Read this First - Jetting 101

[stevo]

Read This First ----- Jetting 101




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Jetting A Sporty


A Carby Works By Mixing Air And Fuel Into A Vaporous Mixture.
We Want The Ideal Mixture So The Bike Runs Well.

Unfortunately It's Not Quite Simple...As A Carburettor Mixes Air By A Set Volume To Set Amount Of Fuel.
The Amount Of Oxygen Density In That Set Amount Of Air Varies And This Is What Gives Variations In Mixture Due To Differing Conditions.

The Required Air / Fuel Mixture Changes Depending On The Conditions

Max Power For Acceleration Is Around 12.5 Parts Air To One Part Fuel

Chemically Correct Or Stoichiometric Is 14.7 : 1

A Rich Mixture Misfire Usually Needs About 6-7 :1
A Lean Mixture Misfire Usually Needs Over 20 : 1

We Want To Try And Get The Most Fuel In There That We Can But The Limiting Factor Is The Amount Of Oxygen That We Can Fit In There And Give The Correct Ratio...

A Number Of Things Have An Effect On The Amount Of Oxygen We Can Get In There..

Just Like You Run Out Of Breath At Altitude Because Of Lack Of Oxygen So Does Your Bike..

The Air Pressure Or Barometric Pressure Is Higher At Sea Level Than At High Altitude.
The More Pressure We Have Means The More Oxygen In The Same Amount Of Air

If The Bike Is Jetted For The Correct Mixture At Sea Level And Then You Go Up The Mountains.
Then The Higher The Altitude The Less Oxygen And The Richer The Bike Will Run.
The Opposite Is Also True.

A Few Other Things Also Have An Effect.


Hotter Air ................................................. = Less Oxygen :- Richer
Colder Air ................................................. = More Oxygen :- Leaner
Higher Barometric Pressure Or Low Altitude....... = More Oxygen :- Leaner
Lower Barometric Pressure Or Higher Altitude ... = Less Oxygen :- Richer
Higher Humidity.......................................... = Less Oxygen :- Richer




One Of Basic Things That People Don't Seem To Understand Is That There Is Different Fuel Circuits N The Carb For A Reason...
Different Circuits Are Used At Different Throttle / Vacuum Positions Not Rev Ranges..... The Revs The Motor Is Doing Has Stuff All To Do With Which Circuit Is Being Used.

At Idle And To 1/8 Throttle Is Mostly Used At Cruise In Town And Just Leaving The Lights ......
This Area Is Governed Mainly By The Pilot Jet And Pilot Screw..... Although The Parallel Portion of the Needle Can Supply a Bit of Fuel Too

Highway Cruise Is Around The 1/8 - 1/4 Throttle Depending On the Speed You're Doing.....
This Area Is Governed By The Pilot Circuit And The Needle..... The Start Of The Taper On The Needle Starts To Come Into Play Significantly Here

Pushing Thru The Twisties Tends To Use The 1/4 - 1/2 Throttle
This Area Is Mainly Covered By The Needle But Both The Pilot Circuit And The Main Jet Have A Small Input

1/2 - Wide Open Throttle ... Hard Acceleration
This Area Is Covered By The Taper Of The Needle And Then The Main Jet Becomes The Main Fuel Supply From About 3/4 Throttle To WOT


A Few Points.. The Pilot Circuit Supplies Fuel Throughout The Whole Range

[stevo]

p From Cold ..............................5 - 6 Parts Air : 1 Part Fuel
Acceleration (Max Power)1/2 - WOT.........Around 12.5 : 1
Idle - 1/8............................................. Around 12 : 1
Twisties 1/4 - 1/2..................................Around 13.5 : 1
Cruise 1/8 - 1/4...................................Around 14.7 : 1




I Have Found That With Later HD's (And This Covers Sporties And Big Twins From About Mid '90's) That The Main Jet And Needle Are Very Close To Correct.
The Main Problem Area Is The Pilot Circuit Is Too Lean In Most, Not All, Instances..
An Increase In Pilot Jet Size By One Size Is Usually All That Is Needed...Sometimes 2 Depending On Intake And Exhaust And Cams
This Means If Your Bike Has A 40 Then Go To 42, 42 Then Go To A 45, 45 -> 48

At The End Of The Day A Carb Does Nothing More Than Meter The Fuel... The Motor Doesn't Care What The Carb Looks Like Or What Jets Are In There.....So Long As It's Getting The Right Fuel Air Mix For The Conditions...




Here's A Few Symptoms/Fixes

When Trying To Tune A Bike You Need To Know What Circuit Is Covering The Area Where You Feel You Have A Problem.
Changing The Main Jet Will Have No Effect On The Idle And Up To 1/2-3/4 Throttle
Whereas A Part Throttle Problem That Requires An Increase In Pilot Jet Size Will Give A Little Extra Fuel Over The Whole Range

If You Have Had The Carb Off Then Check The Float Level ....You Will Need To Refer To Your Factory Manual.... But A Very Close Guide Is The Join Line In The Floats On The Square Float Models Should Be Parallel To The Base Of The Carb When The Carb Is Held About 15* From Vertical.

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Idle - 1/8 ... Pilot Circuit

Signs Of Too Lean

If The Bikes Surges Back And Forward At A Steady Throttle At Around 3000+ Rpm
Excessive Popping Or Backfiring At Closed Throttle Deceleration..
The Bike Runs Better When Hot But Still Not Right
If You Blip The Throttle From Idle And As It Drops Back Down To Idle It Tends To Hang Up A Few Revs Too High And Then Come Down To A Lower Rpm...

Wind Out Pilot Screw By Half A Turn At A Time And See If It Improves.... If You Get Past 3 1/2 Turns Then Go Up To Next Size Pilot Jet


Other Things That Will Give A Lean Condition Are Intake Leaks..... If In Doubt Then Replace Intake Seals And Lube Them With A Silicon/Rubber Grease As They Are Assembled...


Sign Of Too Rich

Sooty Plugs..... Too Large A Pilot Circuit Will Soot Your Plugs Up Way More Than A Rich Main Circuit
The Bike Runs Better When Cold But Still Not Right...
If The Bike Doesn't Require Choke To Start And Idle From Cold Then You Are Too Rich
If The Throttle Is Blipped From Idle And The Revs Drop Below Normal Idle And Then Come Back Up To Normal Idle....This Will Be Worse When The Motor Is Hot.

To Correct... Wind Pilot Screw In 1/2 Turn At A Time ... If It Gets Less To Only 1 Turn Out Then Reduce The Pilot Jet By One Size

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1/4 - 3/4

This Is A Hard One To Pick........
Try Running In 2nd Or 3rd Gear And Crack The Throttle...

If It Pulls Clean ... Leave It....It Takes A Lot Of Experience To Pick The Difference Between A Rich Or Lean Condition Here...

I've Found That Needle Heights From Standard To About 0.030" Raised Is The Range For Most...This Is Done With A Small Shim Under The Head Of The Needle...Make Sure That The OD Of The Shim Is No Larger Than The Needle Head Or The Needle Will Not Sit Properly And Will Float..

A Lean Condition Will Tend To Surge Or Feel Flat.... A Rich Will Tend To Feel A Bit Soggy...

Try And Take Notes On The Weather ... If It Is Better When The Motor Is Cold Or When You're At Sea Level Or The Ambient Temp Is Lower Then You Are Probably Rich

If It Is Better When The Motor Is Hot Or You're At A Higher Altitude Or The Ambient Is Higher ...Then You're Probably Lean


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Wide Open Throttle .. WOT

A Main Jet Guide To Rich Or Lean ...


A Few Points ... Your Lower Speed Circuits Tend To Work More Off Vacuum Than Airflow..... Whereas The Main Jet Or WOT Tends To Work Off Airflow.

WOT Throttle Runs Are Best Done At The Drag Strip And You Want The Jet That Gives The Best Mph Thru The Traps...... I Prefer To Start Rich And Come Back Down While The Mph Keeps Increasing Until The Mph Drops ... Then Go Back Up A Size...But Ya Can Start Lean And Go Up One Size At A Time Until The Mph Drops And Then Come Back One Size..

If It Runs Better At Night .... Ya Probably Rich ... If It Runs Better In The Afternoon, Ya Probably Lean..

If You Don't Have Access To A Drag Strip Then A Dyno Is Good For WOT Runs But Not Very Good For Part Throttle Runs Unless You Have A Good Operator..

Without Access To A Drag Strip Or Dyno, Here's A Few Tips..

As Stated If It Runs Better When It's Cold Ya Probably Rich.....
If It Runs Better When It's Hot Then Ya Probably Lean....

Another Way Is To Take It Past 5000 In A High Gear At WOT
Then Close It To About 7/8 Throttle
If The Engine Accelerates A Touch Then You Are Lean....
If The Engine Hesitates, Then Ya Probably Rich...
It Should Just Slow Slightly...

If The Engine Pops At The Change Rpm At Full Throttle Thru The Gears It Is Usually Lean.


Another Guide For Rich/Lean ...

If You Remove The Air Cleaner And The Bike Improves Then It Is Probably Rich

If You Tape Up Half Your Air Cleaner Element And It Improves Then You're Probably Lean


Turn The Fuel Tap Off And Keep Riding...

If It Improves As The Fuel Level Drops, Then It's Rich
If It Gets Worse As The Fuel Level Drops Then It's Lean


If It Runs Better In That Range When The Air/Motor Is Cold It's Rich

If It Runs Better In That Range When The Air/Motor Is Hot It's Lean

[hawk mechanic]

Thanks Steve, we all do appreciate the knowledge we gain from your threads.

Joe

[semjpm]

Thank you. Your reposting is very much appreciated.

SEMJPM

[jamman]

thanks Stevo, you put it in terms folks can understand..

[racerwill]

tons of good info here.... good job....

thanks Stevo

Ww

[BWP 5p]

Thanks Stevo....Classy of you

[stevo]

I wont be updating this here but in about a month or more when I've had the time to sit down and write some more I'll write this up as I should have done it and I'll put up some articles on tuning and other issues that expand on jetting on my yahoo group, stevos_shed ....

There is more than enough info here for the vast majority of people..in fact more info than most people will ever get anywhere near using.... but those that had read this, feel they understand some or most of it but wish to know more are welcome to come over and chat.... I feel more comfortable in a small tech/information based area, like this place was to start with....



Jetting is often used to correct other areas of the motor when it shouldn't be... and many other parts of the motor affect it's tuning, such as exhaust and ignition timing as major players and compression, cams and other things that also add to the whole system... if you change any of these things it may also affect the jetting...

[david]

Thanks man

[Casper]

Thanks for re-posting it, Stevo. Heaven knows this thread has helped a lot of people (myself included) and saved a lot of repeated questions on this board. Glad to see it back!