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Thread: Honda 250 Carb issue has me bummed out

  1. #1
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    5th June 2018 - 00:29
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    Honda 250 Carb issue has me bummed out

    Hi Guys,

    I ride a Honda CBX250 Twister (known as CBF250 in some markets) and I am from South Africa.

    I am having an issue with the carb that I am unable to diagnose.

    First off, the bike has an aftermarket exhaust on it. The bike is also at 46 000Km. The motor was rebuilt by Honda by the Previous Owner. The carburetor is so clean, I will eat out of it. There's nothing wrong in the carb.

    The bike had a #142 Main Jet in the carb when I got it. (factory is #138 - It was probably changed somewhere in it's life due to the aftermarket slip-on)

    On partial throttle and idle the bike is very happy, no issues at all. After 3/4 throttle the bike starts to starve it feels like you're closing and opening the throttle continuously.

    Now, here's what I've done in order to diagnose the problem.

    1st, I removed the air filter to test - No difference
    At full throttle with choke on - No difference

    So I went up from the #142 to a #145 main jet.

    It feel ALMOST perfect but it still starves at >3/4 throttle. again, air filter out and choke on with full throttle makes NO difference. - Choke makes it feel a bit worse though.

    So I purchased both #148 and a #150 main jets in HOPE that I manage to sort the bike out!....

    Installed the #148, it feels exactly the same as the #145. same applies to choke and air filter removal.

    I then installed the #150. Now. Now, it over fuels! The bike sputters and pops even at partial throttle (anything more than 1/2 throttle).

    BUTTTT with the air filter removed, it revs freely to 10grand with no issues - which means that my #150 main jet is too BIG!

    I am now currently lost for words.

    I have stripped the whole thing down, checked the air box, all the hoses the carb, everything. I found a o-ring in the carb at the idle jet that was perished, It was replaced. Needle works perfectly and moves up freely when air is blowing over it. The diaphragm is perfect. The carb does not have an adjustable air jet and the needle and floats are non-adjustable.


    It is a Keihin VE carb.

    Guys please help me diagnose this issue!?
    https://farm5.staticflickr.com/4344/...3cc5f11ff5.jpg

    Thank you,
    André

  2. #2
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    Have you tried the #138 jet?
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  3. #3
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    Quote Originally Posted by AndreJ View Post
    I have stripped the whole thing down, checked the air box, all the hoses the carb, everything. I found a o-ring in the carb at the idle jet that was perished, It was replaced. Needle works perfectly and moves up freely when air is blowing over it. The diaphragm is perfect. The carb does not have an adjustable air jet and the needle and floats are non-adjustable.



    Thank you,
    André
    Did you do this before or after the #150 jet was tested ... ???
    Sweat wipes off. Road-rash doesn't.

  4. #4
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    5th June 2018 - 00:29
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    I have not tried the 138 jet. But, I think if it under fuels with the 142, testing a 138 would be pointless?

    I have tried all jets, the stripped the bike down and inspected everything (replaced the broken o-ring) then tested all the jets AGAIN with similar results

  5. #5
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    20th January 2010 - 14:41
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    Quote Originally Posted by AndreJ View Post
    Hi Guys,

    I ride a Honda CBX250 Twister (known as CBF250 in some markets) and I am from South Africa.

    I am having an issue with the carb that I am unable to diagnose.

    First off, the bike has an aftermarket exhaust on it. The bike is also at 46 000Km. The motor was rebuilt by Honda by the Previous Owner. The carburetor is so clean, I will eat out of it. There's nothing wrong in the carb.

    The bike had a #142 Main Jet in the carb when I got it. (factory is #138 - It was probably changed somewhere in it's life due to the aftermarket slip-on)

    On partial throttle and idle the bike is very happy, no issues at all. After 3/4 throttle the bike starts to starve it feels like you're closing and opening the throttle continuously.

    Now, here's what I've done in order to diagnose the problem.

    1st, I removed the air filter to test - No difference
    At full throttle with choke on - No difference

    So I went up from the #142 to a #146 main jet.

    It feel ALMOST perfect but it still starves at >3/4 throttle. again, air filter out and choke on with full throttle makes NO difference. - Choke makes it feel a bit worse though.

    So I purchased both #148 and a #150 main jets in HOPE that I manage to sort the bike out!....

    Installed the #148, it feels exactly the same as the #146. same applies to choke and air filter removal.
    Also check the Float level.

    I then installed the #150. Now. Now, it over fuels! The bike sputters and pops even at partial throttle (anything more than 1/2 throttle).

    BUTTTT with the air filter removed, it revs freely to 10grand with no issues - which means that my #150 main jet is too BIG!

    I am now currently lost for words.

    I have stripped the whole thing down, checked the air box, all the hoses the carb, everything. I found a o-ring in the carb at the idle jet that was perished, It was replaced. Needle works perfectly and moves up freely when air is blowing over it. The diaphragm is perfect. The carb does not have an adjustable air jet and the needle and floats are non-adjustable.


    It is a Keihin VE carb.

    Guys please help me diagnose this issue!?
    https://farm5.staticflickr.com/4344/...3cc5f11ff5.jpg

    Thank you,
    André

    If the issue is at 3/4 throttle try raising or lowering the needle.
    Some of the Hondas are not adjustable but you can raise them with a washer under the clip.
    Personally I would put it back to how you got it and put a washer (say 1mm thick) at a time under the needle clip. then try 2mm then 3mm.
    When adjusting it always go back to the original setting afterwards just to confirm it is actually better.
    Carbs have as well as having overlapping circuits also have air corrector circuits that work over narrow ranges.
    But stick with the basics first.
    As it has a CV carb you can also make the air slide admission hole slightly larger or make the Diaphragm springer stronger (but only do this last)



    Kinky is using a feather. Perverted is using the whole chicken

  6. #6
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    5th June 2018 - 00:29
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    Thank you! I was looking at the needle when I had it all apart and wondered if this idea would work. Now you have confirmed that I can try it, I am going to give it a try TONIGHT!

    I currently have the #145 back in the carb as it gives me the best feeling out of all of them. Will try the washer idea with this jet first and see how it goes.

  7. #7
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    You can get some pretty thin washers don't go crazy at first but the needle is very likely the issue due to new pipe. Obviously clean carb also means you can blow through the pilot and air holes. Blow on a tube if you don't have compressed air.

    On a cv carb blowing in the inlet at the top will raise the carb slide if it is not perished.
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  8. #8
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    Sounds like carb may not get enough fuel....
    You say floats are not adjustable?
    Can't bend the tab?
    See at what level floats shut of fuel... ( clear hose from bowl drain up...)
    Also is fuel filter in fuel tap clean...?
    Opinions are like arseholes: Everybody has got one, but that doesn't mean you got to air it in public all the time....

  9. #9
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    5th June 2018 - 00:29
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    Hi guys.

    First of all, I cannot bend the tab on the float, its plastic. I have already replaced the fuel hose from the tank to the carb, new in-line fuel filter - feels exactly the same with or without the filter. the fuel petcock trap is also clean. And yes, I have checked that the slider moves up with air and it does and I have cleaned and blown out all the jets and holes. The carb and jets are CLEAN.


    Ok, I have tried some washers:


    The washers I used is 0.5mm thick.

    With the #145 it feels the same as without one.

    #148 with 1 washer feel dramatically beter! But there is still a little bit of hesitance.

    #150 with or without a washer feels horrible the bike pops and spits with it.

    So I tried the #148 with 2 washers, also feels horrible.

    So I'm back with the #148 with 1 washer and still the bike isn't running as it should, although it is a lot better.

  10. #10
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    Quote Originally Posted by AndreJ View Post
    Hi guys.

    First of all, I cannot bend the tab on the float, its plastic. I have already replaced the fuel hose from the tank to the carb, new in-line fuel filter - feels exactly the same with or without the filter. the fuel petcock trap is also clean. And yes, I have checked that the slider moves up with air and it does and I have cleaned and blown out all the jets and holes. The carb and jets are CLEAN.


    Ok, I have tried some washers:


    The washers I used is 0.5mm thick.

    With the #145 it feels the same as without one.

    #148 with 1 washer feel dramatically beter! But there is still a little bit of hesitance.

    #150 with or without a washer feels horrible the bike pops and spits with it.

    So I tried the #148 with 2 washers, also feels horrible.

    So I'm back with the #148 with 1 washer and still the bike isn't running as it should, although it is a lot better.
    Are you at altitude?
    Next step i would try is to preload the slide spring a little bit.
    Failing that admit defeat and buy a jet kit.
    https://www.6sigmajetkit.com/honda-cbf250-jet-kit.html



    Kinky is using a feather. Perverted is using the whole chicken

  11. #11
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    Quote Originally Posted by husaberg View Post
    Are you at altitude?
    Next step i would try is to preload the slide spring a little bit.
    Failing that admit defeat and buy a jet kit.
    https://www.6sigmajetkit.com/honda-cbf250-jet-kit.html
    Yes, I am at altitude, 1724m.

    How do I preload the slide spring?

  12. #12
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    Quote Originally Posted by AndreJ View Post
    Yes, I am at altitude, 1724m.

    How do I preload the slide spring?
    Stick a big washer under it



    Kinky is using a feather. Perverted is using the whole chicken

  13. #13
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    Quote Originally Posted by husaberg View Post
    Stick a big washer under it
    I'm gonna put the bike on a exhaust gas analyzer first and see what that tells me. Then I'm gonna have to get serious with this carb. I'm getting rather tired and taking it apart and reinstalling time and time and time and time again.

  14. #14
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    Quote Originally Posted by AndreJ View Post
    I'm gonna put the bike on a exhaust gas analyzer first and see what that tells me. Then I'm gonna have to get serious with this carb. I'm getting rather tired and taking it apart and reinstalling time and time and time and time again.
    Cant you just loosen the clamp and twist the carb to one side to access the top or the bottom?

    Odd thing is altitude requires leaner jetting than std.



    Kinky is using a feather. Perverted is using the whole chicken

  15. #15
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    Quote Originally Posted by husaberg View Post
    Cant you just loosen the clamp and twist the carb to one side to access the top or the bottom?

    Odd thing is altitude requires leaner jetting than std.
    Yes, I can, but access still isnt easy and my clamps etc is starting to show wear on the screws which I hate.

    By bike has a air injection system - https://www.motorcyclespareparts.eu/...-control-valve

    I wonder if this might not be the cause of my problems...

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