View Full Version : how to tell if lean or rich for 2 & 4 stroke

17-05-2003, 02:15 AM
how do we generally tell if we are running lean or rich for 2 & 4 stroke machines besides the obvious spooge & color of the spark plugs etc etc,
i mean when we ride the bike can we tell with some degree of
accuracy without doing a plug chop or something?

17-05-2003, 10:06 AM
rich, (on my wr200 2 stroke anyway) tends to go all crackly and crappy sounding. lean supposedly bogs down or detonates, but I think if you're lean at full throttle you can still cause big damage before that point, so it's a concern... :Q

17-05-2003, 07:41 PM
for a 2-stroke check how much smoke is coming out of the exhaust!!! if lots is then u are runnin it to rich. but if none is coming out a little to lean! but if only a little comes out then it should b fine!

well thats how i check!!
correct me if im wrong!

17-05-2003, 08:43 PM
Hi dkwok,

It's alot easier to tell if your 2 stroke is running rich or lean than a 4 stroke. A 4 stroke bike will generally take a bit more pushishment from bad jetting than a 2 stroke because a 4 stroke bike is still getting good lubrication even though it may be running quite lean... do that to a 2 stroke and your motor will eventually become a solid lump!

With a 2 stroke, if your pilot jet is too rich it'll have a hard time idling and will eventually stall. This is what was wrong with my YZWR. My pilot jet was too rich and wouldn't idle properly so i turned up the idle screw (which let in more air) and that fixed the idle problem... but it just created another problem.
As it turns out, my turning up the idle screw was just a band-aid fix for poor jetting.
Next time i started up the bike, i pulled out the choke as usual and kickstarted the bike. Well.. next thing you know my bike was revving it's arse off because the choke circuit was providing heaps of fuel and my band-aid fix was providing heaps of air !!
I knew straight away that my pilot jet was way too rich.
Another symptom you'll find is when you take off, you'll get heaps of smoke and spooge and it'll feel blubbery until it clears.

If it's lean, after you blip the throttle it will hang (rev high) for a while until it settles back down to a normal idle. If it's really lean it'll make a boooowang sound when you take off which means that it'll bog and then suddenly hit hard.

If the needle or main jet is too lean it'll rev out but feels like it has less power and it'll run really hot. After a while your engine will make knocking noises and then eventually seize.
A rich main jet or needle will make it feel blubbery, spit spooge all over your mates and suffocate them with smoke.

When a 4 stroke runs lean it'll feel flat in the power department or bog quite noticeably. This is why they have accelerator pumps on the latest 4 strokes. With older 4 strokes when they're idling and you abruptly open the throttle, it'll bog and even stall. The accelerator pump squirts a stream of fuel down the throat of the carby to, hopefully, overcome this problem. A way to tell if your main jet is too lean is to back off slightly. If it accelerates or feels like it has more power, then it's lean. Another way is to gently pull the choke up while riding along. If it goes better when you do this, it's lean.
When rich they're just slower to respond (sluggish) and have a higher fuel consumption rate. When you rev it, you'll see a black cloud of exhaust fumes coming out your pipe.

Don't assume that your bike is rich or lean due to bad jetting.
Always eliminate all other variables before changing the jets.
Make sure your air filter is clean, carby is clean, spark plug is the correct heat range, and the bike is mechanically ok.
Some mechanical problems like blown head gaskets, blown crankcase seals or gummed up power valves can imitate poor jetting.

When talking about being rich or lean, we're actually referring to air/fuel ratio.. not oil/fuel ratio. When you add more oil to your fuel (eg. going from 40:1 to 30:1) your actually leaning out your air/fuel mixture because the extra oil takes up the space that the fuel would have otherwise taken up. When going from 30:1 to 40:1 you're richening up the air/fuel ratio because now the fuel is taking up the space that the oil used to take up.

Hope this answers your question.

Mike. :bzr

17-05-2003, 11:38 PM
thanks alot,
my bike would not rev out at full throttle,
someone ask me to try a 1 to 2 size smaller mj,
i jus dont wish to fry it???

18-05-2003, 12:09 AM
Some damn good descriptions there, Mike. Easy to understand and follow. :)

18-05-2003, 05:00 AM
I picked this up from the Keihin website... there may be more stuff there about 2-smokers etc.. but this gives a good idea of how the different jets effect the fuel/air flow.

Jetting Your Slide Valve Carburettors

All Keihin carburettors are pre-jetted for bolt-on operation. Carburettors are jetted using stock motorcycles and watercrafts. Any major engine modifications like higher compression pistons and racing exhaust systems may require minor jetting adjustments. The following is a guideline for jetting Keihin carburettors. Perform the jetting in the order given below.

1) Correct Float Height Before changing any jetting parts, check the carburettor floats for correct height. Measure the height from the bottom of the float to the Throttle Opening. Carburettor-body gasket surface. Correct height can be found on the chart. When checking the float height, the float should be resting, but not depressing, the spring-loaded float valve pin. This can be done by tilting the carburettor until the float tab just makes contact with the valve pin. If adjustment is needed, bend the metal tab on the float arm until correct height is obtained. --see example #1Float Height NOTE: (See Jetting instruction #1for correct pressure)
ALL 38 24-38 30-34 36-38
16mm 6.5mm 14mm 20mm 22.5mm
ALL ALL 28 35-39
9mm 14mm 19mm 16mm

2) Idle Set idle speed to proper rpm, by adjusting the IDLE SPEED SCREW. Turn the IDLE MIXTURE SCREW or the AIR SCREW to for correct procedure, achieve highest speed and best response. The IDLE MIXTURE SCREW (FCR) controls fuel delivery to the idle port and the SCREW is located on the engine side of the carburettor slide. Turning the IDLE MIXTURE SCREW out will make idle and off idle richer. Turning IDLE MIXTURE SCREW (CR, PWK, PJ, PE) controls the amount of air to the IDLE and SLOW CIRCUIT. This SCREW is located on the air cleaner side of the throttle slide and turning the SCREW out will lean the mixture and turning the SCREW in (clockwise) will richen the mixture.

3) Off Idle To 1/4 Throttle The SLOW JET and SLOW AIR JET are most effective in this range. When you want a richer mixture in this range, use a larger SLOW JET or a smaller SLOW AIR JET. The opposite holds true for a leaner mixture.
General Jet Needle Changes

4) 1/4 TO 3/4 Throttle The JET NEEDLE is the most effective component in the range. Changing the STRAIGHT DIAMETER (D) will change the calibration in the transition range from the SLOW circuit to the MAIN circuit (1/8 to 1/4) throttle. A smaller diameter will make this range richer and a larger diameter will lean this range. TAPER (A) changes are only made if there is a problem balancing the calibration between 1/4 and 3/4 throttle. If the mixture is rich at 1/4 throttle and lean at 3/4 throttle, a JET NEEDLE with a larger taper is needed. If mixture is lean at 1/4 throttle and rich at 3/4 throttle, change to smaller taper. If the calibration is lean from 1/4 to 3/4 throttle, raise the JET NEEDLE by lowering clip position, or use JET NEEDLE with shorter length (L1). If the calibration is rich, lower the JET NEEDLE with a longer (L1).

5) Wide Open Throttle Changing the MAIN JET affects this range. Select the size of MAIN JET which offers the best W.O.T. performance, then install one size larger MAIN JET for ideal engine durability.

6) Accelerator Pump (for FCR) In normal applications, the ACCELERATOR PUMP should not require any adjustment. If a rich stumble occurs as the throttles are opened, the ACCELERATOR PUMP timing can be delayed by widening the gap on the ACCELERATOR PUMP linkage where it makes contact with the plastic lever. Reducing the gap will cause the ACCELERATOR PUMP to deliver fuel earlier.

7) Throttle Valve Cut-Away (FOR CR, PWK, PJ, & PE) THROTTLE VALVE CUT-AWAY will influence the calibration in the area of 1/8 to 1/4 throttle. What the CUT-AWAY does is change the air velocity over the MAIN NEEDLE JET which changes when the MAIN SYSTEM begins delivering fuel. If the transition is lean change the throttle valve to a valve which has less CUT-AWAY (lower number). If this range is rich use a throttle valve with more valve CUT-AWAY (higher number).


18-05-2003, 12:43 PM
Thanks Mike and Crusty.

I'm going to put a new carby on my bike in a few weeks and I was a bit worried about jetting it correctly. This info should help me out a bit.

23-05-2003, 01:15 PM
damn i get so confused about jetting, i never knew that if alot of smoke blew outa the exhaust it was because your jets were too rich, i thought it was because there was too much oil, i always knew a bit of smoke was supposed to come out because of the burning oil, and when i screwed up on my mixing ratio it blew tons more smoke, anyhow ill pick up some jets and some new oil and rejet it up right.

big fella
31-05-2003, 01:49 PM
Jetting Facts Fiction, or getting to know your FCR

Currently we are riding the crest of a wave …… 4 stroke mania!
The race track and trails are echoing with the sounds of 4 stoke thunder, most of us are getting acquainted with a four stoke for the first time after a life time of racing two strokes.
Chances are you have a new YZ/WR Yamaha, EXC /SX KTM or maybe a new CR450 Honda parked in your garage.
You’ve probably looked up under the tank and caught a glimpse of the carbie, wedged in a impossibly tight sandwich of engine air boots, cables and frame rails! Hose’s galore! Best hope you never have to pull that puppy out!.........................
Sure as death and taxes one day you’ll be pulling it out wether it’s for preventative maintenance, re-jetting for weather changes, that fancy new pipe you just bolted on, or may be to drain the water and mud from a failed creek crossing!
But one day it will have to come out!
Lucky all the above mention bikes have one thing in common, they all have a variant the same carbie fitted, the Keihin FCR .
While how you shoe horn that puppy in or out we will leave up to you we will offer the following advise! Read on.

1/ The FCR History and How does it work?

The FCR carbie is for the most part a lot like every other carb around it has the same type of adjustments or metering circuits. It uses the venturi effect to draw fuel up from the bowl, like water up a straw. Its flat side design makes for fast clean throttle control. With the slides guillotine like shape and thin cross-section barely disturbing the smooth bore design . The slide is also positioned close as possible to the front of the carbie, this helps throttle response. The KTM’s and early YZ’s and WR’s still carry the tell tail reminders of the FCR’s past. The FCR was originally made for road race bikes that used a semi down draft design meaning the carb was tilted forward at the top. This is reflected in the angle of the slide , looking raked back with the carb mounted straight up and down as on most dirt bikes.
The Keihin fitted to the current YZ and WR have a new “Built for dirt bikes” design with the slide straight up and down and two stroke style pilot jets but are effectively the same in every way that counts.
Where the FCR differs from the two stroke carb’s we grew up with is in the accelerator pump. This gismo squirts fuel in to the intake as you open the throttle. A small rubber diaphragm is depressed by a cam/arm/spring arrangement activated by the throttles opening.
The arm holds pressure on the diaphragm forcing fuel up and into the intake much like a water pistol (don’t take my word for it next time the carb’s off try giving the throttle a turn just make sure the bowl is full of fuel , point the carb away from you in a flame safe environment of cause).
This feature supplies fuel at a time that the engine needs it, but does not yet have the vacuum to pull the fuel up and into the motor. This is one of the ways that they made the throttle response so much faster on these modern 4 strokes.

It’s best to remember that which fuel circuits are in play is governed by throttle position not by engine RPM. If your not sure at which part of the throttle opening you having trouble you can simply mark your throttle grip and housing with a paint marking pen or some white out from the office.
First make a mark both on the throttle grip and the housing with a line, when the throttle is closed. Open the throttle fully and make the grip again inline with the original mark on the housing, you have now marked close and full throttle make three more lines on the grip at 1/4 and ½ and ¾ throttle. Please don’t try to look at the markings if you are riding the bike. As this can be dangerous particularly at above ½ throttle! This is just helps to narrow the affected range and jet circuits involved

Remember these circuits overlap each other as the throttle opens.
The metering circuits and their descriptions and available adjustments are:

•Pilot jet (adjustable with optional jets)
*Primarily affects idle through 1/4 throttle opening.

•Air/fuel screw (Screw adjustment at underside front of carbie)
*Controls fuel flow into carbie in front of slide.
* Primarily affects idle through 1/4 throttle opening.
*Should be adjusted between 1 1/2 to 3 turns out. This affects throttle response right of idle.

•Needle diameter (adjustable with optional needles)
*Controls fuel flow primarily at idle through 1/4 throttle. A smaller diameter needle is richer, allowing more fuel flow. A larger diameter is leaner, allowing less fuel.

•Throttle slide cut-a-way
*Controls air flow into the carbie, this primarily affects 1/8 to 1/3 throttle

•Needle clip position
* By raising or lowering the needle clip you can adjust the point that the taper of the needle comes into play

•Needle taper (adjustable with optional Needles)
* Controls fuel flow from 1/4 to full throttle. Has a major affect on jetting. More taper means progressively more fuel, or richer at full throttle. Less taper means there is less of an increase in fuel flow, or leaner at full throttle.

•Main jet (adjustable with optional jet sizes)
* Controls fuel flow primarily 1/2 through to full throttle.

•Accelerator pump (adjustment and diaphragm size)
*This is the feature that sets the FCR apart from the two stroke carbie, the pump squirts fuel into the intake port as the throttle is cracked open. Suppling fuel before the motor builds the vacuum required to daw it up from the fuel bowl

•Choke circuit ( on /off manually)
*rich en’s the Mixter for cold staring.

•Hot start button ( on /off manually)
* leans the mixture for hot starting.

•Idle adjuster screw
*Moves the side up or down to increase /decrease air flow at idle speed

2/ Jetting 101

The first thing you must do be for attacking the carbie and throwing you cash around on expensive Jets is try to identify the symptoms before the cure. The FCR is in fact a simple Fuel mixer (air and fuel) if the ingredients change so do’s the mix, the qualities of the fuel are most important . If the fuel is suddenly thicker or thinner it will not run through the jets as before and will make the engine run lean or rich. If the octane varies, so to must the jetting. We could both grow beards before I could explain the differences between different fuels, leaded un leaded etc. With fuel retailers striving to made a buck now days, the quality and constancy of fuel never been more doubtful.
The fuel its self is the most suspect and variable factor in today’s racing four strokes jetting. For consistency always use a quality fuel, Mobil Synergy 2000 or Shell RF100 (if your not racing ) are my favourites.
Remember the weather or rather changing of the weather can affect how the bike runs. Air pressure, altitude , temperature and humidity has as much of a bearing on jetting as any thing else. In winter the air is mostly denser and contains lots of oxygen, air pressure is high. Bikes run leaner in these conditions and you my have to jet up to keep you bike running well.
And if you bike was running we a few months ago in winter but now is not, it could be because the air is now thinner with lower air pressure and less oxygen. You may have to jet down to get the old girl running right again.

With altitude it is much the same, at sea level the air pressure is high and the bike will run leaner. Up high the air is thinner and the bike will run richer.

Below is listed the three most common problems uncounted in the real world and some guidance to help you fix them.

1/ Bog/Hesitation off the bottom when throttle is cracked open.
*This is the most common compliant on the modern 4 stroke,
The hesitates ion is usually caused by the mixture being too lean in the 1/8 to ¼ throttle area.
Try the mixture screw at the front underside for the carbie. On most, Yamaha’s and SX model KTM’s it’s easy to get to, on electric start KTM’s it’s hard to access.
Remember that this screw controls the flow of fuel, not air into the motor side of the carb. This means clock wise is leaner and anti clock wise is richer.
Try turning the screw anticlockwise ¼ of a turn (from where it is set) and see if the bog is better or worst if you can’t tell, go out an other ¼ turn and try again.

This mixture screw has a fairly wide range of adjustment and effectiveness.
If the problem goes away by winding the screw out a ½ turn or so great! Your done, if you had to turn it out a full turn or more you had better check just where the screw wound up, buy counting the turns from the setting out now have to full closed (wound in) if it’s was more the 3and 1/2 turns out you can go up a pilot jet size and wind the mixture screw back to 2turns out. It’s worth checking, as if the screw is too far out it can loosen and drop right out and you don’t want that!

If the problem is worst or no better the more you wind the screw out, try ¼ in from your ordinal starting point. Again if the problem is gone with a 1/2 turn or less your work is done! If it takes more than that you can drop a pilot jet size and set the screw back around 2 turns out. This means your bike was running too rich.

Small changes in jetting like this, are usually required because of weather/altitude changes ,or because of modifications to the exhaust system.

On a machine that fails to respond to the adjustment of the mixture screw you should look for and eliminate the following.
Eliminate the fuel as a culprit. I always drain the fuel and use fresh clean fuel (the only fresh clean fuel you can rely on is RF100, AV gas, failing that, fuel from another bike that is running fine).
If fresh fuel didn’t help, you must strip the carb and clean the jets.
Water and dirt can block the pilot jet or mixture screw circuit. Also strip the accelerator pump as I have seen some stop working because of mud and dirt stopping the diaphragm from functioning. If the pump stops working the bog is always very bad.

Note; Dirty or over-oiled air filters can some times make life difficult for throttle response by cutting down the flow of air to the motor (this makes the bike run rich).

2/ Backfire, Missing or Pinging under acceleration or load

Wether is just a small hesitation or like some one stabbing the kill button but you can bet its annoying the hell out of you!
First thing is to astatine at witch throttle opening the problem occurs. !/4, ½, ¾ ,or full throttle. Then narrow down the problem more by finding out if it’s running rich or lean.
You can simply guess and start changing jets, but I like to test the condition by making the bike run leaner or richer be for I pull the carb.
This is quite easy just pull the air box cover or seat off the bike and run or ride the bike to see if the problem is better or worse. If it got better it means the bike was running rich, and jet down the appropriate jetting circuit.
If you couldn’t feel any change, put some rags in the air box to cover @1/3 of the air filter and re fit the air box lid or seat this will cut down the air coming in past the air filter and richen the mixture. If this improves the condition it means your bike was running lean, simply jet up the appropriate circuit!

There are other ways to check for riche or lean most notably by looking at the colour of the spark plug this works well when running leaded fuels but now with un-leaded the plug is mostly black all the time and every hard to read.

If you can’t get any improvement ether way you probably have a dirt, water, fuel or maybe an ignition problem.

3/ Backfire or popping while the throttle is closed.
This is an other of the most commonly reported problems and it usually plagues the owners of older bikes or those who have recently fitted an after market pipe or mufflers.

The back fire or crackle while coasting is caused by overly lean mixture
in the pipe system itself not because of the motor or any thing up stream of the exhaust manifold. Simply the exhaust gas is mixing with the air out side of the motor in the pipe and ignite the un burnt fuel that has escaped combustion in the motor. You can argue that the backfire or popping is caused by the jetting being to rich at idle and to be sure by leaning of the idle circuit some reduction of the symptoms can be achieved. (If there’s less unburnt fuel there’s less noise).
But bikes with std (restrictive exhausts with out leaks) rarely suffer from the problem, bikes with open competition style mufflers almost always pop on the run down.
Modern 4 stroke cam timing allows air to be drawn in the open end of the pipe this is because as the piston starts descending after the exhaust stroke with the exhaust valves still opened. Some fresh air is sucked in to the pipe the more open the pipe the more air goes in. That air mixes and burns in the pipe causing the banging and popping this is much worse when the pipe has leaks around joins or cranks.
Idle speed also comes into play here as the higher the idle the more likely the popping is to occur.

Battered Sav
31-05-2003, 11:41 PM
Phew, a lot of good reading in this post, well done.

31-05-2003, 11:53 PM

02-06-2003, 08:24 PM
wat abt 2 stroke carb? or rather carb for 2 strk?
i tink i stick wif 2 stroke for the timebeing?
easier to jet/cure wifout having to tinkle wif the squirt timing
& such?:BH

02-06-2003, 09:17 PM
Thanks for the effort in your reply, Big Fella. :D

big fella
02-06-2003, 11:12 PM
No big effort just a cut and paste of a old adb story i did.....

04-06-2003, 06:44 PM
this should be pinned!