CB125s build. "The Dishwasher"

[honeybadger]

So this is my first real project beyond just trying to fix my hateful old XJ750.


Bought a clean, all original '74 CB125s with 1222 miles.


It kicks over, starts up, idles, all without a problem (also without choke... I need to figure out why ;D )


But when you wrap open the throttle, it sputters and dies if you don't back it off for a second, and then it catches and revs. Also, under firm throttle at higher speeds (like 40mph trying to maintain speed or accelerate) it hesitates ever so slightly. It also tends to foul plugs (though I haven't gotten it to foul one yet to check why)


It's got fresh gas, and the previous owner took the carb apart, cleaned it, and the bike was only ever stored with no gas in it.


Thusfar (bought it today) I've titled it, tabbed it, pulled the wires out of the handlebars, flipped the bars (ordering clubmans tonight) and tomorrow will put together the controls again and re-grip it so I can ride it again. Soon after, I'll change out the signals, and a little down the road is a pod to replace the airbox, cafe seat, custom scrambler exhaust setup, and adjusting the peg position so the bars don't bash into my knees.


here are pics of it after I started pulling apart the controls. All in all, for $800 with a clean title, I'm effing stoked. Not a single screw or bolt is rusted or missing. Just the side covers.

 

[honeybadger]

Re: CB125s build. questions abound!

Carbs are definitely not my forté. I'll pull the filter and check.


How does one check the float level? The only things I know about carbs are how to screw with the idle and pilot jet.

 

[treitz]

Re: CB125s build. questions abound!

Cool bike! Good luck with the carb. It's nice only having one carb to worry about :-D

 

[honeybadger]

Re: CB125s build. questions abound!

sweet, shop manual will definitely help (I'm familiar with FI setups and have built several engines in the past, I've just never had to deal with carbs before :-X )


Thanks so much for finding that!


I took a video of the problem. If i wait for the revs to open up under light throttle and then blip it WOT, it backfires. Sounds like it's just running pig rich, I think (which is also why it fires up ice cold with no choke whatsoever?)


Will post the vid when youtube finishes encoding it. Just don't try to actually -watch- the camera, or you might get seasick

 

[honeybadger]

Re: CB125s build. questions abound!

Here's the video. Every time it sounds like someone just kicked a labrador, I'm blipping it WOT. When it's revving cleanly, that's 1/2 throttle.


CB125s running rough

Also, there is no air filter. It's just running the mesh filter cage right now.

While I'm on the topic, does anyone have some leads on 6v turn signals? I'd love to get rid of the brontosaurus-necked stockers.


Also, where can I find the specs on the carb inlet size? I'm looking to just throw away the box and switch to a pod.

 

[teazer]

Re: CB125s build. questions abound!

Most small bikes will die if you bang the throttle open at idle. That much is normal. The same on the street. If you are riding along at say 30 and bang the throttle open it will die or at least go flat as a witches tit.

Gentle and smooth are the two words that come to mind when riding small bikes.

Starting without choke on a warm day is also normal but starting on a cold morning with teh bike stone cold is indicative of too much gas.

I'd put those stock bars back the way they are supposed to go and pull the carb and filter and clean both.

Is the choke lifting properly or is it hanging down into the air stream?

Is the filter clean?

Is anything blocking the air box entry?

Is the ignition timing spot on and points clean and gapped?

Does it have a clean or new spark plug ignore what the PO said. Sellers all lie. Some lie more than others.

Get a manual and adjust the carb to book spec and make sure the jets look OK and don't look as if someone drilled them out with a blunt nail.

 

[honeybadger]

Re: CB125s build. questions abound!

JESUS CHRIST.


I took the carb apart, adjusted the float level, and now as soon as I kick it over it revs straight up to like 4-5k. The idler screw isn't affecting it at all. o_o scared the everloving hell out of me. It says in the manual to adjust it to where the float is 24mm above the side of the carb. it was way above that (like 35mm)

 

[honeybadger]

Re: CB125s build. questions abound!

Most small bikes will die if you bang the throttle open at idle. That much is normal. The same on the street. If you are riding along at say 30 and bang the throttle open it will die or at least go flat as a witches tit.

It's not behaving properly at all. I know you need to be smooth, but it's hesitating at ~30mph.

Starting without choke on a warm day is also normal but starting on a cold morning with teh bike stone cold is indicative of too much gas.

It's march in Seattle. today was warmish, but the choke had no influence on the idle (or is the choke in this different from the chokes in newer carbs?)

I'd put those stock bars back the way they are supposed to go and pull the carb and filter and clean both.

What's wrong with the stock bars? I've only ridden the bike with the bars in the stock, normal position.


I pulled the carb and cleaned it, adjusted the float valve (At least, I think I did the float valve properly.) The thing I noticed is that the O-ring between the carb and the carb spacer was missing. Could that cause it?

Is the choke lifting properly or is it hanging down into the air stream?

Lifting properly.

Is anything blocking the air box entry?

No, but I'm not running the airbox right now (deliberately, as the filter cage mesh was really dirty)

Is the ignition timing spot on and points clean and gapped?

I don't know about either of these. I'll have to do that tomorrow.

Does it have a clean or new spark plug ignore what the PO said. Sellers all lie. Some lie more than others.

The plug in it during that video was new.


This guy was honest. He said he didn't know much, all he did was pull the carb apart and clean it out.

 

[honeybadger]

Re: CB125s build. questions abound!

Okay, I think I got it!


It now needs the choke to start and idle cold (55 degrees out and dark)


Once it's hot, the choke actually feels like it's doing something. If I just rip it open at low revs, it still chokes, but it feels like a choke that something with a mechanical fuel pump does when you just overdo it. Once it's revving, I can rip into the throttle and it doesn't backfire or complain. I just had the throttle pinched. Once I unpunched it (there's some normal tension on the cable) I was able to dial it back with the idle screw and AFM screw. I have the AFM screw (pilot jet, right?) farther in than I'd expect it to be, but I'm running it with no filter right now. I'm presuming that it's just running lean, and once I put a pod on it, I'll just change the pilot jet out and be more "normal"? guessing the same will go for a more open exhaust when I finally get to that step.


(in the meantime, I'll pick up a new filter at the dealership tomorrow for the stock airbox to fettle it)

I can't test ride it until I drill out the controls for the wires to slide through (or just drill new holes in the bars, which may be a better bet than dicking up my controls.) and change out the grips. But I get the feeling that it's going to run like a stabbed rat now!


Thanks for the help everyone, I've officially popped my carb cherry ;D

 

[honeybadger]

Re: CB125s build. questions abound!

Which is the main jet in the carb? For only having two moving parts, these little bastards seem complicated >.>


When it comes time to actually change my jets, I'll be visiting my mechanic for the official numbers.


I'll keep playing with the throttle, but there's no slack I can find in it (I'll rerun the cable tomorrow when I'm mounting the controls to the bars)


How important is that o ring between the carb and the carb spacer? could that be causing some problems if it's causing leaking at all?


Also, I found some signals I like. they're 12v and use an m7 bulb. is it possible to get an m7 (or something that matches the socket) in 6v?

 

[teazer]

Re: CB125s build. questions abound!

Sounds like you are making great progress. That O ring is important in getting a good air tight seal. You can probably get one at your local NAPA type store.

My comment about the bars was because they look like they are upside down in the pictures.

And the "all sellers lie" comment was a take off from House on TV. Thought you might have caught that one.

 

[honeybadger]

Re: CB125s build. questions abound!

haha, I'm just going to cut slots in the bars and run them halfway up towards the "horns"


Shouldn't be hard. I just don't want to mutilate my controls just yet.


I see, the main jet is what the needle slides into.


And the bars are upside down. I flipped them over and turned them upside down. They're like lower, wider clubmans right now.


And I've got all the slack out of the cable from the various adjustment nuts (There are three) I just need to rerun the cable a bit. I had this problem when I put clubmans on my old XJ750.

 

[surffly]

Re: CB125s build. questions abound!

god that thing must be uncomfortable....i know cause i had clubmans and stock pegs on a cb125...
i would think drag bars would be as low as you can go without making some sort of rearset for the bike


these are small and simple motors but need to be kept in tune as small changes will make a big difference in how they run

if i were you i would pull the carb and manifold. get a new gasket and o ring. they seal the carb to the engine and if they are bad or missing then the carb will not be meetering fuel the way that the motor needs.
these motors just run like crap with out a filter. a GOOD pod will work but will never run as good as stock, even if you mess with the jets.
i have not had good luck with "aftermarket" carb kits. i find they come with gaskets that dont seal right and also the wrong size gets. better to spend the extra money and buy OEM honda carb parts.
is your throttle cable moving freely?
also might want to check your ignition and charging system too. as again you want to be spot on in order to get the most out of the little guy.
you do have a factory honda manual right? you can download it easy and its great to show how to get everything perfect.

what you want to do is start buy doing the timing and ignition. once that is perfect do what i said to the carb. then make sure that your petcock is flowing right. and then go from there. you should either A have a good running motor or B know what parts you need.

always get the bike running 100% as it came from the factory before you start messing around with things, as the "new upgrade" ads a variable that will be work with in a motor thats not running as it should

 

[honeybadger]

Re: CB125s build. questions abound!

Went ahead and bought a new filter, am washing the filter cage, bought new seals for the carb, and a new fuel inlet needle (In case it's worn out)


All from the honda dealer.

Will report back once I've got the controls all hooked up and I run it.


After that I'll go into the timing and clean the points (going to be my first time working on points, too! timing should be a snap.)

 

[honeybadger]

Re: CB125s build. questions abound!

Hmm. I got it to start running and revving smoothly, put it all back together and wrapped a sock around the filter cage to simulate at least -some- basic filter, drilled out the controls (wow that was easy. Though I am grounding the horn, ;D ) and popped them onto a set of clubmans I got at my local scooter store with some new grips. made a makeshift o-ring for the carb out of electrical tape and buttoned it back down. Also found the slack in the throttle I was looking for. Just needed to reroute it. Though the brake cable is too long now, so it's a little tight.


But as I rode it down the road, it was acting exactly the same way after a very short distance. Sputtering and hesitating when trying to maintain about 4-6k RPM, and then it was just running really rough when I pulled back home (drove it maybe a half mile)


Pulled the plug and it's covered in sooty carbon. It was brand new when I put it in yesterday, rode it about three miles, and idled it for about 15 minutes while playing with the carbs. It's the right plug, it's gapped correctly. I put in a new plug and it's purring pretty nicely again. But riding it, same problem. It's ruining plugs.


Haven't had time to check the ignition or points yet. Will probably do it saturday.


I'll take a picture of the plug in a few minutes. All that said, with the clubmans on it's actually very comfortable, my knees are just a bit further forward than I'd like. Will sort that out once I have it actually running like it should.

 

[honeybadger]

Re: CB125s build. questions abound!

picture of the plug. It's dry and sooty. Indicative of rich, correct?

 

[honeybadger]

Re: CB125s build. questions abound!

Here's how it stands after I put it all back together. Pulling the carb again to check the size of the main jet.


Would checking the size of the pilot be a good idea (is it stamped on the pilot like it is the main?) Also, which way is "leaner" and which is "richer" with regards to turning it?

 

[honeybadger]

Re: CB125s build. questions abound!

I think I understand it now. The pilot jet is right next to the main inside the float body


It says "86" on the main jet. The spec for stock says "105"


Is 86 going to make it run richer, or leaner?


it looks like it says "38" on the pilot.


I'm guessing the larger one with "86" stamped on it is the main, and the smaller one to the left is the pilot, right? I for some reason always thought the air/fuel screw on the exterior was the pilot.


In the manual it says "slow jet" = 38. So it looks like that part is correct.

A cold compression test (I know, hot is better. But it'll have to wait until I get the new jet. Hopefully tomorrow) showed 145PSI. Stock is 170. I know air cooled engines expand and contract a lot, and 25psi in a normal engine means something's leaking (based on the small amount of smoke it makes when revved, I'd guess the ring) but is 25psi, cold, in an air cooled engine going to bump me up to a "good" number when the engine's warm?


Cam chain tensioned, drive chain adjusted. Holy fuck I love working on this bike! It's so easy, nothing is seized or broken.

 

[honeybadger]

Re: CB125s build. questions abound!

3/26/11 update - Took the carb into my mechanic just so he could look it over for me. He looked at the plug, it's 100% running way rich.


I also looked, and the carb number is from the next year 125s, 1975. But honda was using a lot of different parts back then, so it's very possible it's still the original carb.


And looking at the settings for -that- carb, the main is supposed to be a 98... If you flip 86 over... guess what you get! so I have a good carb that should be do-able.


But then he popped off the floats and gave them a shake. They're sunk! So that's what's causing it to run so rich! Ordered some new floats, a petcock rebuild kit (it's a bit leaky in the off position) and fork seals, and while I was poking around in the back, I saw that he had a cherry '75 cb125s sitting in the back. So I stole the front fender (mine's banged up) for $25 as well. This project just keeps moving along at such a nice pace! Though I will be stuck for a week waiting for the carb parts to come in. In the meantime I'm going to dip the carb.


Now off to clean the points and check the ignition timing.

 

[jbhowat]

Re: CB125s build. questions abound!

Love this project. The bike is looking nice and the budget is just my style. I love tiddlers, am starting into a CL175 myself right now, and I've owned a couple CB125s in the past (A single and the SS125). Make sure to keep us updated...