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Discussion Starter #1
I have a Honda 1980 CB125S that I picked up that was not running since the owner said it had not been run in years and the fuel tank/carb was varnished up. Problem I am having is that it only runs on choke. Or sometimes it will idle at No Choke, but as soon as you open the throttle a bit, it will die.

Things I have done...

- Checked compression. 120 with throttle closed, 150 with throttle way open
- Soaked/Cleaned carb.
- Removed and emptied gas tank, and flushed tank with gas.
- New spark plug and checked for spark.
- New battery, and have 6.35 volts on it at the moment.
- Checked for air leaks by spraying carb cleaner while running at the joints after carb....no effect. I even put some liquid gasket on the joints between carb and manifold, and manifold and case.
- Bought an after-market carb for like $25, but got same results.
- Put old carb back on, same results. Carb is a Keihin, PD 24A RH

I would appreciate any advice and help!!!

Thank you.
 

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The fuel mixture is probably set too lean. It could be an adjustment in the carb or something still clogged up.

It might be a good idea to check the valve adjustment as well.
 

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Discussion Starter #3
Sorry for not gettng back right away, but I eventually figured out I had to tank the seat and tank off in order to adjust the valves. Well I have the valve clearances at .002" per spec now. Not sure if they were out of spec since I fuddled with them while getting the piston at TDC on the compression stroke.

Since I had the carburetor off, I put on the the after-market carb on since it just seems more pristine.

I was able to get the bike started. And after getting it warm and messing with the idle screw, it now seems to be able to idle without the choke on at all. But it was still acting strange in the it would seem to rev up a bit here and there, and it still would die if you would quickly try to give it throttle, or try to start out in first gear without really making sure you are giving it enough throttle.

I then thought that maybe I sure raise the throttle needle. This needle has five positions of a clip that holds it up. So I lowered it one notch thinking that it sure make it not run as lean. Well after fuddling around some more, and then playing with the pilot screw (screw on bottom of carb on engine side), it seems to idle a lot better. But the symptom remains... of conking out when giving very quick throttle (in neutral) or when trying to start out in first gear with very little throttle.

Not sure how to get to the bottom of this problem. This is the motorcycle I have worked out, and it seems so finnicky. I typical only have played with lawn mower carburetors before and they dont seem so finnicky.

Any help on some next steps would be appreciated.
 

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When you say "soaked,cleaned carb" explain just what you did. If cleaning the carb did not include removing the jets and blowing air through the passages, then your job needs doing again. Some of the jets go through emulsifier tubes,and the holes running crossways through them need to be cleared as well. Occasionally I have to use a wire to clear the gunk,so I use a strand of copper wire to avoid enlarging the passage. Make sure to use a manual to know how to remove the jets. Some screw in and some are just a light press fit..

It goes without saying that you need to be careful with carb soak around rubber parts of the carb
 

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Discussion Starter #5
Well for the old carb I removed the main jet, float and needle, and cleaned it really well. And soaked the parts and whole carb. Then used carb cleaner to clean it again.

In any case, I now have the new after-market carb on the motorcycle now. So I am thinking it is not a dirty carb issue.

Just seems odd how I got the same results with the new carb or old cleaned carb where the cycle just no respond immediately to the throttle at the start of quickly increasing the throttle.
 

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All I can tell you is that I have a CB100 and perhaps we have a very similar carb set-up. The carb passages that gave me fits were the smaller ones and the slow jet was the worst of the lot. Without the slow jet being clean,the air screw has no effect. When I finally got mine clean,it ran like a different bike.

BTW,since I mentioned the air screw,removing it and spraying some carb cleaner there can make the bike run better as well.There is a screw with a needle point,an o-ring washer and spring beneath the screw, so be careful that nothing gets lost.

Another issue can be the seals around the carb letting in air resulting in a lean condition. If you spray some carb cleaner outside of the carb and the bike engine RPMs increase,that is the place it is leaking.
 
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