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I like this info I found on the web, Harley guys have been running these carbs for some time so this applies to harley bikes but principle is still the same. I would give credit to the author but I cant find the original, I have seen this printed numerous times and found it on a different site than I original had.
1. Top end (full throttle / 7.5k to redline - Best Main Jet be selected before starting step 2!
Select Best Main Jet
If the bike pulls harder at high rpm when cold and less hard when fully warmed up, the main jet
is too large. Install a smaller main jet and retest until you find the main jet that pulls the hardest
at high rpm when fully warmed up. This must be done first - before moving on to the other tuning ranges.
If the bike doesn't pull well at high rpm when cold and gets only slightly better when fully warmed
up, the main jet is too small.
In order to properly tune the midrange and low rpm carburetion, THE MAIN JET MUST FIRST BE
PROPERLY SELECTED after 10 to 15 minutes of hard use!
Do not pay too much attention to the lowend richness when you are changing main jets - you still need
to be using the main jets that produce the best power at high rpm. You will deal with the lowend / cruise later - after step 2.
2. Midrange (full throttle /5k-7k)
Select best needle clip position
To get the best power at full throttle / 5k-7k rpm, after you have already selected the best main jet,
If the engine pulls better on a full throttle roll-on starting at <3k, when cool but soft when at full operating
temperature, it is too rich in the midrange and the needle should be lowered.
If the engine pulls better when fully warmed up but still not great between 5k-7k, try raising the needle to
If the engine pulls equally well between 5k-7k when cooler as compared to fully warmed up, the needle height
is probably properly set.
Do not pay too much attention to the lowend richness when you are changing needle clip positions - you still
need to be using the clip position that produces the best full throttle / 5k-7k power in conjunction with the
main jets that produce the best power at high rpm. You will deal with the lowend / cruise next.
3. Low end (full throttle / 2k-3k)
To get best lowend power, set float height so that the engine will accept full throttle in 2nd gear from
2.5k to 3k rpm at minimum.
Float heights, unless otherwise specified in the installation guide, are measured from the "gasket surface"
of the carb body to the highest part of the top of the float - with the float tang touching but not compressing
the float valve spring.
If the engine has a "wet" rhythmic, soggy area at full throttle / 3k-4k rpm, that gets worse as the engine heats
up, lower the fuel level by resetting the float height 1mm greater (if the original was 13mm - go to 14mm).
This will lower the fuel level, making full throttle / 2k-3k rpm leaner.
If the engine is "dry" and flat between 2k to 3k rpm, raise the fuel level.
Example: change float height from 15mm to 14mm to richen up that area.
REMEMBER, since the main jet WILL affect low speed operation, the MAIN JET has to be within 1 or 2
sizes of correct before final float setting.
Warning: If the engine is left with the fuel level too high,, the engine may foul plugs on the street and will
be "soft" and boggy at part throttle operation. Adjust Floats to raise/ lower the Fuel Level.
Base settings are usually given if a particular application has a history of fuel level criticalness. The Fuel
level height in the float bowl affects full throttle/low rpm and, also, richness or leanness at cruise/low rpm.
Reference: a bike that runs cleanly at small throttle openings when cold, but starts to show signs of richness
as it heats up to full operating temperature, will usually be leaned out enough to be correct if the fuel level is
LOWERED 1mm. FUEL LEVEL IS EXTREMELY IMPORTANT!!!
If there are lowend richness problems, even after lowering the fuel level much more than 1.5mm from our initial
settings, also check for needle wear and needle jet (part of the emulsion tube). See Worn Needle and
Worn Needle Jet diagram. It is VERY common for the brass needle jets (in the top of the "emulsion tube")
in 36mm, 38mm and 40mm Mikuni CV carbs to wear out in as little as 5,000 miles. Check them for "oblong"
wear - the needle jet orifice starts out round!
4. Idle and low rpm cruise
Fuel Screw setting (AKA mixture screws)
There is usually a machined brass plug covering the fuel screws. It's about the diameter of a pencil. Plug location.
Set for smoothest idle and 2nd gear, 4k rpm, steady state cruise operation. Set mixture screws at recommended
settings, as a starting point. For smoothest idle, 2nd gear 4000 rpm steady state cruise , and 1/8 throttle high
Pilot fuel mixture screw settings, float level AND pilot jet size are the primary sources of mixture delivery during
4000 rpm steady state cruise operation.
If lean surging is encountered, richen mixture screws (turn out) in 1/2 turn increments. Pilot jets are also available.
Pilot fuel mixture screw settings, float level and pilot jet size also affect high-rpm, 0 to 1/8 throttle maneuvers.
Too lean, will cause surging problems when the engine is operated at high rpm at small throttle openings! Opening
the mixture screws and/or increasing pilot jet size will usually cure the problem.
NOTE: A rich problem gets worse as the engine heats up.
If the throttle is lightly "blipped" at idle, and the rpm drops below the set idle speed, then rises up to the set idle
speed, the low speed mixture screws are probably set too rich: try 1/2 turn in, to lean the idle mixture.
NOTE: A lean problem gets better as the engine heats up.
If the throttle is lightly "blipped" at idle, and the rpm "hangs up" before dropping to the set idle speed, and there
are no intake leaks and the idle speed is set at less than 1000 rpm, the mixture screws are probably too lean: try
1/2 turn out, to richen mixture. Be sure there are no intake leaks and the idle speed is set at about 1100 rpm!
Well, I can't top warrior007's post but here are a few things that help me:
Definately do main jet first and make sure float levels is correct. This affects everything else.
Generally a "flat" feeling is lean and a "bogging" feeling is rich - it's sometimes hard to tell apart.
Plug chops are worthless - despite all the plug chop info people give it's been my experience a 1/4 mile run down the road will tell you nothing after looking at the plugs. They need a LOT longer to get color.
If your engine runs hot for no apparent reason you are lean somewhere - probably midrange or main jet depending on the riding you are doing.
Going rich on the main jet until it bogs then backing off 1 jet size doesn't work that well. You can be pretty rich and still not have a noticable bog on top. This method leads to rich jetting.
Personally, I think getting the main jet right is the hardest - you are dealing with full throttle operation and a little off one way or another is hard to discern.
If it stumbles or bogs in the midrange you are rich - lower the needle a notch (raise the clip). If it goes flat and doesn't want to rev fast enough but doesn't stumble you are lean - raise the needle a notch (lower the clip)
Same thing with the idle. If it stumbles off idle you are rich, if it goes flat but no stumble you are lean. Try the air screws first then worry about the pilot jet.
Backfiring on rapid deceleration (when fully closing the throttle) indicates a lean idle circuit.
Do all this with the engine warmed up.
These are my opinions - I am not an engine tuner but this is what works for me. I'm sure some will disagree about the plug chop statement and about goping rich on the main jet until it bogs not working comment.
I'm sure some will disagree about the plug chop statement and about goping rich on the main jet until it bogs not working comment.
You won't hear me disagree, thats why I want opinons like this. I don't really think these work either, not to get it right anyway. You might get close enought to run well like that but not "spot on".