Mikuni Carburetors

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 it's 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. So I bought the 18mm Mikuni which is available from Dennis Kirk (only $77). I had bought mine from Treatland but it came with the wrong needle jet which made it run lean before 1/2 throttle. It has a 25.7mm inner diameter clamp and so I needed to have another manifold made for it. I am using the Rock Solid reed valve with my 55cc cylinder and so this is the drawing I made for the local machine shop to use to make the manifold out of nylon for me. Treatland also sells a number of good air filters that fit this carbs 32mm inlet. 

Treatland also sells a 16mm Mikuni for $85 that has a 21mm 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. It comes with a #70 main jet and a #25 idle jet. The main jet series for this is different than for the 18mm. It is N100.604. You may need a larger jet. Set needle clip to second to lowest position.

Here's a picture of it on my 55cc engine with Rock Solid reed valve and nylon adaptor. The carb bottom just barely touches the clutch cable which is OK. Not much room for any air filter which is why I recommend the one listed below.
Mikuni 18mm

Here is my filter of choice for this carb for clearance issues. It costs $23 from Treatland.

Here is the Mikuni on my 60cc with shortened intake which angled it upwards some for more clearance from the frame. The noted distances are 105mm length (reed valve, adaptor, carb) and 50mm from centerline to bottom of float bowl.

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 a little bit of fuel comes up by the needle at idle (if it's too high) 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 cutaway peak height from 1.5mm to 2.25mm to make the mixture leaner at the beginning of the opening of the throttle (see drawing below). 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 my 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
Increase the slide cutaway to 2.25mm
Set needle clip to middle clip position.
Don't use a restrictive air filter. (With it on the engine should run almost equally as good as with it off.)
Shorten the throttle cable tubing by around 15mm by use of your rotary tool.
Here's a drawing showing how much free throttle cable should be able to extend past its housing after you modify it:

What the carb comes with
pilot jet #30 (for 48cc piston port use #10 and for reed valve use #20)
needle #4F18
Needle jet #O-0
main jet #80 (for 48cc piston port use #80 and for reed vlve use #90)
slide cutaway #1.0

Here is a drawing for a custom intake extension that provides a 6.3" total intake length (for extra down low power) for use with a Mikuni 18mm carb and an engine with piston port intake (not reed valve). I have one on my non-reed valve engine. It's advantage is the 20mm internal diameter for little flow restriction. The commercially available offset intakes have only 16mm.

 Here are 3 different ways to connect the Mikuni carbs to an engine with piston port intake. The offset intake is available with 32mm bolt spacing (eye to eye), or 40mm bolt spacing. Be aware that without a reed valve the needed jet sizes are smaller than with a reed valve. So a normal piston port intake might need a #10 pilot jet and #80 main jet (it comes with a #70).

Mikuni Tuning Guide