, and 200 engines will bolt in to the 125 frame with just a new top bracket and widening a couple of the back mounting holes. I looked in to this when I had head problems on my 125. Good luck on whatever you choose!
 
I just rebuilt a 125 top end. It is insanely simple. Download a proper manual and dive in. Took me a total of about 4hrs.
 
bitten_by_tarmac said:
VonYinzer,

It is not insanely simple. It is do-able, if you know how to use a spanner, but not insanely simple. But I agree, dive in.....
Click to expand...

Well it is about as simple as your are going to get with a four stroke. Not as simple as a two stroke for sure :), but in comparison to other bikes its pretty simple.
 
Couple hand tools, a clean work bench, and a couple bucks worth of new parts... It is pretty basic stuff. My only point is that you should have no issue doing the work.
 
Thanks for all the input guys! I think I am going to end up rebuilding the top end around the middle of July, which gives me time to clear off that work bench and get some funds for the parts I'll need. Replacing the engine would be nice, but I'm not sure if I am going to call it quits on the original engine just yet. If anyone can hook me up with a link/url where I can download a service manual or instructions for rebuilding the top end, that would be awesome!

The biggest parts of the rebuild which I need to get done are replacing the piston rings (if need be) and valve stem seals, and sizing the piston, etc. I need to stop this engine from burning so much oil.

Right now though I'm focused on getting my bike ready for inspection. I bought some awesome aluminum mini taillights which I've mounted right behind the seat....and I'm working on the wiring, which it appears that the previous owner removed the winker relay...so I'll have to get one of those now. I also got some small mirrors to put on the ends of the handlebars (haven't arrived yet in the mail).

I appreciate all the comments and advice. I will keep you all updated! Next time, I'll add some pics.
 
CALfeRacer said:
I have a Clymer and use this:
http://www.dotheton.com/forum/index.php?topic=20352.msg209939#msg209939
Click to expand...

Thanks for the link CALfeRacer!

I was wondering if anyone might have come across a similar issue when installing turn signals....

Yesterday, I figured out the circuitry for the turn signal installation with a little help from a wiring diagram I found online for a CB 125 S. I found I needed a Winker/Flasher Relay as the original or previous one was removed by the previous owner. I installed the flasher relay and low-and-behold the turn signals work! However, it seems that I am getting some interference with the headlight. When I turn the power on and the headlight is on, it seems to drain the power from both my turn signals and rear brake light. My thoughts on the cause of the issue:

1). Something must not be grounded properly....
2). The headlight is taking up too much of the juice from the battery....

Ways to troubleshoot and resolve,

  • Wire the headlight switch directly to the battery
  • Wire the turn signal circuit directly to the battery

Any thoughts or opinions are greatly appreciated! Even links to previous threads! Thanks!Pictures to come once I get the turn signals finished.
 
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