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Missfire under load - 900SS

Discussion in 'Supersport (1974-2007)' started by Quartermaster, Mar 11, 2012.

  1. Help,

    I fired up my '97 900SS this moring, the weather being half decent. I warmed it up as usual, a couple of minutes juggling throttle and choke and once the oil temp gauge had started to climb I set off for a spin. Trickling along was a bit jerkier than normal and under moderate load going up hill it was clearly missing periodically on one cylinder (the rear one I think). Above 5000 it seemed to clear itself.

    Any pointers? I thought I'd replace the plugs first being the cheapest option, but if that doesn't work what next?

    Your views / experiences greatly appreciated.

    QM
     
  2. Loose ht/coil connection, dodgy plug breaking down under load etc....
     
  3. Has it been standing unused for some time and in possibly damp conditions QM? Could be so many things as usual and the intermittent ones are always the hardest to find of course, but my 400SS tends to do the same particularly through the winter. Mine is due to a small amount of water getting into the float bowls. You only need a droplet of water to play havoc with carburretor jets and the classic symptoms are poor running at low down revs/won't idle even when warmed up and the partially wet plug as a result on the same cylinder only helps to compound the problem.


    Cleaning or replacing plugs is a good place to start regardless of above. I always try running mine in the dark and looking at the plug caps and leads in general if this is suspected(again, more likely in damp conditions). As said above, old plug leads, carbon or copper can break down with age and this, topped off with an old resistor plug cap (particularly if you are already running resistor-type plugs!) can sometimes result in sparks jumping to the outside of the cylinder head.
     
  4. Thanks SRAD will check connections plus Chris' suggestions.
     
  5. Stale fuel ?
     
  6. Thanks Chris.

    It had stood for a long time (mid November) in a dry garage attached to the house so reasonably warm. I started it last month for the first time since November and it wouldn't fire on the rear cylinder, but I swapped for a spare (old) plug and it ran fine. Rode about 20 miles or so and put it back to bed until today. It does have the symptoms you describe for water in the float bowls but I really don't want to dismantle the carbs so wonder if filling the tank with fresh fuel might help. I understand that a lot of petrol has 5% ethanol in it now which is hygroscopic so would attract water.

    Plan is.

    Run it in the dark and observe any untoward sparks

    Fit new plugs

    Check connections esp coil

    Put some fresh fuel in

    Taunt it using witty sarcasm.
     
  7. love the last one and would be great if it works. Re: draining float bowls - there is a provision for doing exactly this in the form of a drain screw (M8 or M7 hex head, be careful not to overtighten when doing back up) on each bowl and even a drain tube from standard although this is often missing now - the retaining clip didn't work that well. It's worth catching the contents of each bowl if you can just to confirm (or not) if water was present - you can't miss it as it will lie lurking at the lowest point of the fluid.
     
  8. Mine ran out of sparks one day a few years ago and it was a corroded connector in the kill switch circuit. Not sure which one exactly as I'm gettin old and can't remember, but it was a block connector at the front end, so I had the fairing, headlight and clocks off and gave every connection a good clean and check while I was in there. Not been a problem since. Mine is nearly 18 years old, so it was probably well overdue a full loom check.
     
  9. Thanks for all your ideas guys. The problem is now sorted and I've had a really nice ride this afternoon to prove the point. It turned out to be the rear cyl spark plug which was number one on the list of things to have a look at. Nice cheap zero hassle solution!
     
  10. pleased to hear that Q.M.
     
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