Jetting 101 - Everything Explained!

randelator · 01-06-2008, 12:19 PM

with this evryone should be able to do their owne jetting enjoye!!!

Overview: Jetting 101

Carburetor fuel circuits and functions:

1) Pilot jet/slow jet - Affects mixture from idle to 1/3 throttle opening. The pilot jet meters fuel to the "bottom end circuits"

2) Air screw - Meters air to pilot jet. It is usually located near the back or air box side of the carb. Turning in clockwise, will richen the pilot mixture. Turning out, counter clockwise, will lean out the pilot mixture. Average setting for most 2 strokes is 1 to 2 1/2 turns out. Refer to owner's manual.

Note: If the adjustment screw is in the front of the carb, it is a fuel screw and not an air screw; it is adjusted the opposite of the air screw. Most 4-strokes are this way.

3) Jet needle aka Needle - Affects the mixture from 1/4 throttle to full throttle. The needle is in the leanest position when the clip is on the top, and richest on the bottom. The needle calibrates the full to the change in throttle valve (slide) opening. The further down the needle is, the later the main jet comes on and the leaner the mixture at that point.

4) Main Jet - This circuit affects the mixture from 1/2 throttle to full throttle. This is your full blast top end circuit. This circuit is most accurately tuned by checking the plug for the correct color, sort of mocha brown.

5) Float/Float valve/Float level - Your owner's manual has the correct level and procedure for calibrating your bike. Here are some symptoms that would indicate the need for float adjustment.

A) If float level is too high, the float bowl overflows out the drain hoses, and fuel often blocks the air passages that allow the carb to de-pressurize. This causes a bog, or hiccup over large hits.

If float level is too low, the engine starves for fuel off idle, causing a "boooooowang" sound very similar to a pilot jet or air screw set too lean.

Jetting Guidelines; Eliminating the variables.

1) Never change more than one circuit at a time. You need this tool to tell you exactly which circuit is affecting performance and at what throttle opening.

2) Use fresh gas. Octane ratings diminish with time and the higher the octane the slower the burn and richer the mixture. The quality of today’s pump gas is decreasing. To compensate for the fuel, richer jets are required. Also note that fuel with ethanol (aka MTBE) also requires a slightly richer mixture.

3) Always start with a fresh air filter. Let a freshly oiled air filter dry at least 6 hours in advance so that the alcohol in the air filter can evaporate. Then match the carb to a motor that breathes properly. Otherwise, the engine will run slightly leaner when the clean air filter is installed.

4) When performing a plug check, use a good used spark plug. New plugs are white and are hard to read accurately for mixture checks. If you have to use a new plug be sure to put at least 15 to 20 hard minutes on it before attempting to get a reading.

5) Always check for clogged carb vent lines. Clogged lines will cause hard starting and bogging when the motor is hot. They can also cause erratic running that seems to mimic a mis-adjusted float, but worse. Performance will seem rich and acceleration will be sluggish off idle.

6) If you’re trying to track down a jetting problem, don’t change your premix oil brand or the ratio. The pre-mix ratio will determine how much fuel is available to be burned because the oil isn’t burned, and in terms of liquid volume, the oil takes up a portion of the liquid (oil + fuel). While the ratio will have a minimal impact when making small changes, remember that you’re trying to eliminate variables.

How to Jet

1) Mark your throttle housing and grip in 1/4 turn increments. Use a marker, razor blade, etc.

2) Now get the motor to operating temperature by riding around, away from the pits.

3) Once the motor is warm, ride in 2nd and 3rd gear from the low RPM to high RPM. This puts a good load on the motor and is an accurate test of performance.

* try to notice if the problem gets worse or better as the motor heats up.

4) Now try to locate the throttle opening at which the problem exists. See details just below.

Rich Jetting Symptoms and Solutions

A) Motor won’t idle and idle set screw is ineffective. Try leaning out the air screw by turning it counter clockwise a quarter turn at a time. If the air screw has no effect, install a leaner pilot jet and return the air screw to 1 turn out. Proper air screw settings are usually between 1 and 2 1/2 turns out. The correct pilot for your bike is one that allows instant off-idle throttle response, and allows the air screw to be effective between 1 and 2.5 turns out. If the problem still exists, check and clean the choke circuit and carb vent lines.

Motor stumbles off idle to 1/2 throttle, then cleans up. If everything in part A above is correct, check the needle. Stock position is usually in the middle, but check your owner’s manual to be sure. To lean out the needle, you need to raise the clip. If the needle is dropped all the way lean and problem still exists, try a leaner needle.

C. Motor will rev through the mid range then becomes gurgley (technical term) at full throttle and power is sluggish. This is a text book example of a rich main jet. This problem often occurs when an air filter becomes clogged and gets worse as the motor heats up. Choose a main jet with a smaller number and lean out the air fuel mixture one step at a time. If the problem still persists even though it’s improved a little, reinstall your original main jet and lean out the needle one position. Now fine tune the main jet with plug checks.

Plug checks are the key to fine tuning the main jet once the other circuits are set.

White Porcelain Chocolate Mocha Brown Black/Wet

Lean Perfect! Rich

Look for symmetrical burn patterns, smell for odors like burnt plastic, which is actually tranny fluid and would be indicative of a blown seal.

Lean Jetting Symptoms and Solutions

A) Motor hesitates off idle with a "Boooooooooowang" sound. This is a lean symptom and often occurs when a motor is cold. Try turning the air-fuel screw clockwise a quarter turn richer and check throttle response again until motor revs without hesitation. If the motor "hangs up" or doesn’t come right back down to idle, install a richer pilot, and reset air screw.

Motor knock knock knocks at idle when hot. Try solution A. If problem persists, perform a "leakdown pressure test" to check for ignition crank seal leakage.

C) Motor revs clean and crisp but runs really hot and lacks power. This symptom indicates a lean main jet and/or needle. Use a richer main jet and/or needle setting. This is the most common misconception about two stroke jetting: When a motor runs excessively hot and lacks power, it is on the verge of seizing. If you are on the trail and don’t have the right main jet, try raising the needle by lowering the clip. It won’t be exact but at least you won’t seize.

Note: To be certain that your problem is lean, pull on the choke and see if the problem gets better or worse. If better, your problem is a lean condition.

Thanks randelator!!!!

--------------------------------------------------------------------------------

blasterman312 · 01-06-2008, 12:22 PM

Jabes!!!!.....sticky this please!

cyphertext69 · 01-24-2008, 02:51 PM

Here are some illustrations for those who can't read.

randelator · 02-02-2008, 11:00 AM

great add-on

04-05-2008, 02:59 PM

Nicely stated and I have added to your rep. The only correction is that MTBE is not methanol it is methyl tert butyl ether. an oxiginator used to elevate octane and the primary replacement when lead was fazed out. MTBE is a carcinogen and found polluting ground water and has not been used in gasoline for 2 years.

cirame1 · 11-21-2008, 08:39 AM

vote to sticky

bentobeto · 02-02-2009, 10:20 AM

i understand but how does it work what does it do:">:">

mr_3_0_5 · 03-02-2009, 11:03 PM

someone sticky this please..

Outlawed Genetics · 06-23-2009, 12:44 AM

could backfiring be a symptom of being lean? backfire is at about 3/4 - WOT in 3rd and 4th gear. if so would a click or 2 on the needle be enough or would i need to up the main jet? as far as i know its still the stock 230 jet and only mod is a reed spacer, and a cheap aftermarket air filter like a ready filter or some shit like that. i dont have 240 jet but do have 250

i will also add that even with the backfire its got lots of power so if it is lean i dont think its by much.this bike is freshly rebuilt and i noticed it while performing the last bit of the break in process

redranger · 06-27-2009, 04:37 PM

Quote:

Originally Posted by cyphertext69

Here are some illustrations for those who can't read.

haha thanks a lot man!

01-06-2008, 12:19 PM	#1 (permalink)
randelator Can't Stop, Won't Stop! Join Date: Nov 2007 Location: quebec,canada Age: 37 Posts: 877 Rep Power: 41 Reputation:	Jetting 101 - Everything Explained! with this evryone should be able to do their owne jetting enjoye!!! Overview: Jetting 101 Carburetor fuel circuits and functions: 1) Pilot jet/slow jet - Affects mixture from idle to 1/3 throttle opening. The pilot jet meters fuel to the "bottom end circuits" 2) Air screw - Meters air to pilot jet. It is usually located near the back or air box side of the carb. Turning in clockwise, will richen the pilot mixture. Turning out, counter clockwise, will lean out the pilot mixture. Average setting for most 2 strokes is 1 to 2 1/2 turns out. Refer to owner's manual. Note: If the adjustment screw is in the front of the carb, it is a fuel screw and not an air screw; it is adjusted the opposite of the air screw. Most 4-strokes are this way. 3) Jet needle aka Needle - Affects the mixture from 1/4 throttle to full throttle. The needle is in the leanest position when the clip is on the top, and richest on the bottom. The needle calibrates the full to the change in throttle valve (slide) opening. The further down the needle is, the later the main jet comes on and the leaner the mixture at that point. 4) Main Jet - This circuit affects the mixture from 1/2 throttle to full throttle. This is your full blast top end circuit. This circuit is most accurately tuned by checking the plug for the correct color, sort of mocha brown. 5) Float/Float valve/Float level - Your owner's manual has the correct level and procedure for calibrating your bike. Here are some symptoms that would indicate the need for float adjustment. A) If float level is too high, the float bowl overflows out the drain hoses, and fuel often blocks the air passages that allow the carb to de-pressurize. This causes a bog, or hiccup over large hits. If float level is too low, the engine starves for fuel off idle, causing a "boooooowang" sound very similar to a pilot jet or air screw set too lean. Jetting Guidelines; Eliminating the variables. 1) Never change more than one circuit at a time. You need this tool to tell you exactly which circuit is affecting performance and at what throttle opening. 2) Use fresh gas. Octane ratings diminish with time and the higher the octane the slower the burn and richer the mixture. The quality of today’s pump gas is decreasing. To compensate for the fuel, richer jets are required. Also note that fuel with ethanol (aka MTBE) also requires a slightly richer mixture. 3) Always start with a fresh air filter. Let a freshly oiled air filter dry at least 6 hours in advance so that the alcohol in the air filter can evaporate. Then match the carb to a motor that breathes properly. Otherwise, the engine will run slightly leaner when the clean air filter is installed. 4) When performing a plug check, use a good used spark plug. New plugs are white and are hard to read accurately for mixture checks. If you have to use a new plug be sure to put at least 15 to 20 hard minutes on it before attempting to get a reading. 5) Always check for clogged carb vent lines. Clogged lines will cause hard starting and bogging when the motor is hot. They can also cause erratic running that seems to mimic a mis-adjusted float, but worse. Performance will seem rich and acceleration will be sluggish off idle. 6) If you’re trying to track down a jetting problem, don’t change your premix oil brand or the ratio. The pre-mix ratio will determine how much fuel is available to be burned because the oil isn’t burned, and in terms of liquid volume, the oil takes up a portion of the liquid (oil + fuel). While the ratio will have a minimal impact when making small changes, remember that you’re trying to eliminate variables. How to Jet 1) Mark your throttle housing and grip in 1/4 turn increments. Use a marker, razor blade, etc. 2) Now get the motor to operating temperature by riding around, away from the pits. 3) Once the motor is warm, ride in 2nd and 3rd gear from the low RPM to high RPM. This puts a good load on the motor and is an accurate test of performance. * try to notice if the problem gets worse or better as the motor heats up. 4) Now try to locate the throttle opening at which the problem exists. See details just below. Rich Jetting Symptoms and Solutions A) Motor won’t idle and idle set screw is ineffective. Try leaning out the air screw by turning it counter clockwise a quarter turn at a time. If the air screw has no effect, install a leaner pilot jet and return the air screw to 1 turn out. Proper air screw settings are usually between 1 and 2 1/2 turns out. The correct pilot for your bike is one that allows instant off-idle throttle response, and allows the air screw to be effective between 1 and 2.5 turns out. If the problem still exists, check and clean the choke circuit and carb vent lines. Motor stumbles off idle to 1/2 throttle, then cleans up. If everything in part A above is correct, check the needle. Stock position is usually in the middle, but check your owner’s manual to be sure. To lean out the needle, you need to raise the clip. If the needle is dropped all the way lean and problem still exists, try a leaner needle. C. Motor will rev through the mid range then becomes gurgley (technical term) at full throttle and power is sluggish. This is a text book example of a rich main jet. This problem often occurs when an air filter becomes clogged and gets worse as the motor heats up. Choose a main jet with a smaller number and lean out the air fuel mixture one step at a time. If the problem still persists even though it’s improved a little, reinstall your original main jet and lean out the needle one position. Now fine tune the main jet with plug checks. Plug checks are the key to fine tuning the main jet once the other circuits are set. White Porcelain Chocolate Mocha Brown Black/Wet Lean Perfect! Rich Look for symmetrical burn patterns, smell for odors like burnt plastic, which is actually tranny fluid and would be indicative of a blown seal. Lean Jetting Symptoms and Solutions A) Motor hesitates off idle with a "Boooooooooowang" sound. This is a lean symptom and often occurs when a motor is cold. Try turning the air-fuel screw clockwise a quarter turn richer and check throttle response again until motor revs without hesitation. If the motor "hangs up" or doesn’t come right back down to idle, install a richer pilot, and reset air screw. Motor knock knock knocks at idle when hot. Try solution A. If problem persists, perform a "leakdown pressure test" to check for ignition crank seal leakage. C) Motor revs clean and crisp but runs really hot and lacks power. This symptom indicates a lean main jet and/or needle. Use a richer main jet and/or needle setting. This is the most common misconception about two stroke jetting: When a motor runs excessively hot and lacks power, it is on the verge of seizing. If you are on the trail and don’t have the right main jet, try raising the needle by lowering the clip. It won’t be exact but at least you won’t seize. Note: To be certain that your problem is lean, pull on the choke and see if the problem gets better or worse. If better, your problem is a lean condition. Thanks randelator!!!! -------------------------------------------------------------------------------- __________________ 2003 blaster g-force axle +3+1 a-arm stainless steel brake line factory7 air box +uni filter factory7 complete pipe motion-pro twist trohtle msr handle bar ct-240 full ported cylinder kits +4 long rod 36mil keihin pwk air stryker carb ricky stator full adjustable timing plate 400ex rear shock banshee front shock(elka comming soon) yz125 cdi +3 swing arm and more and more to come

01-06-2008, 12:22 PM	#2 (permalink)
blasterman312 mmm diesel smoke Join Date: Jul 2007 Location: Parry Sound, Ontario P2A2W9 Age: 19 Posts: 3,272 Rep Power: 71 Reputation:	Jabes!!!!.....sticky this please! __________________ '92 Blaster pro-circuit pipe; 260 main, k&n filter, airbox lid ON, needle on 4th slot, wheel spacers [F&R], maxxis 4SNOW tires, 22x10x9 on warrior hubs, banshee front shocks, 250r rear shock, trailtech vapor, air scoops, aluminum grab bar, rear fender mounted tail light. (2)-20w wall-e lights... LOOKING PISSED OFF!!!!

02-02-2008, 11:00 AM	#4 (permalink)
randelator Can't Stop, Won't Stop! Join Date: Nov 2007 Location: quebec,canada Age: 37 Posts: 877 Rep Power: 41 Reputation:	great add-on __________________ 2003 blaster g-force axle +3+1 a-arm stainless steel brake line factory7 air box +uni filter factory7 complete pipe motion-pro twist trohtle msr handle bar ct-240 full ported cylinder kits +4 long rod 36mil keihin pwk air stryker carb ricky stator full adjustable timing plate 400ex rear shock banshee front shock(elka comming soon) yz125 cdi +3 swing arm and more and more to come

11-21-2008, 08:39 AM	#6 (permalink)
cirame1 Addicted Join Date: Jul 2008 Posts: 381 Rep Power: 28 Reputation:	vote to sticky __________________ 2003 blaster fmf gold series w/ power core 2 vitos carbon reeds with reed spacer mikuni 28mm flat side carb ac front bumper custom grab bar fly handle bars 400 ex rear shock shee front shox +2 swing arm oil block off kit dg nerfs tors eliminated 280 main jet air box lid removed 18" razr rear tires 5 inch wheel spacers all around. 14/40 gearing ported and milled cylinder w/wiseco piston.

06-23-2009, 12:44 AM	#9 (permalink)
Outlawed Genetics Can't Stop, Won't Stop! Join Date: Jul 2008 Posts: 770 Rep Power: 32 Reputation:	Re: Jetting 101 - Everything Explained! could backfiring be a symptom of being lean? backfire is at about 3/4 - WOT in 3rd and 4th gear. if so would a click or 2 on the needle be enough or would i need to up the main jet? as far as i know its still the stock 230 jet and only mod is a reed spacer, and a cheap aftermarket air filter like a ready filter or some shit like that. i dont have 240 jet but do have 250 i will also add that even with the backfire its got lots of power so if it is lean i dont think its by much.this bike is freshly rebuilt and i noticed it while performing the last bit of the break in process __________________ 94 red/black blaster (pictured) FMF gold series fatty vito's 240cc big boremetalic candy red frame vito's performance clutch Flotek ported Vito's reed spacer vito's 3mm crank Vito's kill switch Vito's high flow filter vforce3's black "FLY" hi-bend bars Motion Pro clutch cable Flame grips +4 swingarm warrior stemYFZ450 rear shockBlack "Fast 1" Tri-cross bumperDouglas chrome kenda xcr rubber 14/38 gearing 3" spacers 04 red/white blaster pro circuit pipe boyesen pro series reeds vitos high flow filter black fly bars blk/red flame grips carlisle badlands 3" spacersmikuni 28mm flatside

04-05-2008, 02:59 PM	#5 (permalink)
rix005 Guest Posts: n/a	Nicely stated and I have added to your rep. The only correction is that MTBE is not methanol it is methyl tert butyl ether. an oxiginator used to elevate octane and the primary replacement when lead was fazed out. MTBE is a carcinogen and found polluting ground water and has not been used in gasoline for 2 years.

02-02-2009, 10:20 AM	#7 (permalink)
bentobeto Newbie Join Date: Feb 2009 Location: Mozambique, Maputo Posts: 17 Rep Power: 0 Reputation:	i understand but how does it work what does it do:">:">

LinkBacks (?) LinkBack to this Thread: http://www.blasterforum.com/do-yourself-20/jetting-101-everything-explained-1625/
Posted By	For	Type	Date
Blaster Chat	This thread	Refback	06-03-2008 06:26 PM
Digg - Jetting 101 - Everything Explained!	This thread	Refback	03-01-2008 06:50 PM
Digg / Autos / Upcoming	This thread	Refback	02-20-2008 08:39 AM
Yamaha Blaster Forum - Powered by vBulletin	This thread	Refback	01-06-2008 04:19 PM

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Sponsored links