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91 Supersport Woes.

Discussion in 'Supersport (1974-2007)' started by Phartycr0c, Jan 12, 2020.

  1. Right folks if you could bear with me on this one a little,

    I have a 91 SS that ive owned and maintained for 25 years which has become a little bit of a garage queen.

    A number of years ago she developed an oil leak which was ultimately down to a hairline crack in the rear cylinder head,(replaced).

    Add insult to injury, while being diagnosed, the tank rusted bottom left corner, repaired including
    new fuel filter and pump.

    On to my point, I dearly want to get her running as she was pre rebuild. She was strong, lively and sounded like a symphony orchestra.

    Now however on starting, the horizontal cylinder fails to run correctly, eventually chiming in but coughing and popping like a bastard. I'm surprised the Police firearms unit hasn't been out.

    Starting from the top.
    Heads are all shimmed correctly and within tolerance.
    New valve stem oil seals all round. Heads were refurbed by Geoff at Baines Racing
    New belts.
    Ignitech ignition unit on standard settings.
    New coils and plugs, gapped correctly. Both plugs produce a lovely fat blue spark.
    New pick up coils set to default values.
    Kn filter open box cleaned and oiled.

    Now what I think may be the likely problem.
    Standard mikuni carbs with dynojet stage 2. 144 main jet with needle set 2 from top.
    dynojet springs.
    Straight through exhaust system.
    I have rebuilt the carbs with refurb kit, all seals and o rings, new diaphrams, emulsion tubes and fuel valve.
    float height is at 14mm

    Fuel screw settings are 3 1/4 turns front cylinder and 3 1/2 rear

    I must point out that each component has been dealt with individually in an attempt to isolate the issue. The symptoms have remained unchanged with the bike running like a pig, popping and farting and refusing to smooth out.

    I know a good blast around on the road is required to fully diagnose fueling problems but she is not getting to the point where I feel she won't leave me stranded at the roadside.

    I have trawled forums etc to identify a solution and while there are a number of discussions around this problem, none of the solutions appear to work with my hangar queen.

    Having droned on now, I'm currently inclined to remove the carbs and yet again give them a good blast through with cleaner or ultrasonic clean them.

    There is a lot there I know but i'm not prepared to give up on her, I just want to ride her again ( and the supersport.)
  2. Sounds as though it could be fuel, have you tried simple stuff like taking the mixture screw out to 5 turns on the horizontal cylinder?, failing that I would agree with your plan to have the carbs ultrasonically cleaned then double check everything on the rebuild, if the issue persists at least you’ll know it’s less likely to be the carbs.
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  3. Fuel lines and filters cleaned and clear ?
  4. What are the plug colours? If you can get it running long enough? You mention about the heads being refurbished. Did you check timing after this work? Or are the heads within the exact spec as before? If you say its misifire then two issues to start with either spark or fuel. Work on one first. Would be my plan. Hope you get it going, keep at it. Vinnychoff
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  5. Intake vacuum leak?
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  6. Tried awapping coils over, is it then the other cyl?
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  7. im thinking this sounds terminal.. best thing to do is part ex it on a 620 monster or a 800 scrambler icon...
    seriously though, have you tried the simple things that others have suggested? my money would be on a fouled plug or water in the tank, I say this as an owner of a few ss's and those really being the only probs ive encountered. apart from the reg/rect thing which seems to be all to common on the "older" bikes...
  8. My money would be on an intake air leak of some sort. If and when you can get it running, try spraying a little carb cleaner around the joints between the inlet rubbers and the carbs, and the junction between the manifold and the head, also suspect if there is a drilling for a Scottoiler or similar in the intake manifold. Maybe also worth looking at where the throttle butterfly spindles emerge from the carb bodies. If anything about the engine speed or smoothness changes when you spray it, then you have found your leak. If not, then look elsewhere. It looks like you have changed or fiddled with virtually everything else!

    Other stuff would be swapping ignition boxes and coils over for each other to see if the problem shifts between the cylinders. Hey, the bright side is that all this entertainment is free!
    • Agree Agree x 1
  9. I would set the floats at 12mm to start with. Plug colours these days can be a bit misleading because it is unleaded fuel we have to use. Other than that the suggestions by all above make a lot of sense.
  10. My money in on blocked or partially blocked pilot jets which have gummed up with fuel drying up in the carbs while the bike sat.
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  11. Yep all is well in the fuel line camp. Good strong flow from the pump
  12. As a belt abd braces, i think im going to change the intake rubbers just to eliminate that as a cause.
  13. Plugs are brand spanker NGK dpr8ea8 and the reg rec was changed, upgraded years ago.
  14. Thanks to all for your replies and suggestions.

    Chris, As far as electronics go I have swapped coils, leads are brand new, swapped the ignitech out for the standard cracker boxes, (which I must admit I thought were the original cause of the symptoms). The bike is running standard pistons as it was prior to the head problems.

    To all intents and purposes the ignition system is brand new including battery, although I am pretty shit when it comes to setting up the Ignitech which is on its plug and play setup. Although swapping this with the original cracker boxes makes no difference to the symptoms so while it is default as it were I am ruling this out as the primary cause of my woes.
    In terms of plugs. The horizontal is usually damp with fuel on start, the rear appears sooty but dry, consistent with sitting running and not getting up onto main jetting as would happen on a run out.

    I am inclined to aim for the carbs again and give them a bloody good cleanout. Shame I cant afford a set of FCR's.
    • Thanks Thanks x 1
  15. I am starting to think this too. despite my cleaning, Iv'e missed something!
  16. Before you replace them, you can eliminate the possibility by wrapping them in tape first. You'll know whether the leak is in the rubbers or not.
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  17. It's always the front cylinder that has a wet plug, it seems.

    The whole pilot circuit on these BDST38 carbs has very tiny fuel ways......it is not easy to clean them right out with just one attempt.

    Even sonic cleaning can dislodge crap which won't come out.

    I favour Wynns carb cleaner.

    If you can get enough into the pilot circuit and keep it in there, or just keep doing it, it will eventually dissolve the crap.

    Word of warning.....be careful if you try to remove the pilot circuit gallery plug. They can be an absolute bugger to get back in and reseal, apart from the fact that getting them out can damage the carb body.

    Personally I wouldn't try and would persevere with Wynns cleaner and compressed air.
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  18. Just swap it for an 1100 engine with fuel injection! :p:joy:
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  19. following on from my conversation, I went back to the bike to turn it over this morning full battery and she wont start not even a hint.

    1. initially tried starting without choke. Plugs pulled, damp but not wet checked against body, huge fat spark from both plugs.
    2. plugs replaced and attempt to start with choke. no joy. Plugs wetter with fuel.
    3. plugs dried replaced and bike cranked. no joy until an application of easy start which prompted her into life, ran for a short time and then died.

    So now I'm definately erring towards a fuelling problem so much so that I have pulled the carbs and stripped them. Wynns has been liberally applied to all orifices, left and re applied. 2 1/2 hrs worth of application.
    I did notice that one of the slide guides is standing ever so slightly proud of the carb body so iv'e popped it out and noticed the O ring was stretched. I replaced it with another o ring for it to promptly crack when a ham fisted me went for another quarter turn to tighten the emulsion tube.
    I normally do this stuff by feel priding myself on deft touch but on this occasion its cost me £50 as i thought I may as well replace both slides while i'm at it.

    while the carbs are apart, I will keep applying the wynns at infrequent intervals until they go back in the bike.

    I love this, I now stink of fuel and Wynns, Oh and iv'e discovered and slight oil leak!
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  20. Are you using the Wynns carb cleaner with the white tube attached, so you can blast it into the holes?

    PS...mind your eyes....it stings like hell.
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