1975 Honda CB125S and 1981 CM200T

whynot

~~~If it ain't raining, I'm riding~~{iii}?~~prost~
Today I ordered a new set of carb gaskets because I guess the first set I purchased, even though they listed "1975 CB125S2" as being compatible, must have been for a modern clone of the OEM carb. Bummer when you think you have the parts, but you don't.
Yeah, I ended up having to order a couple gasket sets too, just to get all the o-rings and stuff.
 

SpaceTrucker

Active Member
Good suggestion, pidjones, thanks.
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With this main jet, should I try to unscrew the end to get it cleaned, or is that needed? Right now it is super tight (and stuck), but I haven't tried the heat gun yet.
7srOhwUtSuKyWPnClEfKEw.jpg
 

SpaceTrucker

Active Member
While waiting on carb parts to arrive, I decided to check for spark. Of course this bike has an 18mm plug and I didn't have a socket for it. Nor did the first store I went to. I eventually found one and I pulled the old plug to test. No spark. I had no battery attached, but I had seen online you could still get a (weak) spark. I installed a 6v battery, still no spark, but I did get a strong burning smell. And smoke! The connector from the rectifier was shorting out. When I pulled it apart, the plastic starting breaking and crumbling. I pulled the rest off and put heat shrink on the wires. Good for the moment, but not vibration proof. With this fixed I hooked the battery back up (no sizzle this time) and successfully got a working-ish headlight, break light, no blinkers, but also got what looks like a strong spark. I have no idea why the blinkers didn't work but hopefully it's just the bulbs.

ORQ%TZiBSNyq66Fb4nPsEg.jpg
 

Maritime

Over 10,000 Posts
While waiting on carb parts to arrive, I decided to check for spark. Of course this bike has an 18mm plug and I didn't have a socket for it. Nor did the first store I went to. I eventually found one and I pulled the old plug to test. No spark. I had no battery attached, but I had seen online you could still get a (weak) spark. I installed a 6v battery, still no spark, but I did get a strong burning smell. And smoke! The connector from the rectifier was shorting out. When I pulled it apart, the plastic starting breaking and crumbling. I pulled the rest off and put heat shrink on the wires. Good for the moment, but not vibration proof. With this fixed I hooked the battery back up (no sizzle this time) and successfully got a working-ish headlight, break light, no blinkers, but also got what looks like a strong spark. I have no idea why the blinkers didn't work but hopefully it's just the bulbs.

View attachment 221865
Some Hondas the blinkers won't work unless the bike is running. IDK why but I've had 2 that way. I converted to LED with a digital flasher, then they worked running or not. I think its the old flasher relays
 

Maritime

Over 10,000 Posts
Good suggestion, pidjones, thanks.
--------

With this main jet, should I try to unscrew the end to get it cleaned, or is that needed? Right now it is super tight (and stuck), but I haven't tried the heat gun yet.
View attachment 221863
On this you may be able to get it clean enough but if you need to re-jet for any reason you'll need it apart. Boil it in lemon juice or pinesol (outside if pinesol) it should break free. Or soak it in a small jar with a lid and carb cleaner over night.
 

trek97

No Custom Title
DTT BOTM WINNER
On this you may be able to get it clean enough but if you need to re-jet for any reason you'll need it apart. Boil it in lemon juice or pinesol (outside if pinesol) it should break free. Or soak it in a small jar with a lid and carb cleaner over night.
Maritime is right on both accounts. You can get the jet clean enough plus ensure the small hole in the emulsion tube are clear.
and the signal flasher is probably shot.
 

SpaceTrucker

Active Member
Back on the carb after a short break... I’m having trouble getting the slow jet clean. The side holes are clear, but I can’t see light thru the length of it. I let sit overnight in carb cleaner after working on it last night. This morning I still was not able to clear it so I put it back in and poked at it with a needle to make sure no air bubbles were stuck in there. If that doesn’t work, I’ll try boiling it in pinsol. How big is the hole on the slow jet compared to the main jet?


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Maritime

Over 10,000 Posts
Usually much smaller. a needled may not be small enough to poke through it, piano wire or a guitar string can work, or even a strand from some copper electrical wire can work. Compressed air is good and aerosol carb cleaner with the red straw is good to blast it a few times, wear safety glasses though, it can blast back in your eyes and is no fun LOL.
 

pidjones

Over 1,000 Posts
I use a high E or even a G from a 12 string (you can buy individual strings at many music stores like Guitar Center). On a GL1000 the slow fuel is a 35 which is pretty tiny. Now, this is a 999 cc engine, but the carb is for only ~ 249 cc of that. I have a pin vice that I chuck about 2 inches of the steel guitar string (usually springier than piano wire) in and use that to spin it through. Once it gets through, shooting carb cleaner through it or soaking in carb cleaner will remove varnish, and CLR (Calcium Lime and Rust remover from the hardware store) does a good job of removing deposits left there by water.
 

SpaceTrucker

Active Member
Thanks for the tips. Guitar string was the trick. I’m putting the carb back together and so far, so good. However the gasket kit has a small o-ring that I don’t know where it belongs. My best guess is that it goes on the cable adjuster where it screws into lid of the throttle assembly. I don’t remember a rubber o-ring when I took it apart but that was too long ago to trust. The exploded drawings don’t show this part. Any ideas??

IMG_0050.JPG
 
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SpaceTrucker

Active Member
I ended up just leaving the little o-ring out when I put the carb back together. Reassembled the airbox, rehabbed the throttle cable, and plugged in a long fuel line to act as a mini tank; it was time to see if I had a fighting chance. I had to kick it over several times, but magic did happen. The 125 started up and ran for the first time in at least 15 years! It ran really rough but, man that felt good.
 

SpaceTrucker

Active Member
That video was from last week. This weekend I worked on getting the air mix better because it was running very rich. Still needs work but I leaned it out some. It idles great and now starts up with easiest of kicks. However, anything above idle causes it to run super rough. I assumed it was the carb, but I first checked the timing. I rigged up an LED with some batteries and alligator clips and found out the timing was pretty off. I adjusted that but haven’t been able to check if that helps the rough running.
 

SpaceTrucker

Active Member
I also checked the ignition coil. I could not find the specs for it anywhere online. I did find an aftermarket coil with specs, but my results were very different.

The primary circuit read. 2.0 ohms
The spark circuit read 23.4K ohms

Does anyone know if these show the coil needs to be replaced?
(this is for a 1975 Honda cb125)


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pidjones

Over 1,000 Posts
The small o-rings sometimes go on air/idle adjust screws. Also, make sure fuel can flow freely into the float bowl. Blocked strainers/collapsed and kinked lines, plugged pickups, failed petcocks can just let trickles through. Also, make sure the float level is correct.
 

pidjones

Over 1,000 Posts
Oh, and check the voltage to the coils when running. A poor connection or weak battery/charging system might let it idle but not give enough for higher RPMs.
 

SpaceTrucker

Active Member
Fuel is going directly into the carb currently and seems to flow fine. No tank or petcock currently to worry about. When assembling the float, I adjusted it to spec, 24mm I think. It was at 21mm before adjusting... way off.

I too was thinking the main jet may have an issue but I want to make sure the electrical system is in good shape before I break open the carb again.

Anybody know what the ohms should be on the coil?

Where do I test it’s voltage when running?


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