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Idle Mixtures on a 250 Hornet

Discussion in 'Technical and Troubleshooting Torque' started by VCM, Jul 11, 2008.

  1. Engine idle has always been fluctuating little, even hunting at times. It hadn't bothered me until recently. I then decided to take a look at the mixtures ( pilot screw settings). Now I'm from the old school .. what I did was adjust the pilot screws individually, set each one at the highest obtainable Idle speed, then adjusted the idle speed down to around 1300-1500rpm.
    Was hard to hear a difference in RPMs when adjusting but the end result of adjusting the mixtures did raise it, so I figured all was well.

    I now think Ive set it too rich. WHY?
    1 - choke is not required to start bike on these cold mornings
    2 - Apply choke is finicky ... stalls if I give her anymore than a slight application of choke.
    3 - The bike feels flat around 2,000 rpm

    I have searched here and read the CBR250RR manual, ( can't get my hands on a CB250F manual, so I figured they run the same engine right? )

    Gonna attack her again on Saturday ... Any suggestions ???

    PS: those screws are a BASTARD to get to .. any locating a right angled screwdriver is near impossible locally

    - Vinnie
  2. tuning idle mixture by ear on a i4 is never going to work. Set them all back to x turns out. Then just adjust the throttle stop setting.

    You'll have to find out what x should have been.
  3. Thx mate ... now to find the standard setting 'x' . :?
    I'm hoping its the same as the CBR250RR. I'll keep searching
  4. This might help; it's for the 600 but the principles may be the same...

  5. If you can't find it, try setting them all to the leanest one you have at the moment then see how that go's.

    then adjust 1/4 turn at a time, with a full warm up cycle and a ride between adjustments.

    Your final adjustment may be 1/8th of a turn.
  6. Thanks again Guys :)

    and for that page Paul
  7. Does anyone know where I can find something like this:



    Getting to the mixture screws are a real BUGGER
  8. Mate, Try removing the carbies for better access. Remove the pilot screws completely, used some compressed air to give them a good blow out. refit the pilot screws and set them to x turns out(you will need to find the spec somewhere first) do it accurately by vision, not by feel.
  9. from memory the hornet 250's got the motor from the 1989 CBR250RR. It's the one with the gear driven camshafts, hence the scream those motors emit when spun really fast... it's all those gears.

    try and get a manual on the 89 CBR and go from there.
  10. err. no

    In 1996 the 250 Hornet revolutionised the naked street fighter market in Japan. The roots of this engine is the 1986 CBR250Four.
  11. That has always been regarded as an absolute no-no.

    You must never force compressed air into a carb which still has the float chamber/fuel bowl attached - the increased air pressure can easily damage the floats. Plastic floats are a little more robust than the old brass ones, but I would never do it.

    All the best ;-)

    Trevor G
  12. Here is the correct technique:

    1) Determine whether the pilot screw setting is a fuel adjusting or air adjusting device.

    Most vacuum carbs use fuel adjusting - the adjustment screw is on the engine side of the throttle slide, often just underneath the front-most edge of the carb, and awkwardly requiring a short, stubby screwdriver to adjust.

    Some use a screw on the side of the carb which is easy to get at, such as on the VTR250, Spada and even the NSR150. These all use fuel adjusting screws, too, because they are located on the engine side of the throttle slide.

    2) Make sure the valves have been adjusted.

    If you have never done it, or had it done on an old bike such as this, then be warned that valve clearances usually close up, and this will really affect the way the engine performs low down, and also the idle.

    3) Make sure the engine is warmed up properly - you should ride 3 km at least, in this weather.

    4) Once adjustment has started do not allow it to continue for more than a couple of minutes - if the bike idles for too many minutes the temperature rises and so the fuel mixture setting will not be as accurate as it should be.

    5) Adjust one carb at a time.

    Screw the mixture screw in (this leans the mixture on a fuel adjustment pilot) until the engine runs less smoothly.

    Then turn the screw out and listen for it to increase speed and run more smoothly. Count the turns from the too-lean position - it might be 1/2 to 3/4 of a turn, only.

    Keep turning the screw out until it starts to run roughly again - this is the too-rich setting and might be another 1/2 to 3/4 of a turn out.

    Screw it back in to the smooth point, and then count the turns again to where it runs roughly at the inmost, or lean setting. Then turn it back 1/4 turn - this should give smooth running for that cylinder

    6) Don't go in and our more than once at this stage - you will overheat the engine, not so that it is damaged, but so that the adjustment is no longer appropriate for the normal temp that the engine will sit on.

    Move on to the other carbs, and adjust the idle speed to the correct setting as you go.

    7) If you take longer than 3 minutes you most probably need to ride around the block to cool the engine down. Remember that switching the engine off actually increases its temp initially.

    8) Once you establish the smoothest setting for each cylinder, but no more than 1/4 turn back from the lean-running position, go for a ride.

    The pilot screw mixture setting really only adjusts the idle speed mixture, but I have noticed on vacuum carbs on J bikes that opening it up to another 1/4 turn richer can help with low end throttle response, as each carb adjustment point overlaps the adjacent one.

    However, remember that if you adjust for this, you will be running richer at idle.

    The most likely reasons for a bike not needing choke to start are:

    1) The engine is warm

    2) It's a hot day

    3) The idle mixture is set too rich

    4) The needle jets and jet needles are worn, making the low to mid-range mixture richer than it should be. Believe it or not, this also seems to allow more fuel in at starting time.

    All the best

    Trevor G
  13. Thanks Trev! Your a Champ :beer:
    Reading through I again reminded of the one item I have been putting off that will prevent me from correctly setting idle mixtures ...
    Valve Clearance :shock:
    Building Heads for a living I know too well the importance of this.
    Doing clearance on a work bench is a pain, I can't imagine having to do them on a bike :cry: .. why I've been avoiding it.
    That said, I'll have to set a day aside and do em.
    Once that's done I'll attack those pilot screws again.

    Question: I'll need to purchase a 'shim kit'
    Any clues on where best to buy these? I'm guessing it'll have to be a honda dealer right?
  14. I agree with Trev's summary above, but wil add some litle tricks I have worked out.
    As said, always tune EVERYTHING else up first, including valve clearances and carb balance. Otherwise you are just shooting in the dark.
    I always turn the idle mixture screws till I start to get a lean misfire (usually inwards, check your workshop manual), then back it off till it just runs smoothly.
    The idle mix screws are very sensitive, a small movement will do a lot, and they can take a few seconds to make a difference to idle quality, so take the process nice and slowly.
    I also walk to teh back of the bike and listen to teh exhaust occasionaly, you can hear things much better this way.
    Expect to have to go back and readjust teh mixtures again when you get the last carb done. Adjusting the screws will affect idle speed, and if you have ben tweaking idle speed up or down as you go along, you'll have to do some more minor tweaks on teh carbs you have done.
    DO NOT ADJUST IDLE MIXTURES until the bike is completely warmed up. This is where teh engine spends most of it's time, so adjust it for this condition.
    You can definitely tune an I4 to have a very smooth idle by ear, it just takes time and patience. My bike idles smoother than an injected VFR 800.......

    Regards, Andrew.
  15. It's not really hard to check clearances on teh bike, it just takes more time.
    As for a shim kit, I just figure out what shims I need and buy those particular shims. Buying kits only ensures you will have lots of shims you'll neve use.
    Sometimes shims you already have on the engine can be moved around to fit another tappet on the engine almost perfectly.
    Fortunately for me, Kawasaki blessed the GTR with screws and locknuts.

    Regards, Andrew.
  16. Thanks for all the valuable tips guys !
    I'll attack the clearances this week which may mean being a cager for a cpl of daysif I need to order shims as I go :cry:
    Still trying to find an angled driver to get to carbs 2 & 3.
    You'd think it'd be pretty readily avail in most stores and tool outlets :cry:
  17. Not sure about the 250/4 Honda.

    The usual method, if shims are used, is to measure all the clearances and then also record the shim size at each valve.

    You then use simple maths to work out which shim needs to go where to get the clearance required.

    Record the readings in chart form, and do this every time you adjust. You can frequently just shift shims around to get most clearances correct, and maybe only replace 2 or 3.

    I used to just order the shims I needed after taking the measurements, and then replace them the following weekend.


    Trevor G

    PS But I was working on Kawas, so it might be a little different for a Honda...maybe. ;-)
  18. Adjusted my valve clearances last week. Amazingly it was not as difficult as I had thought. Found the intakes were on or a little under bottom tolerance.
    Then proceeded to sync the carbs and reset mixtures.
    The difference it made to the bike's mid to high range was a surprise :shock:
    So much smoother and respsonsive. The only thing that has been bugging me is the fact that no choke is necessary for cold starting. To me this indicates a rich idle mixture. I've been tinkering with the mixtures and even have em set to about 1.5 turns out ( lean ). Still does not require choke :?
    Idle is smooth and steady. It will handle the slightest of choke, any more and she wants to die. Air cleaner is clear.
    I have yet to remove the pilot screws completely and blow out the jets.
    Should I be concerned ??
  19. Given it's a Honda, I'm guessing it's running relatively large pilot jets. This means 1.5 turns out may not be that lean at all.

    go in 1/4 of a turn and see if it has any ill effects. pull a plug afterwoods (sit on idle for a bit before stopping)and see how sooty it is.

    It does sound like your pretty close, however.