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Honda CB125T Extreamly high idle

297 Views 7 Replies 4 Participants Last post by  mike in idaho
I have had problems with the bike for some time. At high rpm bike started stuttering and losing power and the problem got worse the hotter the bike got. Cold starting has also been really difficult but when warm, it starts on the first kick. I tried all the basic stuff and changed the whole ignition (points, condensators etc.).

After winter I did some basic maintenance: Cleaned the tank, new fuel lines, cleaned the carbs and switched the main jets from 88 to 90 to see if made difference at the high rpms. When the bike was running right before i tried 90s because they were suggested for more torque, but I noticed no difference so I switched back to 88 then.

Bike was really hard to start (50 kicks) and when it started the bike immediately overrevving. I though a slide was stuck or something and I tried a lot of other stuff. At first idle is around 5k and when the bike warms it gets closer to 8k.

I put new o-rings on the intake manifolds and tired to seal them with better clamps and duct tape with no effect. Also tried spraying intake manifolds with starting fluid, which had no noticable effect. I also tried sealing the slide caps better.

I tried to feel the carburetor slides through the air filter and played with throttle and both slides seem to go all the way down. Slides are adjusted to go as down as they can.

When the bike was warm, I tried to start the bike with right spark plug wire disconnected and it had no life. When I did the same, but removed the left wire, bike started immediately and was idling 5k on only the right cylinder.

When I ajust the left carb or block the left air filter intake, it has no effect. When I block the ride side air filter intake the bike starts decreasing in revs and shuts off.

Tried to move the bike but it had no power.

Both spark plugs were black(burning a bit of oil and they are not new)

What should I try next? I don't seem to come up with anything that could help anymore. Seems like the air is somehow just going through the slide. Also I find it hard to believe that this could be caused by changing the jets considering they bike has ran just fine on 90s before.
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Had a similar problem with a 74 cb125s I fixed not too long ago. Bike started but the idle was way too high and adjusting the idle screw or pilot fuel screw did nothing. Turns out the throttle housing was incorrect for the bike and it was never letting the slide in the carb fully close.

So the issue ended up being something as simple as that. But if your bike is shutting off when you block the air filter…there’s probably an intake leak so double check everything make sure it’s all sealed up.
I doubt the problem is with slides not closing because:
1. Because bike has no power
2. Revving feels sluggish
3. Slides feel closed through the air filter
4. There have been no such problems before

On the intake leaks:

1.I changed both o-ring on both intakes
2. Rubbers seemed intact but I put some duct tape around them just to be sure
3. I checked that everything is thigt
There was no improvement after this. If the main problem was intake leak after the carburetor I would imagine there would've been even some improvement.

I noticed that both pilot jets seem plugged but im not sure. Also i don't know if plugged pilot jet could cause high idle. Should I see light though the pilot jets in PD26 carbs?
Also the pilot jets have no groove or anything to screw them off. Theyre just round cylinders with a hole in the bottom. Are they pressed and if yes, how should I remove them?
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Have you dipped the carbs? Or cleaned them in an ultrasonic?
Clened with 10bar air and brake cleaned. It feels like air flows everywhere but not sure. Ultrasonic is mostly out of picture because I dont even know anyone who has the machine. Dipping I could try. Do I need carb cleaner or is there some makeshift liquid that I could use.

Best would be if i could just remove the pilots
See if an auto parts store near you has a carb dip for sale and try that out. Remove all the rubber from the carb prior to dipping.
Pilot jets dont have thread. You just simply need to grab them and pull (maybe with little twist)
I have the same issue with start (kicking 50 times).
Did you install the slides the right way around, beveled side towards the air cleaners?
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