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Carburetor improvements

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  • Carburetor improvements

    I thought I would ask what you guys think of some modifications I plan to make to my bikes carburetor.

    overall my goals are to ever so slightly improve performance by reducing turbulence.

    A: this is the outlet of the choke enrichment circuit
    B is pilot(idle?) jet, C is main jet
    E is a hose connector for venting the float bowl
    F is some kinda of auxiliary air intake linked to D via the float bowl gasket, and nothing else. It leads to a caped off hose connector on the carburetor. It also links to H on the next picture. It goes into the side of the main needle well.
    G is the pickup for the choke circuit (There is a jet in the float bowl?)
    K is.. I have no idea. its caped off. Oil injection? Nitros? (Hah) On my bikes carburetor, I have already pulled that tube and epoxy filled the venturi smooth.
    I seems to be the air passage that leads to the pilot jet
    J is the actual pilot/idle air intake, you can just see the tip of the air screw.
    L is the air intake for the choke circuit

    A: I plan to fill this up with epoxy using some kind of plug (wax?) to prevent the hole from clogging. Basically to smooth it out. As well as doing a little cleanup work of that ridge.
    H/I/J: I plan to carve these away to be as smooth with the venturi as I can. Do I even really need J/I at all? If I removed the ball bearing, Couldn't the air screw act as a gate valve instead of a needle valve? The bike does suffer from too rich at idle and running out of screw adjustment range, so extra intake air and a 'leaky' air screw are not bad things.

    Also, that brings me to the slide. I was thinking of making a new one out of aluminum and making the bottom more or less smooth instead of so recessed, to lower turbulence. I realise it would need to be cut out somewhat on the air intake side, but if I can lean out the idle jet I think I can minimize the cut out needed.
    Last edited by Black_Moons; 03-19-2012, 05:30 AM.
    Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

  • #2
    You will never notice any difference other than pyschosomatic.


    • #3
      Does AMAL make the pyschosomatic carb or is it Japanese ?

      Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.


      • #4
        Mods that add 5 or 10% won't be noticed on their own, But combine a few of them and they can add up to noticeable improvements. Also note I ride this thing WOT basically all the time, So any improvement would be good.

        I also have the jets and needle adjustment range needed to compensate for the change in mixture.

        Stock is a 0.70mm main jet. Due to all the airflow improvement mods I have done, I am currently running a 0.94mm in my carburetor (Not pictured, pictures are of a spare).

        I have already opened up the engine and ported it.

        And I still need to lean it up at lower throttle/rpms to get better power, one of my goals in attempting this mod that would change the idle circuit and slide valve.
        Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.


        • #5
          It's awfully easy to turn a functional carburettor into scrap. I've got a draw with several carb bodies in that are totally stock, but just don't work properly becuase of some subtle internal problem. They've been properly cleaned and the passages blown through, but just don't work.

          It's possible to make all sorts of airflow improvements to carburettors, but by far the best way it to fit a larger one, unless you are competing in a race formula where the stock carb has to be retained.

          You may spend most of your time at WOT, but you still have to get there. A progressive throttle response counts for an awful lot.
          Paul Compton


          • #6
            Been reading the following. A good read for any gearheads. He doesn't provide any "how to's" but after you've read twenty or thirty pages on mixing gasoline and air and lighting it, you have a new found respect for the equipment that works and the people who developed it.



            • #7
              Originally posted by John Stevenson
              Does AMAL make the pyschosomatic carb or is it Japanese ?
              Italian, I think. Kind of like those noise makers they put on little 4 cal card. Makes more noise so it must be more powerful, right?

              But I say do what EVGuru says and just buy bigger carbs. I had a set of del'orto's on my old 71 BMW that replaced the old Bing's. That made a difference.


              • #8
                I guess there's always improvements that can be made to anything but with certain appliances, carburetors for one, I can't see how seat of the pants engineering can improve over an engineering facility's resources, flow benches, vacuum equipment and above all knowledge of the product.
                Having worked on motorcycles since the late 60's I have found that the best thing a person can do to a carburetor is keep it clean (pilot circuit) and adjusted. I have swapped out larger carbs for people who wanted more performance only to spend untold hours riding, rejeting, riding, rejeting, well you get the picture.
                You say you only want marginal improvements. How would you measure these?


                • #9
                  Before too many mods were done, I'd say look up the carb and find out as much as you can about the passages.

                  The way K sticks in there, it almost looks like a midrange enricher. normally used so idle doesn't have to run rich. some carbs have idle/mid/main circuits, others only have idle/main and the idle/main circuits over lap, making it a real pain to get a good balance in all three ranges. Often leading to compromise, idle mid high, pick any two.

                  as a rule of thumb, don't do anything that makes the throat 'bigger'. smoothing it off and getting rid of steps is perfectly fine and will improve flow.

                  It's more important that the bore is smoothly blended from the air filter side towards the engine side. If you can smooth that step seen in your third pic without enlarging the bore, it should help with flow. (not certain that it's an actual step, or just the other side of the slide bore....) Steps from the engine side are not as critical and can often help flow.

                  To see which holes can be plugged, check gaskets, if a factory gasket plugs a hole, do what you want to seal it up and smooth it off. (unless it would restrict air or fuel flow like around a needle, if you restrict the space you have essentially screwed the needle in too far and cut the flow)

                  Otherwise, I agree with the others, you'll often see more returns with a new carb, but larger is not always better.

                  There are many sites on cleaning up carb airflow, getting scarcer now with EFI being about the only thing you can get, but well worth looking into. (as is a home built flow bench if you get serious)

                  I've done it on about every carbureted engine I've had, and while it does help performance, it's not going to peel your eyelids back. One thing I noticed is that it does seem to help mileage a bit.


                  • #10
                    Excellent advice from all who contributed already. I see we have people here that have already tried or who have been around those that did try and improve on an already proven concept.

                    Not to discourage you BM but what has been said comes from a vast and experienced source of personal knowledge.
                    The carburetor you have before you is not the first of it's kind or type, it works over a wide range of operational parameters. Just about anything you do to change that delicate balance of air and fuel flow will compromise that situation.
                    If you need more air and fuel flow through the engine go up in size a couple of millimeters, it will be more productive even if it does narrow your rpm operational window somewhat. Not that what you have in mind would increase it.

                    There are just too many interrelated aspects to your project to have much hope for an improved performance. Even small modifications such as another home made slide can cause problems like sticking, without a special coating on it to prevent this. Aluminum and aluminum don't make for a nice sliding fit.
                    Home, down in the valley behind the Red Angus
                    Bad Decisions Make Good Stories​


                    • #11
                      What's WOT on a Mo Ped 37 MPH??


                      • #12
                        If you wanted to go the new slide route how would delrin go? Its slippery and easier to work than aluminium and machines nice.


                        • #13
                          Didn't see this mentioned but what about a velocity stack (gently curved and tapered inlet shape) just ahead of the carb. Should result in less turbulence/more flow if that shape isn't already present in your inlet tract. This is less likely to disrupt factory calibration than internal carb mods.


                          • #14
                            Originally posted by moe1942
                            What's WOT on a Mo Ped 37 MPH??
                            What does "WOT" mean?


                            • #15
                              wide open throttle