Jetting Info

[Guest dugdthug]

This is some of the info i received after i ordered my jet kit. If there is anything to add feel free

Please follow this procedure, so we can better determine what is the need exactly, and take corrective action. Note: It is also very important to verify that valve adjustment is correct, as that will cause havoc with jetting. Make sure your throttle cable has slck, and that the slides are closing fully.

1. Top end (full throttle / 6k to redline -
Best Main Jet must be selected before starting step 2 (needle height)!

• • • Select Best Main Jet
  • To get the best, most even top end power (full throttle/after 5000 rpm), select the main jet that produces the hardest pull at high rpm.
    • 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 low-end 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 low-end / cruise later - after step 2.
• 2. Midrange (full throttle /4k-6k)

Step 1 (Best Main Jet) must be selected before starting step 2!

• • • Select best needle position
  • To get the best power at full throttle / 4k-6k rpm, adjust the needle height, after you have already selected the best main jet.
    • If the engine pulls better or is smoother at full throttle/4k-6k in a full throttle roll-on starting at <3k when cool but soft and/or rough 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 4k-6k, try raising the needle to richen 4k-6k.
    • If the engine pulls equally well between 4k-6k when cooler as compared to fully warmed up, the needle height is probably properly set.
      • Do not pay too much attention to the low-end richness when you are changing needle positions - you still need to be using the position that produces the best full throttle / 4k-6k power in conjunction with the main jets (Step 1) that produce the best power at high rpm. You will deal with the low-end / cruise next.

• 3. Low end (full throttle / 2k-3k)

Step 1 (Best Main Jet) and Step 2 (needle height) must be selected before starting step 3!

• • • Float height (AKA fuel level & How to..)
  • To get best low-end power, set float height (fuel level) so that the engine will accept full throttle, without missing or stumbling, 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. Check out and RESET all: Needless to say, FUEL LEVEL IS EXTREMELY IMPORTANT!!!
    • If there are low-end richness problems, even after lowering the fuel level much more than 1.5mm from our initial settings, check for needle wear and needle jet (part of the emulsion tube). 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 or aluminum cap over the fuel screws on all but newer Honda. It's about the diameter of a pencil. Newer Honda carbs have no caps, but use a special "D" shaped driver.
    • 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 rpm operation.
      • Pilot fuel mixture screw settings, float level (but, you've "fixed" the fuel level in Step 3 - which you have already done!)
      • If lean surging is encountered, richen mixture screws (turn out for fuel mixture screw, turn in for air mixture screw) in 1/2 turn increments.
      • 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 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 (turn in for fuel mixture screw, turn out for air mixture screw), 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 (turn out for fuel mixture screw, turn in for air mixture screw), to richen mixture. Be sure there are no intake leaks and the idle speed is set at less than 1000 rpm!

[Guest]

this makes no sense. the jetting in a cv carb is cumulative. each circuit adds to the next as the velocity increases and the slide rises when the butterfly is opened. to say you need to pick a correct main then work backwards makes no sense. once you pick a main, then change the next jet back, the fueling at full throtlle will change making the main the wrong size. really the best thing you could do is do a dyno run with a gas anlyiser to see the air / fuel ratio through the rpm range. knowing what jets effect what rpm's will tell you which way you need to go with the jets. the whole seat of the pants thing has always pissed me off. some guy whith 1000's of hours tuning bikes writes a how to for common folk like us to jet their bike. his perception of what "feels right" and joe bikers perceptions will be vastly different. if you don't have access to a dyno, get a jet kit with specific directions like "if you have A exhaust with B filtration then use these jets", makes more sense to me.less guess work and you'll be closer to correct from the start.besides, pulling the carbs on these things are a pain in the ass. the less you need to do it the better. YMMV...

[timmythecop]

..and remember:

-The pilot (or slow) jet handles start and idle and a wee bit of off-idle

-The main jet is where most of the fuel comes from and is responsible for mostly wide open throttle

-The needle (dont worry about taper too much as they sorta got standardadized for each bike) and its height, handle mid range and the transition from idle to mid to WOT.

Hints: most bikes come lean, lean lean to pass emissions. this is most evident in the pilot jet. Any bike made after 1980 could do with a bump up to at least the next size pilot jet. If the needle isnt at the right height, you will feel it either flutter (too rich-too high) or get thin (too lean-too high) The main jet is kinda easy because if it pulls cleanly at very large throttle openings and full throttle, it is most likely good. If it is overall too lean the 2 signs are the revs hanging up before it settles back down to idle and the dreaded "lean-pop" where the overrun sounds like Beruit. Too rich is a little harder to decipher, but if it doesnt hurt your eyes or nose and pulls cleanly, then a little too rich is always better than a little to lean. Also remember that if you take off the covers and start moving the idle mixture screws, thay are for idle only and shouldnt/wont affect anything else.

This being said, it isnt all that easy because as stated above, all 3 systems sorta work together and feed off each other. Then you throw in bad gas, ethanol, torn diaphrams etc, it gets frustrating. A jet kit should get you very close and then it is just fine tuning. Only make one change at a time, and only take apart one carb at a time. dig in and you'll get it.