1981 CB125S won't start

papadapalis

papadapalis

New Member
We're working on a friend's 1981 cb125s that won't start, and I'm not sure what to do here.

First off it has the CDI ignition instead of points. I can't find much info on timing these little guys out there, but I set the crank to the F mark on the compression stroke and lined up the marks on the ignition. I'm not sure this is correct, and if it is I do not know if i need to set the gap between the stationary and spinning parts.

Secondly, the original carb was beyond repair and replaced with an aftermarket carb. I'm not sure where the air and fuel screws need to be, nor what the float height should be set to.

Any help would be much appreciated, we would love to get this thing on the road.

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The only ones I can find only show setting of the older points ignitions, and only showing carb adjustments.

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Okay according to info on similar honda singles (namely this video https://youtu.be/XuSaIsP_Kx0 ) the timing and gap should be right. We're making sure the carb is fully clean right now.

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It is a 26mm oemstd he ordered from ebay when he first got the bike. The bike at one point fired and ran with this carb, albiet poorly.

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becdeaf6dd0a1ff736941f8288cfd9ac.jpg
this is the carb.

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Well the pilot on the carb is clogged, we have the ultrasonic cleaner going at the moment. The ignition switch is good I believe, lights work with key on. I just took the tank off to set the tappets while the carbs getting cleaned. Fuses look to be good as well.

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Just tossing a carb in an ultrasonic cleaner won't necessarily unclog all the passages. My process is to strip everything off the carb especially remove all brass jets etc, soak the carb body or ultrasonic. Spray carb cleaner through each jet, the emulsion tube and all the air passages, and then blow out with compressed air until you see a steady stream. Hold the jets up to a. Light and make sure the passage is completely clear, sometimes a magnifying glass is helpful but you should be able to tell whether the inner passage is clear and circular or has any crud in it.


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Carbs off... shoot a little WD init and see if it pops off, did you look to see if it has spark?
 
Well it lives, kinda. It will not idle properly unless the choke is on, but it did manage to carry me around the block. The timing was off so we adjusted that, which confirms the procedure in the video I linked will work for these. I also adjusted the valve clearances while the carb was being thoroughly cleaned. It still won't idle with the choke off even with the idle screw turned the entire way out. Would sourcing an oem keihin carb be the best solution?

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Yes. If you have a bigger pilot jet put that in and see if you can get it to idle, but bottom line... that carb is not the best.
 
papadapalis said:
Well it lives, kinda. It will not idle properly unless the choke is on, but it did manage to carry me around the block. The timing was off so we adjusted that, which confirms the procedure in the video I linked will work for these. I also adjusted the valve clearances while the carb was being thoroughly cleaned. It still won't idle with the choke off even with the idle screw turned the entire way out. Would sourcing an oem keihin carb be the best solution?

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Click to expand...

I'd install a stock Keihin Honda carb.(Made in Japan)after cleaning it up real good.
 
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