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Help Twister Carb issue. Really need help!

Discussion in 'Honda 250cc Singles' started by AndreJ, Feb 13, 2019.

  1. AndreJ

    AndreJ Member

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    Hi guys,

    I have introduced myself in the newbies corner.

    I am from South Africa.

    Guys, Seriously I am starting to become seriously irritated with a carb issue I have.

    Now, the bike has an aftermarket exhaust with a stock air filter, no mods to the air box. Bought the bike like that, 2 years ago. At idle and riding normally the bike runs absolutely perfectly!! However. As soon as you take it over 6500RPM at WOT it starts to sputter and cough all the way to the 10 000RPM redline.

    I’ve played with the jets but no joy. First off, the bike had a 142 jet in it (installed by a previous owner - although 136 is factory) and I’ve had a 146, 148 and 150 jets in it. It feels the best with the 148 (but STILL sputter and cough) and with the 150, it feels absolutely HORRIBLE. I suppose it over fuels massively with the 150. THE CARB IS CLEAN. Choke or no choke, airfilter or no airfilter makes no difference on any of these Jets.

    This has me thinking, maybe it over fuels with all of these jets and maybe we should go down to 136 again??

    The needle and everything seats as they should. The bike is CLEAN, it’s serviced and exceptionally maintained. The air filter is basically new also.

    The issue is however, not the needle, the air, nor the float level is adjustable. So it MUST be the main jet.

    As indicated before, the bike runs 100% perfectly when you just ride normally. (primary jet working fine) There is absolutely NOTHING wrong with it at normal cruising. It’s only when you crack that throttle further than half way open to get power and after 6500 RPM that the bike starts to cough. Very irritating, especially when you’re trying to go uphill. (The bike will rev to 10 000RPM with no load in N)

    A brand new spark plug was fitted on Thursday, old plug came out looking fine, no gap issues, but the coughing remains. There is no way that the standard pipe can be fitted as the bike was purchased with this setup.

    The carb is the diaphragm needle type, needle is not adjustable (albeit I put a small washer in there also). Diaphragm is working as it should and has no tears or leaks in it at all.

    I started to think that I might be just dumping WAY to much fuel to such an extent that the choke-test just doesn't even matter. Yesterday, I got a 140 jet (smallest one I've ever tried) NOW, the bike sputters alot. BUT, with the choke test this time, there is a difference!!! The bike wants to stall with the choke applied.. This indicates to me that I am on the right track to go down in jet sizes as even with the 140 it is over fuelling.

    Next I have a 132, I'll put that in and see.

    ANY ADVISE PLEASE GUYS?

    Here's my Bike. It is absolutely prestine:

    IMG-20181130-125628.jpg
     
  2. kiffsta

    kiffsta Administrator Staff Member Dirty Wheel Club Contributing Member

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    our of curiosity , how many kms has the bike done ? After a spirited ride , can you please pull the plug and post a few pics , when I say spirited not revving to 10k at idle,
    But taking it out on a decent ride

    I’d hazzard a guess there is an air leak which is causing your issue, how is the manifold that holds your carb to your head
     
  3. AndreJ

    AndreJ Member

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    Manifold is fine, looking good, no cracks. I have checked for air leaks, couldn't find any. The bike has done 47 000Kms but the motor was rebuilt by Honda just before I bought it.

    Taking the bike on a spirited ride is next to impossible with the sputtering going on. it doesn't want to rev. It goes to 10 000 but it's difficult to get it to do that.

    On top of that, For some reason I am unable to log in to my Flickr account on the PC so I cannot share photos. The photo on the previous post was borrowed from a pic my brother had on his posting site...
     
  4. GreyImport

    GreyImport Administrator Staff Member The Big Cheese Contributing Member

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    I would put in a stock main jet so u can rule that out
     
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  5. AndreJ

    AndreJ Member

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    I have a 132 at home, will install it after work. I need to get my hands on a 136, will see if I can get one. I am at altitude
     
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  6. AndreJ

    AndreJ Member

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    I read on a South African forum from 2011, one owner that had the same issue with his bike after installing an aftermarket exhaust.

    Unfortunately he decided to sell the bike for a CBR600RR and the issue was never resolved instead he just fitted the std pipe back on. So no info there...
     
  7. ShaneP

    ShaneP Well-Known Member Premium Member Contributing Member

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    It seems you're on the right track with the main jet, given you're at altitude it might be good to go small and take the washer out from under the needle.
    Make sure the carb slider is actually moving the whole stroke, too. I've seen them with folded springs and they only slider part way. Usually you blow in the top section of filter side to see if it functions properly, but even lifting it with your finger or screwdriver will work - the return should be smooth and steady.

    Also, float level is adjustable, you simply bend the tab on the float that pushes the needle up. Seems crude, but it is in the manuals. There are aftermarket needles for a lot of carbies, usually I have to lean them out for high-flow exhausts.
     
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  8. Murdo

    Murdo The Good Doctor Staff Member Premium Member Ride and Events Crew Contributing Member

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    Go back to the stock jetting so you have a starting point. Some aftermarket exhausts work better/flow better than stock and need leaner jetting than standard. Jet to suit what the engine wants rather than what you think it needs.
     
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  9. my67xr

    my67xr Bike Enthusiast Staff Member Premium Member Contributing Member

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    I agree with trying out a standard main jet, maybe even 1 size smaller with the altitude.
    Another thing to try is to remove the air filter, it will run leaner without it, if it improves how the bike goes at higher speed's then that comfirm's your thought's about needing a leaner main jet

    When you try the 132 main jet if it feel's a lot better and has less spluttering then you're on your way to a better tune.
    Make sure you remove the washer under the needle clip before fitting the new main jet too like @ShaneP mentioned .

    If you find that the 132 is better but feel's flat in the higher rpm's, go up 1 size and try it again it should be better, otherwise go back to a standard size jet.

    My FZR250R has an aftermarket pipe and modified air filter but the carby's still have standard jetting, it goes very well at all throttle position's
     
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  10. sharky

    sharky Well-Known Member Premium Member

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    Are the floats adjustable or not?
     
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  11. GreyImport

    GreyImport Administrator Staff Member The Big Cheese Contributing Member

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    This is from the CBX250 manual :idk:

    float cbx.png
     
  12. AndreJ

    AndreJ Member

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    Nope, The whole float is a single unit made with plastic. I know how to adjust float levels but this one is no go.. unfortunately.
     
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  13. AndreJ

    AndreJ Member

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    Guys, I have NEWS!

    I installed the 132 main jet Yesterday. What a difference! The bike feels like a different bike!

    There is still a SLIGHT hesitation at 7200RPM but it is really, very slight. After I took the bike for a ride, I took it home and removed the washer under the needle. I also read somewhere that it makes a huge difference with the aftermarket pipe when you remove the fine mesh around the air filter. It is wrapped twice. I did that too.

    There is still a slight hesitation so I have one more ace up my sleeve.. A 128 Main.

    I will try that this afternoon after work. I am at 1700m (5577ft) altitude so I think a bit smaller than the 132 might be the cure. Unfortunately I don't have a 130, however if the 128 is too small I can get a 130 which should work!

    Will report back

    I am so excited to have a bike that actually has a usable rev range!!
     
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  14. Murdo

    Murdo The Good Doctor Staff Member Premium Member Ride and Events Crew Contributing Member

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    Manual says 'floats cannot be set'. Sounds like a challenge to me. :D
    What about heating the plastic float arm and bending to correct height.
    What about using different length needles, drilling the seat deeper or grinding a bit off the length of the needle to raise fuel level, putting washers under the seat to lower level.
    Just some of the ways in which I have 'adjusted' the non adjustable float heights.
     
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  15. AndreJ

    AndreJ Member

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    @Murdo Yes, I know there are ways and means of how to cheat the system, but based on my findings Yesterday (as described in my last post) I don't think that the float level is an issue at all.
     
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  16. Frankster

    Frankster See the World before you leave it Staff Member Premium Member Ride and Events Crew

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    In my opinion, you're on the right track with this line of thinking. Just to "play" with the formula, take the air filter out with the 132 and see how that runs. Pay attention to climate on the runs e.g. is it humid, overcast, cold etc. You're doing a good job diagnosing and fixing your issues.
     
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  17. AndreJ

    AndreJ Member

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    Morning guys,

    Right, I installed the 128 jet last night, the bike revs right through to 10 000RPM. There is the slightest slightest hestitation albeit barely noticeable, and the choke makes no difference. I think I just need to adjust mixture screw in front of the carb at the bottom. Manual says 2 and a quarter turns, which is what it is at.
     
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  18. AndreJ

    AndreJ Member

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    Oh JOY! My flicker is sorted! Here she is:
    47079549941_94bd38929b_b.jpg cbxxx
     

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  19. Murdo

    Murdo The Good Doctor Staff Member Premium Member Ride and Events Crew Contributing Member

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    Sweet bike. :thumb_ups:
     
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  20. ShaneP

    ShaneP Well-Known Member Premium Member Contributing Member

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    Luckily it is only a single cylinder engine, not a 4! Well done.
     
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