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Pilot Jet Issue?

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  • Pilot Jet Issue?

    This past spring I changed pipes (one aftermarket brand to another, so the carb had already been rejetted), but rejetted again per recommendations here. The bike runs great, but the problem is my gas mileage has been terrible and it seems to be running rich. So today, when I got home I tried and experiment. With the bike at operating temp and idling at 880 to 920 rpms, I adjusted the PMS, which was at 2.5 turns out. First I ran it out until I could tell a difference in the idle. Then I set it back at the original setting and ran it in. Here's the problem. I ran it all the way in and the idle never changed. I did some Googling and found a pretty good article about this issue. What it said is that if the PMS can be run all the way in and the engine doesn't stall, the pilot jet is too large.

    So here I am. My current jetting is a 172.5 main jet and a 40 pilot jet. My old jetting was a 175 main and a 35 pilot. No change in needle clip position. The difference between the two; with the old jetting I had a little hesitation when blipping the throttle, but gas mileage was between 38-43 mpg. With the new jetting, no throttle hesitation but terrible gas mileage. And running the PMS all the way in doesn't change anything. So would changing the pilot jet to a smaller size be a reasonable place to start in trying to figure this out? If so, what size? Go back to the 35, or try a 37.5? Or are there other issues at play?


  • #2
    Year miles and what air intake system?what all modifications?


    • #3
      2005 Silverado
      AIS and fuel pump removed 2.5 Suzuki needle valve (to run pumpless)
      Dynojet needle
      Max Air PMS
      Ported manifold in good shape
      Mikuni K&N filter with Arlen Ness Big Sucker cover (pics attached)

      Been running with this set-up since 2007.

      Two years ago swapped out the stock ignition coils with Dyna coils and wires.

      Recent exhaust change;
      Replaced HK Dual Sideburners with Cobra HP 2-into-1.

      After checking my records regarding my jetting, I did move the needle clip from the 3rd groove to the 4th with the recent rejetting.

      Attached Files


      • #4
        how are u measuring miles, and did u average 4 or 5 tanks?


        • #5
          Reset my trip meter with every fill-up, then when I have to switch to reserve, divide the mileage by 4 (because I've run through 4 gallons of gas) to get the mpg. That way I have an idea of how much farther I can go on the remaining gas. Then when I fill up, divide the total miles by the amount of gas it takes to fill up to get the mpg. Is there another way?

          And yes, I've run through at least 6-7 tanks of gas.

          And when I say terrible mileage, I'm averaging 27 mpg. Down from an average of 38-43 mpg, depending on the type of riding I did before rejetting and changing the exhaust. So it only makes sense that something in the current jetting has caused the change.


          • #6
            can't say much for validity of calculation but at least its consistent. what speeds you doing? ive found the only way for me to hit 39ish mpg is to sustain lower speeds below 70 mph.

            while i still have the stock exhaust, the afr guage reads on the rich side of things since it got hot outside, around 12.5-13 when on the needle. clip on the 4th position. ive got half a mind to pull the shim.

            im also tempted to do the front pulley if someone in the dallas ft worth area with a bike lift can help me out.


            • #7
              Its possible the choke plunger is not properly seating. I had that problem and it killed my MPG's. Not sure if you removed it when you were messing with the carb.


              • #8
                Choke cable along with the plunger were replaced. If it weren't seating, I would be smelling unburnt fuel, especially at idle, but I'm not.

                Shotround, educate me on how to calculate mpg. How do you do it?

                I've had the bike for 16 years and nothing has changed besides the jetting and different pipes to cause the drop in mpg.


                • #9
                  didnt mean to say your way was wrong or there was a better way. just saying it was consistent such to allow observation of trends.

                  that said i hope something gets figured out as i was thinking of getting the cobra sometime down the road.

                  forgot to add that i have similar observation with the 40 pilot also.


                  • #10
                    with a dynojet needle you can remove the shim under the clip and in some cases even leave the shim and try the 3rd groove on the needle. Leave the 40 pilot jet and don't worry about making a difference when turning it in. That difference will never be important because it will never be turned all the way in. If you change the pilot you will just need to turn it out to around 3 1/2 turns to achieve the same AFR at idle and will lose the benefit of the larger pilot on the off idle position of the pilot circuit.


                    • #11
                      i have 3 RS, reserve remaining fuel varies from bike to bike, . better to base it on actual mileage, and actual fill up gallons. also, pumpless bikes tend to call for reserve at even less gallons; it is hypothesized the pump is more efficient at that point. id do that B4 messing with it


                      • #12
                        Brianmac, I understand that each bike is different. But I only have one and since going pumpless in '07, I know without a doubt that when I switch to reserve, I've gone through 4 gallons of gas, +/- 1/10 of a gallon. So for the sake of quick and easy math on the fly, I divide the mileage on the trip meter by 4 and that lets me know how many more miles I can go before I better start looking for gas. Then yes, when I fill up I I divide the total miles by the number of gallons it took to fill up.

                        Davej, appreciate the input, but now I'm a little confused regarding the PMS. According to articles I printed and saved from the "old" RSC when I first joined and the Road Star Riders forum, the explanation on how to dial in the PMS is to run it out until the engine stumbles, then put it back to the starting point, then run it in until the engine stumbles. Then half way between the two points is where you want the PMS. And if it's turned completely in, shouldn't there be some effect on the engine? Thanks!


                        • #13
                          The original guys were wrong and were only adjusting the PMS and using the pilot jet according to jet kit instructions. I personally experimented with many different jetting configurations and I use a live AFR gauge to see where the AFR is in all ranges.. Trust me when I say don't worry about what the old time articles say about the pms. Use a 40 pilot jet.

                          The articles you have are assuming you are using a stock Mikuni pilot jet. With the 35 pilot that would be a good way to adjust it. Also with the 35 pilot you will have a lean spot and stumble just off idle. That is why research was done by me and the 35 pilot was replaced with a 40 pilot on a 1700 and a 37.5 on a 1600. It gets rid of all the bad issues that a 35 pilot does to the bike. If everything is bone stock a 35 pilot is usable but then again rejetting of any kind is not needed with a bone stock bike


                          • #14
                            Thanks for clearing that up. And that explains why, with my previous jetting using the 35 pilot jet, the bike had a hesitation right off idle.

                            I'll try putting the clip back in the 3rd groove on the needle and see what happens.

                            Thanks again!


                            • #15
                              I've made a couple of changes that seem to have made at least one immediate difference.
                              1) Dropped the needle by moving the clip back to the 3rd groove (from the top).
                              2) Ran the PMS in to about 3/4 to 1 turn out. I know davej said not to worry about the PMS, but I was reading through some of those old RSC articles and found something I'd either forgotten about or overlooked. It says that if you're using the Max Air PMS (which I am), you should "halve" the number of recommended turns, due to the different taper of the screw. For instance, if it says to go to 2 1/2 turns out, using the Max Air screw that should only be 1 1/4. So I started experimenting, turning it in and out while watching my rpms and the sweet spot seems to be at about 1 turn out.

                              The immediate difference I can tell is in around town cruising, in 3rd gear at about 1,500 rpms. Before (with the recent jetting) it seemed like it was hard to find a comfortable speed/gear. Like I was needing to constantly up-shift or down-shift to maintain a steady speed. That doesn't seem to be the case now.

                              While I had the tank off, I pulled the spark plugs and it was definitely running rich, especially on the rear cylinder.

                              As for how this is going to impact my mpg, only time & running through a few tanks of gas will tell. And the annoying off idle hesitation with changing the needle position hasn't returned and there is no change in hard acceleration.

                              Thanks and I'll post an update after I've had a chance to see what my mpgs are.


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