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Pilot screw adjustment with CV carbs

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    #1

    Pilot screw adjustment with CV carbs

    Hi all, I read in the past that you set the pilot screws for the highest idle possible. Well, I turned my bikes idle down to 900 and gave it a shot and found that 2.5 turns is right in the middle. Anywhere between 2 and 3 gave no change, anywhere else and the bike would begin to die.

    Sooo, do I set it closer to closed or closer to open. Or, do I check the plugs once in a while and adjust? That would mean opening the screws out some if I have black plugs and closing them up if I have white?

    Right now my plugs are whitish but I think it's because of my fuel filter which I can see is not full when the bike is running.

    Thanks in advance, Steve
    Tags: None
    #2
    From your description, I would set them right at 2.5 turns.
    Why do you want to adjust them? Did you do something to effect mixture? The screws don't move themselves, so if your plugs are running whitish it's because of something else. Don't cause more problems by trying to compensate for the real problem.
    Were the plug colors correct recently? Is so, what has happened between then and now? Your carbs could be out of sinc' or need cleaning. If your air filter(s) is supposed to be oiled, have you oiled it lately? Are the clamps tight on the filter to carb boots and also the manifolds? Ignition timing good?
    And on the seventh day,after resting from all that he had done,God went for a ride on his GS!
    Upon seeing that it was good, he went out again on his ZX14! But just a little bit faster!

    Comment

      #3
      I did a bunch of things Keith. I'm trying to get more power out of my bike so I stripped the carbs and changed the float level from 23.4 to 22.4 for starters, then I changed the carb boot o-rings, adjusted one tight exhaust valve, and added a fuel filter. Before doing all this I had perfect coloured plugs but wasn't happy with my idle or top speed. The fuel filter is gone as of tomorrow though, it is messing things up on me and probably creating the white plugs since they both changed.

      The idle was a problem as the bike would start with choke but once it started to warm up it would choke itself to death and wouldn't run without the idle either. The idle is now perfect, 900 is achievable when the engine is hot. I'll have to see what it can to cold in the morning. I think about 1000 as I haven't synchronized the carbs yet. 900 is a little rough when cold and not reliable right now. After my work the idle speed would climb with the temp of the engine and I also noticed it was warmer than usual, that's how I found the white plugs.

      Top speed is below what I'd like too, by about 25 mph or 3000 RPM in top gear. I was hoping a bit higher fuel level in the carbs would make it easier for it to get sucked into the engine. I had been getting bad mileage before the work too, I don't know how it is now since I lose so much when removing the fuel line to work on the carbs.

      I'll set them back to 2.5 then, it's been great there but I've always wondered what would happen if I changed the number of turns.

      Thanks buddy,
      Steve

      Comment

        #4
        If your bike is in good shape, I've never seen the need to install a fuel filter. But the fuel filter should not be causing your plugs to run white. I can understand you're thinking "clogged filter=lean mixture" but I doubt it. The fuel starvation symptoms would be obvious. I can tell the difference between fuel starvation and running lean.
        If your idle is climbing with the heat of the motor, then you still have an intake leak somewhere. Can you describe how you adjusted your floats? Maybe even a pic of your floats? It sounds like you may have done something wrong.
        And on the seventh day,after resting from all that he had done,God went for a ride on his GS!
        Upon seeing that it was good, he went out again on his ZX14! But just a little bit faster!

        Comment

          #5
          I set my calipers to 22.4 and then slid them across the lower lip of the carb. Once the calipers stopped hitting any part of the float I switched to the other side to see if the float was bent and fixed one bent one too. The needle spring wasn't compressed so I think I did it right.

          Steve

          Comment

            #6
            If your fuel filter wasn't full, it's not the fuel filter, it's the petcock, or hose. Not being full means that it's able to drain faster then it can pull fuel in. It also may just be a trapped air bubble, and not an issue at all.

            Comment

              #7
              Luke, I got rid of the fuel filter and put on a clear fuel line and I get the same result. I don't think it was a problem after all. With the tank off the bike I can set the petcock to prime and it dumps fuel everywhere. When it's connected to the carbs and I set it to prime nothing happens inside of the line. The needles and seats don't even let the fuel push the air out of the line. It's kind of weird but I can live with that. Once I start the bike you can see fuel and bubbles in the line.

              I will re-check for intake leaks but I hope I don't find any because I just sync'd the carbs 10 minutes ago.

              Steve

              Comment

                #8
                Steve almost an fuel filter will cause the lean problem you have experienced, as almost all are designed to work with a fuel pump which carbed bikes dont have, there are fuel filters that will work with a gravity fed system as on a bike, but I have no idea where you will find them.
                Dink

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                  #9
                  Fuel filters that are used on Briggs and Stratton riding lawnmowers will work fine on a GS. The Briggs engines utilize a pulse diaphram fuel pump that only produces 1 lb of pressure, so restriction is minimal from the briggs fuel filter.

                  Earl


                  Originally posted by Dink
                  Steve almost an fuel filter will cause the lean problem you have experienced, as almost all are designed to work with a fuel pump which carbed bikes dont have, there are fuel filters that will work with a gravity fed system as on a bike, but I have no idea where you will find them.
                  Dink
                  Komorebi-The light filtering through the trees.

                  I would rather sit on a pumpkin and have it all to myself than be crowded on a velvet cushion. H.D.T.

                  Comment

                    #10
                    I actually stole the fuel filter off of my 1100 It's just a straight through thingy with 5/16" spiggots and in the middle a 1" around piece of flat window screen for the filter.

                    Steve

                    Comment

                      #11
                      Steve, after your last post I'm thinking it is fuel starvation now.
                      I've seen some lean running bikes but never one that lost an estimated 25 mph off its top end. Only a lack of fuel could do that. I would think you would have other symptoms to describe. From your petcock/prime test it sounds like your gas tank is venting. You worked on the carbs. I assume the carbs/jets are clean inside. Floats set. Mixture screws are set. Carbs synched. Are you sure the floatbowl vents are clear? Can you blow into them without too much resistance? Could you have accidently semi-plugged them during your cleaning? Could you have semi-plugged any of the fuel passages with a piece of q-tip or something during cleaning? How about the carb diaphragms? Could they be seated wrong and leaking? Did you replace the vacuum line? If you changed the fuel line, what size did you buy?
                      Just trying to think of anything that would cause fuel to flow too slow.
                      And on the seventh day,after resting from all that he had done,God went for a ride on his GS!
                      Upon seeing that it was good, he went out again on his ZX14! But just a little bit faster!

                      Comment

                        #12
                        I'm going to burn a tank or two tomorrow and see what happens. I think everything is under control now. Nothing was dirty in the carbs, in the end all I did was clean the outsides despite having them completely apart. I left the vent tubes off just to see what would happen too. One of my diaphragms had fuel on top of it when I took it apart...I found that strange but put it back in after inspection.

                        Steve

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                          #13
                          Description how to adjust pilot screws http://www.mikuni.com/pdf/vmmanual.pdf Page 14 and 15

                          Comment

                            #14
                            Karl: I read through the PDF and this did not work for me. I found the screw's range of highest RPM and chose a position less than the midpoint yesterday and I had some popping on the overrun. Mostly on really hard engine braking but it was quite loud.

                            Steve

                            Comment

                              #15
                              Okay, I put 250 km + on the bike today and I got 6.7 litres / 100 km for mileage. That's 35 MPG? The problem here is that my speedo turned out to be way off so it's hard to know my mileage.

                              I've always thought my bike was slow but it turns out I was doing 160km/h with an indicated 120 or less! Soo, if I fudge the numbers the mileage goes up to 47.5 MPG which is pretty good considering we were flying today. :twisted: :twisted: :twisted: I had no trouble chasing down a 2003 and 2004 SV650 but I was no match for the SV1000. At one point an SV650 gave me the right away because I was all over him Average speed was about 120-140 km/h hitting 160 whenever possible with lots of hills and twisties so I never used 6th gear.

                              Idle did not change at all during the day.
                              Now I have to check my plugs and see what they have to say.

                              Steve

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