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Bike not running right

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Georgek591
Derestricted Danger



Joined: 02 Jan 2017
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PostPosted: 16:49 - 02 Jan 2017    Post subject: Bike not running right Reply with quote

I have recently purchased a Honda CBR125 cheaply off a family member. Before they owned it a previous owner had crashed it and the bike had been written off. Someone brought it and repaired it using parts from other bikes (Electronics, front light, speedo, temp, rev count Etc as the front was smashed to pieces). I know the bikes engine has been rebuilt and the exaughst has been replaced by a Yoshimura USA (Off the topic I need to get a baffle for this at somepoint as it's very loud and most likely illegal so if you know where I could pick one up cheap it would be much appreciated). The bike is also missing a lot of faring which I hope to replace at some point.

Getting to the point, I know not to expect much from this bike as it's really a hunk of junk but the bike struggles to reach 60 on a flat surface with no wind (I'm aware there's many threads discussing this). I believe the bike is running too lean as it often dies when cold after starting (If I don't rev the bike a lot it will take about 5-10 starts) also when decelerating it makes some lovely popping noise but I believe this is due to it being too lean.

What I need to know, what screw can adjust the fuel to air ratio and where is it. I've read many posts but have been confused by them as I can't tell if their talking about the idle or the air/fuel screw or if their the same thing.

I'm planning on replacing the spark plug but I don't have the correct tools to take it out at the moment (would of checked the spark plug to confirm if it's running to lean but I can't). I am planning on cleaning out the carb and getting a new rear sprocket and chain and replacing the fairing.

I'm aware this bike probably isn't worth the hassle but it's all I can afford to get me around at present. I'm not sure what parts have been replaced on this bike but recently the throttle cable was replaced as it snapped so I'm assuming the bikes got a fair amount of worn parts.

Bike info(if it helps) - CBR125R 2007, Rebuilt engine, Yoshimura USA exaughst, all electronics on the handle bars are gone apart from an only Suzuki speedo (No temp gauge, rev count or mileage), a lot of the elecrronics have been rewired as there from other bikes(Indicators, headlamp, ignition), I believe the rear brake has a cable and the front is hydraulic.

Any help much appreciated. Can supply photos if anyone needs. Thanks
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MotorcycleMD
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Joined: 17 Jan 2017
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PostPosted: 07:19 - 17 Jan 2017    Post subject: Reply with quote

It definitely sounds like a fuel starving issue. If the bike is equipped with a working choke when the bike starts to cut out or die at the higher mph try applying the choke to see if cutting air and allowing more gas through clears it up. If so, then the problem is definitely fuel starvation. If not, than it may be an ignition problem.

At that wide open throttle position, your pilot mixture screw will not make a difference as it does not feed that power curve.

Regardless, it may help the decel backfire a bit. Pilot mixtures are typically located on the lower portions of the carb body, just before the float bowl. It will either be closer to the engine side or closer to the air-box side (its exact location will be the deciding factor on how to make the adjustment)

The only other exception is when they are located directly on the belly of the carb on the forward most section of the bowl. ( I don't know what carb you are using)
Idle rpm screws or adjusters are typically found either dead center on the carb body.

Hope this helps, sounds like you have a real piece of work!
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kgm
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Joined: 04 Jun 2015
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PostPosted: 09:29 - 17 Jan 2017    Post subject: Reply with quote

As MotorcycleMD says the pilot screw controls fuelling at low revs so is unlikely to be the problem here but for information purposes this is how it is adjusted (copied from one of my other posts)

Code:
...All you need to do is get the engine up to normal temp (take it for a ride) then adjust the pilot screw which will change the idle speed slightly. As you back the screw out the idle speed will increase but only to a point. You want it set at the point at which the fastest idle is achieved. Once you have done that you can adjust the idle speed screw until you have the correct idle speed set. Note that the pilot and idle screws are differemt, it's the pilot screw you want to change first. The idle screw is set afterwards and this one has a greater affect on the idle speed.


I think this is the correct carb but I may be wrong:
Carb Diagram
If so I believe that 4 is the pilot mixture screw and 5 is your idle adjuster. I could be wrong though, I'd have to see it to confirm.

If it has a baffleless Yoshi system on it then it's likely that the carb will need to be rejetted to suit. I would start by checking that it is stock, and going back to stock if it is not and work from there. I can't advise on sizing I'm afraid.
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