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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).
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).
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