Engine running Rough and loading when hot

Discussion in 'Engine, Fuel and Exhaust' started by Bigkahuna69, Feb 25, 2009.

  1. Bigkahuna69

    Bigkahuna69 New Member

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    (2004 Sportster) Bike runs fine but after running for 15 minutes or so the minute you let off the gas the bike loads up back fires through the carb as if getting to much fuel, sounds like a fuel problem but?. Have tried everything short of replacing the carb could it be electrical? or any ideas? Everything in the carb is new short of replacing the whole carb!
     
  2. glider

    glider Veteran Member

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    Lot's of things to check before going for a carb. Is the carb clean inside and the needle and seat area, how about plug wires or an intake leak?
    Stock push rods or adjustable?
    What else is done to the bike?
     
  3. Bigkahuna69

    Bigkahuna69 New Member

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    Everything has been replaced with new car dipped and cleaned rejetted up and down, new plug wires, intake seals, honestly everything as for pushrods its and 04 I have no idea.
     
  4. fin_676

    fin_676 Well-Known Member Staff Member Moderator Contributor

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    i get a somewhat similar problem here with the 2 big twins at this time of year
    runs nice on the road when you get to a town and have to slow down by closing the throttle when i open throttle slightly get a lot of back firing
    if i have to slow to a stop the bike cuts out
    we seem to have a lot of moisture in the air at this time of year when the throttle is closed the air in the throat of the carb gets compressed this causes the droplets of water in the air to freeze blocking the pilot and slow jets in the carb sit at side of road for 5 mins and heat from motor melts and then happy for the rest of the day
    never had the same problem with the sportster (drawing less air through carb)

    just something to consider

    Brian
     
  5. M' one.

    M' one. Active Member

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    Carb icing, pretty common phenomena in the venturi of most carbs. Normally unusual in bikes as there should be enough heat from the engine to prevent it. Interesting that you got it.
     
  6. fin_676

    fin_676 Well-Known Member Staff Member Moderator Contributor

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    only seems to happen in feb and mar but then a lot of moisture in the air at that time of year took a while to figure out what was going on once i understood i made allowances seems to be normally in the first 15 miles of riding but has caused a problem for over 25 miles but this was with a stock setup which has a lot of restriction in the air flow getting into the air cleaner
    very common occurance on early 80s kawas (thinks fitted with kehin carbs)
    was driving a landrover on a9 near aviemore once and cut out opend bonnet and carb was a block of ice had been driving over 200 miles before it happend but its all down to moisture content in the air

    Brian
     
  7. M' one.

    M' one. Active Member

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    Carb ice formation depends a lot on the ambient conditions. In very cold air its never a problem as cold air tends to hold less moisture, warmer ambient conditions are usually problematic ( usually in the range of about 4C to 15C, it occurs outside this range but is normally at its worst within it). Cars with carbs usually have a vacuum controlled mechanism that sense the pressure difference in the venturi when the ice forms and then opens a flap/valve in the air intake system allowing hot air from around the engine to be directed through the carb, melting the ice. Its also most likely to form at certain throttle settings (usually about 3/4 open is worst). Bikes do'nt have the room for additional ducting to direct warmer air from around the engine through the carbs but carb icing happens to every carb as they all use venturi's. Different manufacturers used different designs and placed the carbs in different positions to try and reduce the icing, most of them worked. The only way to reduce your icing is to try and deflect some of the hot air from around your engine into the carb but this reduces the charge density and thus the performance of the engine. Brian, I guess you know this all already,but humour me .... I'm really bored at the minute, sitting around waiting for spares to arrive:D:D
     
  8. porky

    porky Member

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    Interesting. I seem to get these symptoms, problems with rain and early mornings. Open the throttle up and problem goes. Come to ride home in afternoon , no problem. Was about to strip carb down to check pilot/ idle. Be interested to see what i find keeping in mind your explaination. Cheers.
     
  9. fin_676

    fin_676 Well-Known Member Staff Member Moderator Contributor

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    Well i stripped the carbs and checked everything trying to find the cause of the problem checked over the whole loom and even tried a differnt coil and cv carb then went into serious thought mode to come up with carb icing as being my running problem early in the year
    had both the big twins out for a shakedown run today dyna was difficult with a lot of backfiring and felt like a real pig unless on full throttle after a 5 min break at 10 miles was just great
    then took the softail out it was great from the start no problems at all think this was down to the reserch i did last year and found that the best answer to this problem was a mikuni hsr 42 didnt get it fitted till end april last year but now comparing between the dyna and the softail the softail is a whole lot better different design of carb

    temp today about 13 c but late afternon took dog for a walk over the top of the hill down to the top of the cliff on looking along the coast could see what looked like low cloud or high mist at about 50 to 200 ft so lot of moisture in the air
    grumpy has tomorrow off and wants to ride her dyna think she may demand a mikuni carb after a wee while

    Brian