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Carburettor genius needed!

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  • Carburettor genius needed!

    Hey Team,
    Im in the process of getting an 81 CB250 ready for rego so i can at least ride something with 2 wheels. Anyway, its got a niggling issue that im having trouble fixing, which i assume is to do with the carbies.

    Bike starts and idles well on choke when cold and normally when hot, however;

    Has a fair bit of hesitation from a standing start and when any more than 1/4 throttle is applied when riding
    also when you pull the clutch in to come to a stop, the engine will almost stall and sometimes even does, with the revs slowly picking back up to idle rpm.(set at 12-1300 as per handbook)

    I have a keyster tuning kit that has 6 main jet sizes (99, 101, 105, 109, 113 and 118. 105 is standard) it also has 4 different needle sizes depending on what main jet you select. The kit also came with new float valves and these have also been replaced.

    I have the standard size jets in the carbs and the largest needle in the kit and this seems to have settled the issue a little. Air screws are set at 3 turns out, which is 1 more than standard.

    Spark plugs are standard and brand new and i have also fitted a brand new coil.

    Before fitting all the new parts to the carbs i stripped them down and cleaned them out with a lot of carby cleaner and as far as i can tell, the valve clearances are within spec. ( I havent as yet checked them as i dont have feeler gauges).

    Id like to get this issue sorted before i try and take the bike over the pits. For a 30 year old bike, its in suprisingly good nick!!

    Cheers in advance,

  • #2
    Sounds like the pilot jet could have some crap in it.


    • #3
      You mentioned you changed the float valve.

      Is the float height right?
      Every one has a story.....


      • #4
        Yeah. Float height is correct. I checked them then rockingham Honda checked them also.

        Is the pilot jet the one behind the main jet?


        • #5
          This sounds like a similar problem to what I'm having. I'll be keen to hear if you get it sorted.

          Does it need to be fixed to take over the pits?
          Go soothingly on the grease mud, as there lurks the skid demon! Press the brake foot as you roll around the corners, and save the collapse and tie up.


          • #6
            Emulsion tube blocked??
            ''It's not how many times we go down,but the amount of times we get back up''


            • #7
              .. and/or air leak leaning things out.

              While your at it check & clean the petrol tap assembly as it can accumulate crap; my old Honda 175 from 1972 had a brass mesh filter inside the tank as part of the tap.

              Is their an old fuel filter in the line that's stuffed? Are the fuel hoses in good nick (internally & externally)?
              .. and thats Racer # 193 to y'all; my fabulous sponsors (who all do good shit) are: Graeme Fleming IT Consultants, Vision Image, Pacific Safety Wear, Excess Power Equipment, Pro Photo Booth

              .. and according to Sean'o: 'get the Kwaka (never thought i would say that!)'


              • #8
                Are you still using the stock airbox or exposed pod filters? Bit of reading i've done show's they really need the back pressure an air box supplies to be smooth.


                • #9
                  Im running the stock airbox and the fuel flows fine into the carbs. I have a clear, inline filter anc can check flow. The only thing i can think of, that hasnt been done is stripping the carbs right back again, getting them ultrasonicly cleaned and going from there. That way i know ive got a good base to start off with. My confidence in using a spray can of carby cleaner to do the same task is lacking a bit. Anyone know of someone in the Rocky/ Secret Harbour area that could do that?


                  • #10
                    The last time I cleaned some carbs I just bought an ultrasonic jewellery cleaner off ebay large enough and did it myself.

                    It worked well for me.
                    In sterquiliniis invenitur.


                    • #11
                      Have you looked at the basics first valve clearance and comp test ??



                      • #12
                        Could be a worn camchain affecting the valve timing, pull the cam chain tensioner off and see if it is at its full extension.
                        I'm Living the dream. Really I am.


                        • #13
                          My bet is fuel starvation due to carbs being old/dirty/etc.

                          conversely, excessive air due to some sort of air leak could be a problem too, as smeghead implied.

                          check perhaps the carb mounting rubber, any breather hoses, etc for cracks. on a bike that old, i'd be surprised if there aren't any.
                          “Crashing is shit for you, shit for the bike, shit for the mechanics and shit for the set-up,” Checa told me a while back. “It’s a signal that you are heading in the wrong direction. You want to win but crashing is the opposite. It’s like being in France when you want to go to England and when you crash you go to Spain. That way you’ll never get to England!” -- Carlos Checa


                          • #14
                            Originally posted by Crobbo View Post
                            The last time I cleaned some carbs I just bought an ultrasonic jewellery cleaner off ebay large enough and did it myself.

                            It worked well for me.
                            How big is the cleaner, and how much did you spend on it?


                            • #15
                              It was some el cheapo Chinese pos off ebay. Cost about $70 plus shipping.

                              The tank was a bit tight but the carb was only small and in bits, id post up picks but I lent it to a mate and it got knocked off a bench.

                              Once the carb went in all this shit started appearing in the water so in knew it was working, it managed to take off most of the varnish in the bowl and brought the aluminium to a shine so I imagine it cleaned out the passages because it fixed the little stutter I had.

                              In sterquiliniis invenitur.