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Missfires/stumble below 5000 rpm

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by Tobbera, Feb 27, 2012.

  1. I start off by saying Hi and that I'm glad to have found this forum. I'm from Sweden but are staying in Sydney for a year. A couple of month ago I bought myself a Yamaha FZX250 Zeal 1999, about 50 000 km on the clock. It has served me well, even though I have had some minor things that I have fixed along the way. I do all mechanic work myself.

    However, there is one issue that I have not been able to sort out or understand. The bikes misfires, stumbles, hesitates, under 5000 rpm after I have rev'd it hard. Starting it up cold, no problem, can run it for a long time without any issues, as long as I don't rev it hard. It pulls smooth all the way from idle at 1600 rpm. But if I decide to rev it, from a green light for example, all the way to 15 000 rpm then shifts up to 6th gear to cruise, it stumbles. Lack of power, feels like its running of 2 out of 4 cyls. If I gear down a few clicks and keep it over 5000 it pulls as mad again. This conditions persists everywhere between 10 min and the rest of my ride. As you can figure out the driving situation becomes very uncomfortable, having to rev it past 5000 to be able to launch form a red light uphill.

    Counter measures:
    • Removed carbs and cleaned them.
    • New spark plugs.
    • New ignition coil for cyl 1 & 4 (wasted spark system).

    • Old spark plugs were really black and soiled. Probably what you call "fouled". I replaced the ignition coil for the two plus that were worst.
    • The bike seems to be 100% stock engine wise.
    • The problem went a lot better after my counter measures, but is still there. For ex the problems seems to go away quicker after I have revved it. Like after 10 mins or so.
    • Same issue no matter what fuel.
    • I have to have the idle mixture screw (pilot screw) set at like 3½ turns for it to idle clean and respond proper on the throttle from idle. The spec's say 1,75 turns.

    Grateful for all input on this matter!

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  2. Welcome [MENTION=37161]Tobbera[/MENTION], your post got auto moderated because of your number of posts. I have marked it approved now. Once you get a few more posts you won't have this problem.

    Regarding your bike issue I don't have anything to offer there I am afraid :( Perhaps now that you gave replaced the spark plugs pull them out again and check their colour, it might give you a clue.

    Others may have some idea. :)
  3. Did you do a balance when you cleaned the carbies? These small 4s are extra sensitive to that
  4. No I did not sync or balance them. I left theese settings alone when I cleaned the carbs. The puzzling thing is that it runs fine then after a hard rev, it stumbles on low rpm's. Something is happening when I rev it.
  5. Clean / replace air filter.
    check chokes are returning to the completely open position when the choke is off.
    check ignition timing
    tune and balance carbs

    Left field.....are you using the correct spark plug ?...not one colder than recommended... ( usually the number is the heat range ie. b9es is colder than b8es )

    Your description suggests it is running rich ( black and sooty plugs )
  6. You need to do the balance to eliminate it. They often wonder even if you don't touch them.
  7. I don't know if this is relevant but I was having a very similar problem in my car. There was a leak somewhere and the leads to one spark plugs was filling with oil and increasing the resistance.
  8. I'm using the corrrect spark plug accorning to spec. The air filter was cleaned about 4000 km ago. I have not checked the timing, something to look into. Balancing carbs requires special tool that I dont have, thats why I'm little reluctant to start digging into that part. This would be alot esier if it ran bad all the time. But this just happends sometimes when I have revved it...
  9. a lot can happen in 4000kms, especially so if the bike has fallen over. Even a stationary lay down.

    Reason : most crankcase breather hoses run to the air box. When a bike falls over some of the oil from the crankcase ends up in the airbox where it contaminates the filter which makes the bike run rich, similar to partial choke on all the time.

    Carb tune and balance while requiring specialist tools and maybe specialist knowledge, will be a worthwhile thing to do. Maybe some of the Sydney riders can point you in the direction of a reputable ( non stealer ) workshop.

    Again left field....check the HT leads for wear or damage. run the engine at night looking for 'leakage'...ie sparking to ground ( lead to engine case or frame member ).
  10. Hmm.... An inline 4 cylinder 250?

    I expect you don't have any power below 5K RPM, especially so if your reline is 15K :)

    Set you idle screw back to standard, it sounds like it may be very rich.

    Have you ridden many other bikes & is this a problem that comes & goes?
  11. Pilot screw is 'air bleed' so screwing it 'in' makes it richer.

    3 1/2 turns out as opposed to 1 1/2 - 2 turns ( standard ) is compensating for a problem rather than correcting the cause of the problem.....
    ( making te assumption here that when you disassembled / re assembled the carbs you didn't alter the needle position ( circlip ) or incorrectly set the float height )

    which adds weight to the theory that the intake is partially blocked

  12. Really? I've got it in my head that it's the opposite... Disregard if so!
  13. On my bike I dont think the crank case breather goes to the airbox, but rather out in the air under the bike. Its not inpossible that the air filter needs another clean though.

    I'm more into the ingnition line. I replaced one of the coils becouse I thought that it maybe burnt out and got "tired" after a heavvy high rev. And as I said, the problem got better after my counter mesuares, wish point me in the ingnition directon.

    When the bike runs fine, there is plenty of power all the way from 1600 rpm. Not a race machine, but it pulls nice and smooth. However, when it gets into trouble mode, it just stumbles below 5000 rpm.

    If I set the idle screw back to standard 1,75 turn it wont idle nice and not respond to throttle when going off idle.

    I ridden some diffrent bikes. This probmlem comes and goes.

    Another obeservation in this troubleshooting is that throttle position is totaly inrellavant. It stubles no matter what under 5000 rpm.
  14. Mike, are you sure about the pilot screw? I was convinced that it make it richer the further out it goes on 4-stroke carbs, and the opposite on 2-stroke. The screw goes in under the bowl and defenetly feels like it goes richer the further out I turn it.
  15. It's not divided along those lines. You really need to know which you have on your bike. Keinan carbs do air bleed more than Mikunis.

    If it is the idle screw causing the problem, then it will come up as a problem when you crack the throttle at speed. A hesitation or flat spot.

    Also did you spray the fuel and air jets with carbie cleaner?
  16. Ok, if it's that far out of whack, sounds like it's making up for something else.

    Like others have said, start with float height, needle clip height & balance, go from there.

    Makes sense, I've only worked on Mikunis.
  17. I'm almost absolutly sure that its a fuel screw. If I go back to 1,75 turns it hesitates when turning the throttle from idle. Its hardly drivable when cold, need full choke to keep running. The RPM falls slowly down to idle rpm when closing throttle when warm. At 3,5 turns it responds nice on throttle, as fast as I can twist it from idle. If runs ok when cold. And has a resonable stable idle without floating around too much.

    I'm hesitating to change jets and needle since it runs absolutly fine when it runs as it should. Its just after a heavy long rev it starts to act up.

    I blowed throu the jest when cleaning the carbs.

    BTW, I'm gonna try to attend to the Monthly Putty Run this Sunday if anyone want to test ride it.
  18. I thought roughly it depends on which side of the slide the screw is. If it's on the airbox side, its an air/fuel screw, and if its on the other side its a fuel/air screw. In the former, the screw adjusts the air introduced (screw out = leaner), and in the latter the screw adjusts the fuel (screw out = richer).
    At least thats my understanding ](*,)

    I also read it functions differently for 2 & 4 strokes, with 4 strokes its to supplement fuel in the pilot circuit as they move less air, and for 2 strokes its to regulate and bleed off the amount of air required for the pilot circuit setting. Makes sense but I can't confirm it.
  19. Mine's on the airbox side (Mikuni's) & I'm pretty sure it's screw out for richer. My old CX500 was the same, can't remember on the ZZR...