Monster 900 Starts But Dies On The Throttle

Discussion in 'Monster' started by Pablo Pirate, Dec 7, 2017.

  1. Howdo!

    I've got a 1999 Monster 900 with 39mm FCR's and Exactfit coils, it has been stored for the past 5 months since Ive last used it. Bike starts fine however when you blip the throttle it occasionally back firings which can be seen in the carbs and then it dies.

    Below are what I've done so far.
    1. Checked the fuel and it did show signs of water, so drained the fuel and replaced with fresh.
    2. New fuel filter.
    3. Plugs looked rich but not totally drenched in fuel, new plugs fitted anyway.
    3. Checked the vacuum hoses from the petcock to the carb intake & to the vacuum pump.
    4. Changed all the seals in the vacuum pump.
    5. Fuel hoses checked for kinks, fuel seems to getting to the carbs (Although I thought this was the initial problem).
    6. Twisting the throttle the accelerator pumps ok.
    7. Removed carbs and checked Main jets and pilot jets clear.
    8. Float heights 9mm, no signs of damaged or obstruction and move freely.
    9. Floats changed 10 months ago (Allen's had serviced the carbs, however bike had been used in some seriously grotty weather).
    10. New petcock fitted.

    To do........
    1. Change the coils back to standard and see if theres an issue with coils breaking down.
    2. Compression test.
    3. Ordered some SEA Foam see if that does owt??
    4. Sacrifice chicken!

    Any advice most welcome.

    Cheers Pablo
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  2. The water thing seems too much of a coincidence.
    Don’t forget that fuel floats on top of water, so anything left in the tank or pipes is likely to have lots of water in it.
    Also, the pilot circuit is in operation at idle and when you blip the throttle, the main jets start to come into play, so water or even debris in the fuel can get sucked into the main jets and cause the engine to die.
    Mikunis suffer from icing in a way that FCRs may not.
    My experience of carb icing though is that the motor will idle but that’s about all.
    Imho, it’s much more likely to be a fuel or carb than electrical issue
  3. agree with your final point, but given the OP has taken the carbs off and checked the jets, then my gut-feel is that a very thorough fuel system strip/empty/clean has happened, then what more can be done on the fuel side of things? maybe I am just assuming that a full fuel system strip and clean has happened based on the OP.

    my thoughts went straight to fuelling and the jet being cacked up a bit - but that's as far as my lay-person mechanics go!

  4. I've emptied the tank completely. No signs of water in the fuel filter.
    Hoses flushed through and carb removed and cleaned. Jets removed and checked clear.
  5. Have you had any headwork done or are you running high compression piston’s? Depending on whether this is the case and by how much higher, Initial thoughts on your symptoms are that timing needs to be retarded, approx 2° typically ought to do it.
    If it was running fine last time with no changes, obviously ignore all the above.
  6. Engine is bog standard. Not adjusted the timing. 600 flywheel to go some stage.
  7. Iirc, FCRs have no choke.
    Could it be that the carbs don’t like cold weather/air and everything needs to warm up properly before you can crack the throttle open?
  8. could well be a variation on above ^ if you are running without air filters then might be worth partially blanking off inlet(s) to see if it will run for longer - be careful that hands are out the way though.
  9. Very strange. I recently put together a 900ss with 41mm FCRs, a 600 flywheel, 944 conversion etc. When I first fired it up it ran exactly like your description. It would idle, albeit slightly roughly, but if I tried to open the throttle it back fired through the carbs and stopped. I too was sure it was a carb problem, especially since it had never been run with the FCRs before.
    It turned out to be the ignition pickups that had the wires swapped, changing the phase and advancing the igntion. The ignition module triggers off the negative going edge of the pulse, so reversing the phase means that it now triggers off the leading edge of the pulse instead of the trailing edge, advancing the ignition, apparently by around 8º. No wonder the engine wouldn't take any throttle. At idle it was already running too much advance and increasing the revs advanced the ignition so much that it wouldn't run at all. Whoever extended the pickup wires must have swapped round the colours but I couldn't see this beneath all the heatshrink that had been added.
    However, since your bike hasn't been touched that rules out the pickups, or at least the wiring to them.
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  10. Bike is in garage and engine fully warmed up. I am edging towards coils/pick ups or something electrical, will have another play tonight.

    Thanks for all the input, keep the ideas coming!
  11. Sounds uncannily similar...
  12. If it’s excess advance causing the problem, then presumably any increase in engine speed will make it worse, no matter how gently you open the throttle.
    If the engine will accelerate with gentle throttle inputs, maybe that would indicate a carb or fuel problem then?
  13. And as if by Magic!!! She lives!!!
    Well I striped down the FCR's and checked the mainjets (155) and pilot jet (55) free from obstruction. Checked float height (9mm). All seemed good. Used some STP carb cleaner and gave it a blast of air. Stuck them back on and she starts and revs a goodun. Just need to test ride and make sure my success is not short lived.

    Goes to show the smallest piece of dirt/debris can bring the bike to a halt.

    Thanks again for all the input :upyeah:

    Just need to sort the suspension, brakes and 200 rusted fasteners.
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  14. Just for info here's the diagram I used for fault finding the fuel system. MONSTER - Fuel System Layout.jpg
  15. Ha, Hilary would have diagnosed your problem immediately.
    Hilary is my mate’s wife and her stock response to any engine problem is “blocked jets”.
    This time she would have been right... ;)
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  16. Well that was short lived, bike starts and rev'ed like a gudun in the garage. One very short test ride and she spluttered to a halt. Back to the drawing board. I must admit I'd like to meet the previous owner and give him a slap, found some right bodges on the poor old bike.
  17. Did it idle ok when warm and what revs? How was the pickup from idle to mid-throttle when riding? Do the carbs have software that uses a TPS?
  18. I have never liked the float chamber arrangement on these, the horizontal cylinder carb has a really compromised bowl and you could never call it a downdraft. Unless everything is like new and working correctly, you can end up with over fuelling if the floats and needle valves play up even slightly. When it spluttered to a halt do you think it flooded/was there a strong smell of fuel?
  19. The carbs had been completely refurbished by Allens in January (£250!, needles, float valves, jets ultra sonic clean gaskets etc). I also removed and cleaned and checked, all seems good.
    Haven't had time to check plugs and carbs didn't smell flooded.

    1. Intention is to remove Exactfit coils and fit standard (Just incase they are breaking down).
    2. Check fuel isn't contaminated again?
    3. Remove all the crap (Petcock, fuel tap, vacuum pump) and just have a fuel hose see if that works, then add a bit at a time.
  20. Bike idles perfect. No TPS. As for riding.....didn't really get that far.