motoz/HMF
06-01-2004, 12:06 PM
The easiest way to determine if you are jetted properly is to do a plug check for each "range" of the carb.
There are three ranges to adjust in the carb.
Low speed: Is off idle to 1/4 throttle position. The pilot jet (is inside the carb float bowl and is removable to adjust the mixture by changing the jet to a smaller or larger number size) or fuel screw found under the carb outside between the float bowl and the intake manifold.
The smaller number on the jet = a leaner mixture. The bigger number on the jet = a richer mixture. The fuel screw turned OUT is richer turned IN is leaner.
Old time round slide carbs use an air screw (on the outside side of the carb) turning the screw IN richens the mixture and OUT leans the mixture. The air screw is NOT common anymore these days.
Mid range: Is adjusted by the needle clip and is 1/4 to 3/4 throttle position.
There are slots on the needle where a clip can be moved up or down to change the mixture. The top position on the needle is leanest (letting the needle stay lower in the bore of the needle jet that it fits into. The lowest position on the needle is the richest letting the needle stay higher in the needle jet bore.
Top end: 3/4 to wide open throttle is the main jet. It is a removable jet in the float bowl. Just like the pilot jet, it is numbered and the bigger number is richer the smaller is leaner.
Checking jetting doing a base ring plug check:
You will need to mark the throttle housing (twist throttle) or remove the top of the thumb throttle cover so that you can watch which part of the throttle opening you are in.
Using some gray tape place it on the throttle housing so that you can mark the position of the throttle. Mark the sweep of the throttle from closed to wide open. Then divide the sweep into 1/3's. Zero to 1/4, 1/4 to 3/4 and 3/4 to wide open. Then make a mark on the grip or throttle lever inside the housing (thumb throttle) so that you can make note of the position when riding.
Install a fresh plug and find an area with a slight incline about a football field long.
You will want to get into 3rd gear and hold the throttle in which ever area of the carb you want to check for the length of the field. Do NOT vary the throttle, you want to hit the kill switch and pull in the clutch, coast to a stop then pull the plug.
Make note of the plugs base ring color (the area just above the threads, see the link for better pictures)
http://www.dragstuff.com/techarticles/how-to-read-plugs.html
A light tan or charcoal color is a safe setting, a cleaner "light tan" is optimum but only for the experienced tuner because you are on the edge of leanness, a dark ring is too rich and the air fuel ratio needs to be changed.
You can do this check for al three areas of the carb.
This test will need to idealy be done in 55 to 65 degrees on a not so humid day. Hotter and more humid or colder days will be totally different as far how the jetting will be.
The hotter it gets outside the richer it will run. The cooler it gets the leaner it will run.
This should give you the basic understanding of how things work in the carb and how to check the ranges of the carb to be able to adjust the jets or screws.
********ALWAYS wear the proper safety gear when doing a throttle chop/plug check, do your testing in a SAFE CONTROLLED RIDING area where the high speeds needed to do the test will not be dangerous to you and others. Be carfuel of removing spark plugs, the engine and related areas will be exceptionally hot. IF you have ANY DOUBTS on your ability to preforn this test have a qualified dealer do it for you..******
There are three ranges to adjust in the carb.
Low speed: Is off idle to 1/4 throttle position. The pilot jet (is inside the carb float bowl and is removable to adjust the mixture by changing the jet to a smaller or larger number size) or fuel screw found under the carb outside between the float bowl and the intake manifold.
The smaller number on the jet = a leaner mixture. The bigger number on the jet = a richer mixture. The fuel screw turned OUT is richer turned IN is leaner.
Old time round slide carbs use an air screw (on the outside side of the carb) turning the screw IN richens the mixture and OUT leans the mixture. The air screw is NOT common anymore these days.
Mid range: Is adjusted by the needle clip and is 1/4 to 3/4 throttle position.
There are slots on the needle where a clip can be moved up or down to change the mixture. The top position on the needle is leanest (letting the needle stay lower in the bore of the needle jet that it fits into. The lowest position on the needle is the richest letting the needle stay higher in the needle jet bore.
Top end: 3/4 to wide open throttle is the main jet. It is a removable jet in the float bowl. Just like the pilot jet, it is numbered and the bigger number is richer the smaller is leaner.
Checking jetting doing a base ring plug check:
You will need to mark the throttle housing (twist throttle) or remove the top of the thumb throttle cover so that you can watch which part of the throttle opening you are in.
Using some gray tape place it on the throttle housing so that you can mark the position of the throttle. Mark the sweep of the throttle from closed to wide open. Then divide the sweep into 1/3's. Zero to 1/4, 1/4 to 3/4 and 3/4 to wide open. Then make a mark on the grip or throttle lever inside the housing (thumb throttle) so that you can make note of the position when riding.
Install a fresh plug and find an area with a slight incline about a football field long.
You will want to get into 3rd gear and hold the throttle in which ever area of the carb you want to check for the length of the field. Do NOT vary the throttle, you want to hit the kill switch and pull in the clutch, coast to a stop then pull the plug.
Make note of the plugs base ring color (the area just above the threads, see the link for better pictures)
http://www.dragstuff.com/techarticles/how-to-read-plugs.html
A light tan or charcoal color is a safe setting, a cleaner "light tan" is optimum but only for the experienced tuner because you are on the edge of leanness, a dark ring is too rich and the air fuel ratio needs to be changed.
You can do this check for al three areas of the carb.
This test will need to idealy be done in 55 to 65 degrees on a not so humid day. Hotter and more humid or colder days will be totally different as far how the jetting will be.
The hotter it gets outside the richer it will run. The cooler it gets the leaner it will run.
This should give you the basic understanding of how things work in the carb and how to check the ranges of the carb to be able to adjust the jets or screws.
********ALWAYS wear the proper safety gear when doing a throttle chop/plug check, do your testing in a SAFE CONTROLLED RIDING area where the high speeds needed to do the test will not be dangerous to you and others. Be carfuel of removing spark plugs, the engine and related areas will be exceptionally hot. IF you have ANY DOUBTS on your ability to preforn this test have a qualified dealer do it for you..******