jetting 101

[UncleBeaner]

Pilot Screw                           - Idle
Pilot Jet                               - Idle to 1/4 throttle
Slide Cutaway                      - 1/8 to 1/4 throttle
Needle Jet and Jet needle     - 1/4 to 3/4 throttle
Main Jet                               - 3/4 to full throttle

1) Idle Screw
Allow the engine to reach operating temperature. turn to smooth idle. If unable to obtain smooth idle, go to Step 2, Pilot Screw

2) Pilot Screw
If unable to obtain smooth idle: turn the pilot screw in all the way GENTLY until it bottoms, then turn the pilot screw 1/4 turn at a time until achieving a smooth idle. The pilot screw should be correct between 1- 2.5 turns. If idle is between 1-2.5 turns, the jump to step 4, Main Jet. If idle is NOT smooth between 1-2.5 turns, then jump to step 3, Pilot/ Slow jet.

3) Pilot/Slow Jet
If idle is not smooth with pilot screw between 1-2.5 turns, then the pilot/slow jet needs to be changed. There are 2 choices. either A or B
A - If the pilot screw has been turned more then 2.5 turns to smooth the idle, then install a bigger Pilot.
B - If the pilot screw has been turned less the 1 turn to reach smooth idle, then install a smaller Pilot.

Note! A larger pilot jet will eliminate the popping sound that may occur when letting off the throttle.
A larger will improve cold-weather starts.

4) Main Jet
Start with the richest (largest numbered) main jet and test at full throttle. the engine should "stumble" at wide open throttle (If engine does not stumble, then the engine is not rich enough and a larger main jet should be installed). Once you have a rich "stumble", install the next smaller jet one size at a time until full throttle results in normal operation. Once main jet step is complete, go back to step 2, Pilot Screw, and perform pilot screw adjustments one last time.

Note! If quad runs faster at 3/4 throttle then at Full throttle, then the jets are lean and a larger jet must be installed.

Needle
If carburetor has an adjustable needle, this step CAN be performed. If you have a slight hesitation but NOT a hard stumble between 1/4 -3/4 throttle, then the needle is probably lean. Raise the needle by lowering the clip.

Slide Cutaway
The slide cutaway controls the transition fron the low speed pilot jet, to the main jet. Replacing the Slide Cutaway with one that has a smaller number, will decrease the amount of air creating a richer mixture.

Reading Spark Plugs
Once the main jet is set correctly by dropping a main jet size after the "stumble", confirm the jetting of the engine by reading the spark plug. This works best in top gear in a slight uphill area. After running at full throttle, do the following at the same time: pull in the clutch and stop the engine, DO NOT LET THE ENGINE IDLE! Remove the spark plug amd examine the color. It should be a Light Tan. If it is white, then the engine is running lean and a bigger Main Jet must be installed. If the color is Black, then the engine is running rich and a smaller Main Jet must be installed.

any questions just ask

[spdraycr]

i have a question-or two...i have an 03 660.yoshi exhaust,k-n filter.had 146/148 jets and 22.5 pilot.all was ok until i took my lid off and tried to run it that way.instantly i noticed my exhaust begin to glow fire red at idle(it never has before),and i have to leave the choke lever in the middle instead of far right or it wont idle.seems like its always racing now.when i ride and open up a little bit,it stalls in high gear then kicks in.went to 150/155 jets and the same.played with a/f screw and no help.moved needle back and forth from 3rd to 4th clip.still the same problems.when i give it gas while idle,it revs then comes down.when i cut it off,it backfires.spark plug tip has been white which i understand now means its lean.what do you think i could do to help fix my problem?and with the jets,which number goes in which side.oh and as far as a/f screw,i have it at 3 full turns from seated.i also put my lid back on and went back to 146/148 jets(was in there when i got it and ran perfect-untili messed it all up).i live a few miles from daytona beach if florida.please help if possible,thanks

[Peelz]

smaller number needs to be on the left(passenger) side...I think  aaah I can't temember pretty sure....

155 160 with airbox lid off. May need a bigger pilot too with no lid. I had trouble running my 660 with no lid. Ran like crap. SO I put i back on and drilled some holes in it. May have to turn out those a/f screws a couple.

[spdraycr]

ok cool,thanks.so im guessing i dont need to use the bigger jets with the lid on...whats the diffrence in performance with lid on/off if there is any?and what brand/size pilot should i buy?any and all help is appreciated,thank you

[Segkast]

I would try some 25 pilot jets, and something close to that 155/160, maybe 2.5 turns on the a/f ?

You can buy Mikuni jets at most Yami shops. Sometimes if they don't have the right size I hit up the Suzuki/Polaris dealer as well. If you don't have anything close to you, you can always use http://jetsrus.com/ as well. Never used them myself, but I've heard good things from a lot of others.

Peels is correct, the smaller jet goes on the left carb.

No airbox lid simply allows for better airflow. Same with drilling holes like Peels did, just more so.

Is your K&N a drop-in filter, or the kind that you had to attach an adapter to the box for ?

[spdraycr]

i have the billet aluminum adapter from k-n.filter is attatched with screw on clamps.question-my header glows bright red right after i turn it on and when i push the throttle,it seems to do something like an over rev then dies down.does that mean its too rich or too lean?and i have the clip on the needle in the 3rd position down

[Peelz]

Peels is correct
 

 

  doesn't happen often, so I am basking in the glory

Removing the airbox lid will give you much low end on the 660. Makes you go really really lean though. We were out at the dunes, mine was jetted perfect for my mods with holes drilled. My buddy convinced me to pull it off to see what happens. BAD IDEA without bigger jets. Pilot jet needed to be bigger. My rappy sounded like rice krispies right off throttle. snap crackle pop   And wouldn't wheelie, which is a deal breaker for me. Guess it has to do with how fast the intake has air available to it.  Full throttle was okay it seemed, though I am sure it was a little lean. 

Seg knows what he is talking 'bout. Mikuni jets. Get a bigger pilot, maybe even bigger mains. THe pilot might be what is making the motor feel like it is dying when you stab the throttle. Geting a ton of air, not enough fuel. Starving for a second, then catching up when the main takes over. put lid back on, if it gets better, you need bigger pilot.    in my opinion. needle position sounds right....

Glowing headers is normal on the 700, does it happen on the 660? I never noticed, mine were always caked in mud

[spdraycr]

ok cool.im gonna go get a bigger pilot jet.when u say put holes in it,how big and approx where at?sorry if i sound kinda slow and goofy,but im trying to soak up as much info as possible.

[Peelz]

ok cool.im gonna go get a bigger pilot jet.when u say put holes in it,how big and approx where at?sorry if i sound kinda slow and goofy,but im trying to soak up as much info as possible.

start with 2     .5 inch holes evenly spaced near the back of filter. If more is needed, drill more toards the snorkel. Or you can keep trying to jet with no lid, you can get there, just takes a minute. 


found this old info... It says 1.5 inch holes, but that seems huge to me, might as well remove it.

Holes # 1 & # 2: 1 ½ inches from rear of airbox lid and 1 ¾ inches from sides.

Holes # 3 & # 4: 7 inches from rear of airbox lid and 2 ½ inches from sides. Note: these holes must be drilled through both layers of airbox lid. WIll be drilling near snorkel.

[spdraycr]

ok cool,thanks for all the help.will be back soon with my results...

[spdraycr]

ok guys,heres my new deal.i put my lid back on and put the jets in it that it had before.i turned my a/f screw out approx 2 1/2 turns.i can get it to idle ok and stop all the things it was doing before(including the cherry red headers)by adjusting the choke lever fully to the left.when i put it all the way to the right like i used to before,it wont idle and shuts off.this is also when the headers glow bright red.i used to only have this lever to the left when i started it up and it was possibly cold.this kinda doesnt make sense.anybody have any helpful info for me.my mains are 146/148 and pilot is 22.5.

[Peelz]

ok guys,heres my new deal.i put my lid back on and put the jets in it that it had before.i turned my a/f screw out approx 2 1/2 turns.i can get it to idle ok and stop all the things it was doing before(including the cherry red headers)by adjusting the choke lever fully to the left.when i put it all the way to the right like i used to before,it wont idle and shuts off.this is also when the headers glow bright red.i used to only have this lever to the left when i started it up and it was possibly cold.this kinda doesnt make sense.anybody have any helpful info for me.my mains are 146/148 and pilot is 22.5.

you have a full exhaust and intake?  You run it with the choke on? does it run with it not on?

[spdraycr]

yea full exhaust and intake.yea with the choke on,it idles and stays on.when i push the throttle,it revs up and right back down.on my test runs in front of my house,seems to have a flat spot in my mid band,but otherwise is ok.but as soon as i put the choke lever to the right where its supposed to be,it wont idle and headers begin to glow cherry red.basically it does the exact opposite...

[Segkast]

You needs bigger jets.

[Peelz]

You needs bigger jets.

blunt and to the point and correct

choking it is making it richer

jets 150 and 160 IMO with no lid maybe even a new 25 pilot