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Help GJ74A GSF250 bogging down at high speed?

Discussion in 'Suzuki 250cc In-line 4's' started by geemannz, Oct 25, 2020.

  1. geemannz

    geemannz Member

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    Location:
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    My Bike:
    Suzuki GSF250 Bandit
    So for a bit of backstory my motorcycle was misbehaving recently, only running on 3 cylinders, no power, backfiring like a Parkinson's patient with a semi-auto handgun, and just generally just not running right. This weekend I decided to sort this with new plugs and a good ole carb clean. So I took apart and cleaned the carbs with a few decent sprits of carb cleaner, put the new plugs in (NGK CR7E) and put the thing back together. It improved in that it was running on 4 cylinders (WOO) but would struggle to go faster than about 80 km/h and refused to break 100.

    Got it back into my garage and noticed that the carb slides weren't moving at all. This was due to my ham-fisted reassembly of the carbs resulting in the springs not going back into 3 of the carbs correctly. Whoops.

    So I rectified this mistake, reassembled the carbs properly, and made sure that the slides moved freely. Took it for another test ride and I appeared to have more power, but it still struggles to get any faster than about 100km/h and still seems light on power. Feels like riding a slightly more powerful version of my old GN250. Also sounds like the supercharger from Mighty Car Mod's Nissan March super turbo when I flick my wrist to full throttle (HONK)

    Any suggestions on why this might be happening? I'm struggling to figure it out and its doing my head in...
     
    geemannz, Oct 25, 2020
    #1
  2. gj74a

    gj74a Active Member

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    My Bike:
    1990 suzuki bandit 250 GJ74A
    Check your spark plug wire to boot connection. The boot will unscrew from the wire. If the connection is loose, cut 5 to 10mm off the end and screw the boot back on.
    Over the years the connection becomes loose and the screw terminal inside the boot does not make a good connection with the wire.

    A quick adjustment for the carb is to set the pilot screw 1.5 turns out from seated. Any more than that and i find it too rich at light throttle and idle.
    Make sure the slide diaphragms are not torn and are seated properly in the groove.

    I also recommend the Litetek carby kit. Have a look at the website mikuni BDST rebuild to see how to rebuild them properly. My gsf250 was running really rich all the time. I replaced all the o-rings from the kit and it made a huge difference to the way it ran.
    My main issue was the slide o-rings had in all 4 carbs had fallen apart and were letting fuel past them resulting in a very rich mixture.

    REgards
    Michael
     
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    gj74a, Oct 27, 2020
    #2
  3. Linkin

    Linkin The Mechanic Premium Member Contributing Member Dirty Wheel Club

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    I suggest checking fuel flow. I had intermittent dramas with my bandit, carbs were spotless, inline fuel filter installed, but I neglected to replace the fuel tap or check the strainer for blockages.
     
    Linkin, Oct 27, 2020
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