Mikuni 18mm carburetor

Discussion in 'Performance Mods' started by jaguar, Mar 1, 2013.

  1. jaguar

    jaguar Well-Known Member

    I finally got tired of my non-adjustable Dellorto SHA carb and so switched to a Mikuni. I must of just been feeling lazy when I got the Dellorto because it supposedly doesn't need any adjustments other than getting the main jet correct, and its clamp on size is the same 19mm that the Grubee engines have at the intake manifold (and also the Rock Solid reed valve). But I am very performance oriented and its just not possible to get the most out of these engines unless you can adjust all three carb circuits (idle, needle, main) to perfectly match your engine and exhaust pipe. I bought the 18mm Mikuni from Treatland (only $70) which has a 23mm inner diameter clamp and so I needed to have another manifold made for it. I am using the Rock Solid reed valve with my 60cc cylinder and so this is the drawing I made for the local machine shop to use to make the manifold out of nylon or aluminum for me. Treatland also sells a number of good air filters that fit this carbs 32mm inlet.
    My Experience with the Mikuni 18mm
    At first it wouldn't start and the choke made no difference so I figured it was too rich for idle speeds. Since some fuel comes up by the needle at idle I decided to lower the needle a couple notches. That worked and it started, but was too lean at mid range speed (since cracking the throttle open at medium speed caused it to die), and way too rich at high speed. Since these carbs usually come with a number 3 slide cutaway and this one was only a 1, I decided to increase the peak cutaway height from 1.5mm to 2.25mm to make the mixture leaner at the beginning of the opening of the throttle. That helped but I eventually (since no pilot jets are available locally) soldered the jet inlet shut and drilled a new hole 14% smaller. I figure a #25 pilot jet would be just right for this 60cc engine. I did the same trick to the main jet till it ran right at top speed but then it was slower than it was previously with the Dellorto 14mm so I knew the homemade air filter was too restrictive and I adapted a high flow filter I had laying around to clamp to it. That enabled me to make use of the #80 main jet that I also ordered. The mid range was finally made right by lowering the clip on the needle to the second to the lowest position (which raised the needle). So after all the changes the main difference this Mikuni made was getting the engine to idle right, whereas the Dellorto was a nightmare since it has no idle air screw or replaceable idle jet. Top speed was the same between the two carbs but I think the mid range power with the Mikuni was better since the needle position is adjustable. Why the engine is not faster with a bigger carb is because the reed valve is restrictive and probably is equivalent to the flow from a 14mm carb. If I had a bigger reed valve I would of had more top end power from using a bigger carb.

    Final Recommendations
    Use your rotary tool to increase the slide cutaway to 2.25mm and shorten the throttle cable housing by 15mm.
    Also buy a #25 pilot (idle) jet and #75 and #80 main jets.
    Set needle clip to second to lowest position.
    Don't use a restrictive air filter. (With it on the engine should run almost equally as good as with it off.)

    What the carb comes with
    pilot jet #30
    needle #4F18
    Needle jet #145650-0
    main jet #70
    slide cutaway #1.0

    Treatland also sells a 16mm Mikuni for $85 that has a 20mm inner clamp diameter. This size carb is more in line with all the Grubee engines that aren't modified to run at high rpm like mine is. Also buy an air filter with 28mm clamp on size for it. I think the jetting should also be like what I list here even if your engine doesn't rev high.

    Here' a picture of the Mikuni 18mm on my engine:
    The length of the cylindrical part of the adaptor is 25mm (1 inch).
    Last edited: Mar 2, 2013

  2. Fabian

    Fabian Well-Known Member

    Attaching a carburettor with an 18mm throat size onto a reed valve with a 14mm internal diameter is a pointless exercise as the intake flow is governed by the smallest I.D. in the system.

    To make your carburettor usable, it's best to contact Rock Solid Engines and have Tony make a new intake extension for the reed valve intake, sized for an 18mm internal diameter, as well as your required O.D. of 23mm
  3. jaguar

    jaguar Well-Known Member

    You just stated what is painfully obvious. It was somewhat of an experiment. With this engine size and revs the ideal carb size is 17.5mm. I had guesstimated that the reed valve is equal in flow area to a 15.5mm carb but I wanted real results to back that up. I have that now and so I know if I can make my own carbon fiber reeds for this reed valve, that are more flexible and will allow more flow, then maybe buying the 18mm won´t of been an error.
    It was also an experiment to see whether or not the idea that too large causes speed loss. Put that old wives tale in the trash. Unless you go way oversized you can alway make it work if you can dial in the jetting. I am getting the same speed as with the previous carb which proves my new belief, that its not an error to use an oversized carb as long as you know how to jet it.

    Fabian, the main limiting factor for the Rock Solid reed valve, since it extends into the intake channel, is the width of it which is pretty much at its limit right now. The only way it can be made to flow more is to use reeds less stiff as long as they aren´t so flimsy that they flutter at top rpm.
  4. Fabian

    Fabian Well-Known Member

    For what you are trying to achieve, your best results will come from a custom machined RSE cylinder that has the intake port area angles and volume optimised and opened up to suit the increased flow of the 18mm Mikuni.

    You would also want to contact Tony at RSE to design and manufacture a custom reed valve intake with lager volume dimensions to suit the custom cylinder.

    The other option is to purchase an Arrow Cycles reed valve intake and have the cylinder custom machined to suit this different type of reed valve intake, because you are asking far more of what the standard cylinder is capable of delivering, even with mild modifications.
  5. Fabian

    Fabian Well-Known Member

    It's at the point where your engine tune and power band is no longer in sync with the lower rpm design specs of the standard cylinder. Time to look for a scooter piston and cylinder or pit bike piston and cylinder and intake system; modifying it to fit onto the bottom end of the Chinese bicycle engine.
  6. Fabian

    Fabian Well-Known Member

    If you are chasing more power, you can always do it in style with these power plants:

    Last edited by a moderator: Dec 15, 2015
  7. jaguar

    jaguar Well-Known Member

    This design is limited to a single transfer channel on each side but with my modifications does pretty well. Of course I'd like to experiment with other cylinders and may eventually but I'm too busy to actually make any concrete plans just yet. The idea from the start was to see what all could be done with the original design to improve it.
    As far as what I'd recommend now- I think people should consider the combo of Walbro carb and Rock Solid reed valve with adaptor which JNMotors sells for $220. The Mikunis and Dellorto PHBG (another adjustable carb) all have clamp on sizes that aren't 19mm (the standard for Grubee). But I think Rock Solid is still ready to make their reed valve part that the carb clamps onto whatever size you ask them to make it when ordering their reed valve from them ($65). Best to double check by email first though. So if you want this nice $70 Mikuni 18mm you can mount it without having to go to the machine shop like I did. Mikuni's used to be the number one carb of choice for many years.
  8. Fabian

    Fabian Well-Known Member

    What about this more avant-garde option for your motorized bicycle:

    Last edited by a moderator: Dec 15, 2015
  9. jaguar

    jaguar Well-Known Member

    4 cylinder tuning nightmare. but at least all the vibrations should counteract each other for smooth running.