Mikuni jetting

[Mickey Sergeant]

I admit I have never been any good at jetting carbs.So here it goes.Brand new mikuni 26mm.At high rpm runs good.Idles good.Coming off idle runs rough,lots of hesitation.acts like to rich?Plug looks a little dark and little wet.Went through all my needle settings and air screw adjustments.Didn't make any improvements.Running good race gas.I have that print out of the spec sheet that was posted.Need recommendation on where to go from here.Thanks in advance for help.

Mickey Sergeant
74 Berkshire

[Ron]

Hi Mickey,
I would start by marking your throttle position 1/8, 1/4, 1/2, 3/4, use a piece of tape on the housing and use a magic marker on the grip.
Since the bike is already idling good, you can move on to the needle jet / needle combo.
BTW, the air screw only controls the idle mixiture setting. Once you crack the throttle, it's running on the pilot jet and the throttle slide cutaway.
Start by removing the main jet and place the needle clip on the needle at the top position (needle all the way down).
Ride the bike on level ground while opening the throttle slowly.
The motor should go completely rich by 3/4 throttle.
If it goes rich by 1/2 throttle, the needle jet is to large. If it continues to run fairly clean to 3/4 or more, the needle jet is to small.
Without this step you won't know if your needle jet / needle combo is to small.
No matter how big the main jet is, the restriction is still in the needle jet / needle combo.
Once you get the needle jet close, you can work with different needle tapers to clean up the part throttle running at 1/8 thru 3/4 throttle positions.
Once you get past 3/4 throttle your running on the main jet.
The reason I follow this sequence is that each of the circuits (ie: pilot jet, needle jet combo etc) contribute to the total volume of fuel being fed through the carburetor.
Previously selecting the main jet would be upset by any changes made to the lower circuits.
BTW, start with a main jet that is to large and back it down until you find a good set-up.
You could easily sieze your piston by starting with a main jet that is to small.

[Ed Chesnut]

Mickey:

Ron's advice was great - but skipped over identifying whether the slide cut-away is "right on", "rich" or "lean".

As you slowly roll on the throttle, slide cut-away is a primary factor from about 1/8 to 3/8 throttle.  As you come into that range (which is where you reported a problem) - does it "stumble" (fall on its face, stop pulling) or does it start "blubbering" (lots of exhaust smoke if you hold 1/4 throttle?).  "Stumble" is lean and you'll need a richer slide (example 1.5 instead of a 2.0).  "Blubbering" is rich and you'll need a leaner slide (example 2.5 instead of a 2.0).

If you have trouble identifying whether it is "stumbling"(lean) or "blubbering"(rich) as you get to 1/4 throttle --- there is another technique to help you identify which is which.  But let's not pull it out of the bag unless you need it.

Ed

Keep the rubber side down!

[gooddirt]

His Carb 26mm MIC was sent with the stock or standard internals from the supplier.

[Mickey Sergeant]

I would call it stumbling[lean].Got stock Jetting info from supplier I purchased carb from.It has a 182n-8 needle jet.He said thats on the leaner side,ordered next 2 sizes up to richen up.Will see how that does
Mickey Sergeant
74 Berkshire