Carburetor Jetting Procedures
Proper jetting is important not only to performance, but the life of your engine. An engine that is a little rich will perform just as well as a properly jetted engine! But an engine that is a little lean will hurt parts, and cost you money. Jetting too close, will yield an engine that may live at 70 degrees outside air temperature, but will stick the piston at 90 degrees! Always jet to the rich side for a happy engine.
The information provide is only guide lines and if done improperly will cause severe engine damage. There are many opinions on how to read spark plugs for jetting. This procedure is meant to provide a basic approach to the subject for the enthusiast. The best way to set up the Air/Fuel ratio for any engine is to use a wide band O2 sensor. If you have questions, please contact Kennedy Power Sports
The chart above provides you with what changes will affect the engines mixture vs. throttle position and its effectiveness.
The following procedure addresses the carburetors main jetting only.
1) Ride the bike for 10 minutes at a moderate pace, vary the rpm and shift up and down the gears. Once the engine is up to operating temperature you can make a jetting pass. Start by replacing the spark plug with a new one. Ride the bike at a fast pace until last gear is achieved, once in last gear hold the throttle at WOT for 3 seconds. At this point, pull in the clutch, kill ignition and brake the bike to a stop.
2) After a short cool down, pull the spark plug out. Now look at the upper 1/3rd of the insulator, if there are any tiny black specks or shiny specks of aluminum that have fused to the insulator, this indicates the first/beginning signs of detonation that will lead to severe engine damage. Detonation can be caused by excessive engine timing, wrong fuel, overheating or jetting too lean. If jetting is the cause, increase the jetting 4 sizes and repeat step 1. If the upper 1/3rd of the insulator looks good, go to step 3.
3) With a flashlight and a magnifying glass first look at the insulator part of the plug, viewing the plug from the center electrode look down the length of the insulator to its base, at this point there should be a light brown/tan colored ring. There was not sufficient time to thoroughly color the whole plug, so the nose of the insulator may still be white. Now look at the spark plug base ring (the base ring is what the ground electrode is welded to). It also should have a light brown/tan color. If the insulator color is white at the base and no color on the base ring, increase the jetting 2 sizes, if dark brown, decrease the jetting 1 size. Return to step 1 as required. Go to step 4 once completed.
4). Start by replacing the spark plug with a new one. Ride the bike at a fast pace until last gear is achieved, once in last gear hold the throttle at WOT for 6 seconds. At this point, pull in the clutch, kill ignition and brake the bike to a stop.
5) After a short cool down, pull the spark plug out. Look at the upper 1/3rd of the insulator; insure there is no indication of detonation as described in step 2. Next, with a flashlight and a magnifying glass look at the insulator part of the plug, viewing the plug from the center electrode look down the length of the insulator to its base and the spark plug base ring. There should be a light brown/tan color as described in step 3. Increase or decrease jetting as required. Go to step 6 once completed.
6) If you ride trail, motocross or sand you are done! If you run most of the time flat out in 6th gear, you should repeat a jetting pass as in step 4, but this time run through 6th gear. When running an engine at full throttle for extended periods, be sure to chop the throttle decisively to slow down, just rolling out a little can cause a lean condition and seize a well jetted engine. Repeat as required.
Note: An engine that is set up to run flat out for long periods of time, like down the road or in the desert will require richer jetting. A jetting set up for the trail, motocross or sand will be too lean if ridden as described. There are many variables that come into play when jetting, always jet a little rich!
Tip: To address 1/4th to 3/4th throttle positions mixture adjustments, substitute the throttle setting of interest for the full throttle requirement in the above procedure. For mixture adjustments from idle to 1/8th throttle, you want the engine to idle and run “clean” without loading up (too rich), but rich enough to provide sharp throttle response when quickly opened. A stumble or flat spot during a quick opening indicates a lean condition, where as lazy acceleration indicates a rich condition. Use the chart to determine the change that affects the mixture for the throttle setting your working on.
Tip: Jets are mass produced leading to diameter differences between jets or identified as the same size. Kennedy Power Sports has found it is not uncommon for a jet to be plus or minus 1 size. This makes it very difficult to jet properly because a jet change to richen may in fact lean the mixture! Kennedy Power Sports recommends the use of a pin gage set to determine size. This is the procedure we use at our shop.
The chart is located on my flickr account
Proper jetting is important not only to performance, but the life of your engine. An engine that is a little rich will perform just as well as a properly jetted engine! But an engine that is a little lean will hurt parts, and cost you money. Jetting too close, will yield an engine that may live at 70 degrees outside air temperature, but will stick the piston at 90 degrees! Always jet to the rich side for a happy engine.
The information provide is only guide lines and if done improperly will cause severe engine damage. There are many opinions on how to read spark plugs for jetting. This procedure is meant to provide a basic approach to the subject for the enthusiast. The best way to set up the Air/Fuel ratio for any engine is to use a wide band O2 sensor. If you have questions, please contact Kennedy Power Sports
The chart above provides you with what changes will affect the engines mixture vs. throttle position and its effectiveness.
The following procedure addresses the carburetors main jetting only.
1) Ride the bike for 10 minutes at a moderate pace, vary the rpm and shift up and down the gears. Once the engine is up to operating temperature you can make a jetting pass. Start by replacing the spark plug with a new one. Ride the bike at a fast pace until last gear is achieved, once in last gear hold the throttle at WOT for 3 seconds. At this point, pull in the clutch, kill ignition and brake the bike to a stop.
2) After a short cool down, pull the spark plug out. Now look at the upper 1/3rd of the insulator, if there are any tiny black specks or shiny specks of aluminum that have fused to the insulator, this indicates the first/beginning signs of detonation that will lead to severe engine damage. Detonation can be caused by excessive engine timing, wrong fuel, overheating or jetting too lean. If jetting is the cause, increase the jetting 4 sizes and repeat step 1. If the upper 1/3rd of the insulator looks good, go to step 3.
3) With a flashlight and a magnifying glass first look at the insulator part of the plug, viewing the plug from the center electrode look down the length of the insulator to its base, at this point there should be a light brown/tan colored ring. There was not sufficient time to thoroughly color the whole plug, so the nose of the insulator may still be white. Now look at the spark plug base ring (the base ring is what the ground electrode is welded to). It also should have a light brown/tan color. If the insulator color is white at the base and no color on the base ring, increase the jetting 2 sizes, if dark brown, decrease the jetting 1 size. Return to step 1 as required. Go to step 4 once completed.
4). Start by replacing the spark plug with a new one. Ride the bike at a fast pace until last gear is achieved, once in last gear hold the throttle at WOT for 6 seconds. At this point, pull in the clutch, kill ignition and brake the bike to a stop.
5) After a short cool down, pull the spark plug out. Look at the upper 1/3rd of the insulator; insure there is no indication of detonation as described in step 2. Next, with a flashlight and a magnifying glass look at the insulator part of the plug, viewing the plug from the center electrode look down the length of the insulator to its base and the spark plug base ring. There should be a light brown/tan color as described in step 3. Increase or decrease jetting as required. Go to step 6 once completed.
6) If you ride trail, motocross or sand you are done! If you run most of the time flat out in 6th gear, you should repeat a jetting pass as in step 4, but this time run through 6th gear. When running an engine at full throttle for extended periods, be sure to chop the throttle decisively to slow down, just rolling out a little can cause a lean condition and seize a well jetted engine. Repeat as required.
Note: An engine that is set up to run flat out for long periods of time, like down the road or in the desert will require richer jetting. A jetting set up for the trail, motocross or sand will be too lean if ridden as described. There are many variables that come into play when jetting, always jet a little rich!
Tip: To address 1/4th to 3/4th throttle positions mixture adjustments, substitute the throttle setting of interest for the full throttle requirement in the above procedure. For mixture adjustments from idle to 1/8th throttle, you want the engine to idle and run “clean” without loading up (too rich), but rich enough to provide sharp throttle response when quickly opened. A stumble or flat spot during a quick opening indicates a lean condition, where as lazy acceleration indicates a rich condition. Use the chart to determine the change that affects the mixture for the throttle setting your working on.
Tip: Jets are mass produced leading to diameter differences between jets or identified as the same size. Kennedy Power Sports has found it is not uncommon for a jet to be plus or minus 1 size. This makes it very difficult to jet properly because a jet change to richen may in fact lean the mixture! Kennedy Power Sports recommends the use of a pin gage set to determine size. This is the procedure we use at our shop.
The chart is located on my flickr account
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