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Why float level setting is so important!

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  • Why float level setting is so important!

    I just saw a post on one of my Facebook off road forums from a legendary engine builder, Bubba Ramsey. Here he explains why float level and venting is so important in any engine (though he specializes in 2-strokes) Read on:

    BHP Bubba's House of Power

    How about a Christmas Eve tech tip? A post made about an engine failure from a bad vent cap has inspired me to write this article. Let's talk about Bernoulli's principle as it relates to carburetor function, specifically a standard 2-stroke carburetor (PWK, lectron, Mikuni etc.), and how the vent issue could kill an engine. I will try to explain this as simply as I can to try and help people that do not know understand. Have you ever wondered why most carburetors taper toward the center from the rear bell? (where the air filter attaches) They call that a venturi for a reason, the air speed increases as it goes through the carburetor. As the air flows across the needle jet (some call it a dump tube) Bernoulli's principle tells us that there is lift generated (as a result of pressure drop) That lift is what pulls the fuel out of the carburetor bowl and into the air stream, which in turn gets pulled into the engine. Now there is another factor in play here that most do not know, which is the fuel level in the carburetor bowl vs the lift by the air speed. When the float bowl is full the fuel can be lifted with little or less effort. As the fuel level drops in the float bowl, the lift from the air speed over the needle jet will not pull as much fuel into the engine. So to put it in plain terms........a plugged vent cap will begin to create a slight vacuum in the fuel tank over a period of time. As the fuel level drops in the float bowl, the engine cannot pull the same amount of fuel. The engine becomes leaner, but gradually in most cases. Eventually (especially on the 2-stroke) the engine will lean out to the point of scuffing a piston or sticking a ring(s). I hope this helps explain the reason why.
    2000 Roadstar 1600 - BAK, Pumpless, Curt's manifold, Mikuni HSR42 Freedom Combat Pipes Jumbo Strong bags, PPG 'HellFire' Paint.

  • #2
    Brilliant. It's always the little things, isn't it? I've got a bit of a head scratcher. I just rejetted my stock Mikuni 40 for sea level and some Cobra 2 into 1 pipes and am having some starting troubles now. The needle is a Barron's set on the fourth slot from the top with one washer underneath. I took the main from 165 to 182.5 and the pilot from 35 to 40. The needle is an aftermarket Barron's at two turns out. Choke is MaxAir. Intake/air cleaner is stock. Ignition is Dyna 3000 set to the 32B advance curve, stock rev limit.

    From cold, I pull it to the first choke position, give it two half squirts of accelerator pump, and sometimes it starts, runs for three or four seconds at around 1000rpm, then dies. I hit the start again, but no dice. It just proceeds to flood. Should I drop the pilot jet down to 37.5? Or just keep screwing the PMS in? This is a 2004 1700cc. Our average temps are mid 40's to mid 50's this time of year. And I have noticed a couple wet spots on the lower lip of the exhaust after running for a bit. The carb rebuild kit for this motor recommended the 182.5 main with the 40 pilot. That seems like a big jump from 165 main, 35 pilot with PMS out only one turn. (This was the original tuning where the bike used to like in Eastern Oregon). Air jet is stock 100, and the starter jet seems to just be a plug. The float level is 5mm above the bowl lip measured with the bike level. It was always a good starter with the previous jets installed... And never backfired on deceleration.

    The Cobra's have their baffles in still. And it still has it's AIS intact and functioning. Thanks for any recommendations you might have.

    Comment


    • #3
      Why did you rejet and kept stock intake/filter? Try taking intake filter off and try to start. Full choke, no throttle and should start. These are cold blooded rigs and need full choke.

      Comment


      • #4
        So the reason I haven't changed the intake was both because I haven't found an air intake that seems weatherproof (I live in Seattle, we had over three inches of rain one day last week) and because I'm not entirely sold on K and N style filters which seem to dominate the aftermarket. Couple gear heads told me they aren't all that good at filtering... I was waiting to do a bit of research one of these weekends. Maybe you have a personal favorite?

        Comment


        • roadiemort
          roadiemort commented
          Editing a comment
          That's the trouble with gear heads their opinions are just that, K&N has been around longer than sliced bread and offer a great product if used correctly as per K&N instructions.

        • Jaybo
          Jaybo commented
          Editing a comment
          I'm down near Salem, so I know about that storm (it missed us) but were you riding in that storm? K&N works fine, filters fine as long as you oil per instructions. I've ran them on my quads for decades but I do put an outerwears cover over them because we ride the dunes a lot. Outerwears sells water shedding covers for the filters that work great and will keep the water out.

      • #5
        Do a search for k&n filters part numbers on this site. They can be had for cheap and simply attach to the carb mouth 2-1/2” opening i am pretty sure. A rain sock can be bought from them for whatever style of filter. I have model RC 1980.

        Comment


        • #6
          Why did you think you needed to rejet the carb? You do know that the stock 40 Mikuni is a Constant Velocity carb Right? Unless you open up the air intake you will be fine at sea level.. You are beau coup rich put it back where it was unless you change air cleaners. .

          Comment


          • #7
            OK. Thanks for the tips. So the 182.5/40 combo was in the carb rebuild kit, but they must have assumed pipes/BAK/porting. So I took the 182.5 back to 165, and the pilot to 37.5. That improved things. It also turns out the fuel pump was intermittent. So I removed it, took the two plugs out of my gas cap, ran a fuel line (with factory wire protector) to the carb after removing the inlet screen. Reset float level back to 5mm above lip. Runs lots better now. I went ahead and ordered the Baron BAK, and will port my manifold in a couple weeks. I wasn't aware the Barons had rain guards available for them, so that's the current direction. Also going to do the Baron's AIS removal/plug procedure and take the main to 172.5 when I install the Grizzly float valve/needle combo. That seems to be the most popular setup for R's with 2 into 1 baffled Cobra pipes. PMS is down around 1 turn right now, no popping on deceleration. I'll pull a plug tomorrow and take a gander. Does anyone make 1) a spiffy choke holder for us tankless types, and b) a battery operated O2 sensor for tuning that we can plug in the tailpipe and watch while we test ride? Thanks again for the info everyone.

            Comment


            • #8
              Oh and no, I wasn't riding in that 3inch/24 hour storm last week. Luckly, the scooter was laid up. :-).

              Comment


              • #9
                Very good article Jaybo. Thank you.
                It really makes it understandable about fuel level in the needle and jet area, and how fuel is pulled up into the carb throat as vacuum is increased.

                Part of this system that wasn't talked about, is the emulsion tube, and the effect it has on the fuel being pulled into the carb throat. There is a lot of adjustments, "hole size and locations of the holes in the emulsion tube", that can be made to help with fuel delivery at different fuel levels, and/or different vacuum signals.
                I find that the emulsion tube is one of the best, but hardest tuning pieces in our carb. It's not for beginners doing carburetor adjustments.

                Comment


                • #10
                  thats how a spray gun works also, of course it is important, but i can ride on down the road just fine for a 1/2 mile with gas off, and then lean backfire,lean backfire , she dies, thats when u want a FUEL PUMP! for optimal performance, and to use our RSC recommended jetting advice, float level needs to be spot on as they say over the pond.

                  Comment


                  • Duke
                    Duke commented
                    Editing a comment
                    Even with a fuel pump, it won't do you any good if the gas valve is off.

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