Jetting Guide by Spanky !

Spanky's Jetting Guide:

A correctly jetted carb makes a tremendous difference in the torque, midrange pull, top-end pull, and over-rev of your engine. If you have never jetted your bike correctly, you will almost certainly gain some performance at some point in the bike's powerband.

A cleanly jetted pilot circuit can be the difference between having to clutch the bike out of a turn or not. The needle can make all the difference in the world for the power of the machine in most situations, as it controls the throttle range that most riders spend most of their time using.

A correctly sized main jet could mean the difference between being able to rev out high enough to not have to shift one more time at the end of the straight, or the power falling flat on top and requiring you to make that extra shift.

Are you fouling plugs? Many people will tell you all sorts of band-aid fixes, from running less oil, to running a hotter plug. Both are incorrect fixes for plug fouling. It's all in the jetting.

The only way to know what jetting changes you will need is by trial-and-error. No one can give you jetting specs, because every bike is different, every rider has a different style, and jetting is totally weather dependent. Unless the person telling you what jets to use is riding an identical bike, on the exact same track, at the same time, his recommendations are meaningless.

Jetting is fairly simple, and is a useful skill to learn if you ride a two-stroke and want it to perform at it's best.

It's very important that you start with the pilot circuit. The reason is simple. The pilot circuit affects the entire throttle range. When you are at full throttle, the main jet is the primary fuel metering device, but the pilot is still delivering fuel as well, adding to the total amount of fuel that your engine is receiving.

Before you start to rejet your bike, you need a clean air filter, a fresh plug (actually you need several plugs to do plug-chop tests for the main jet), and fresh fuel.

One important detail: Make sure the engine is in good mechanical condition. If your engine has a worn top-end, fix it first. Trying to jet a worn out engine is a waste of time. The same goes for reeds that don't seal properly, and a silencer that needs re-packing. Worn reeds will mimic rich jetting, and worn rings will mimic lean jetting.

Before you start the jet testing, Install a fresh plug. Set the float level to the proper specs, an incorrect float height will affect your jetting all across the throttle range.

Warm the bike completely, and shut it off.

As already stated, start with the pilot circuit. Turn the air screw all the way in, then turn it out 1.5 turns to start. Start the engine, and turn the idle screw in until you get a slightly fast idle, or hold the throttle just barely cracked, to keep the engine idleing. Turn the airscrew slowly in, and then out, until you find the point where the idle is fastest. Stop there. Do not open the screw any farther, or your throttle response will be flat and mushy, and the bike may even bog. This is only the starting point, we will still have to tune the air screw for the best response.

Now is the time to determine if you have the correct pilot installed in your carb. The air screw position determines this for you, making it very simple. If your air screw is less than 1 turn from closed, you need a larger pilot jet. If it is more than 2.5 turns from closed, you need a smaller pilot jet.

Once you have determined (and installed it if it's neccessary to change it) the correct pilot jet size, and tuned the air screw for the fastest idle, it's time to tune the air screw for the best throttle response. Again, make sure the bike is at full operating temperature. Set the idle back down (the bike should still idle, despite what you read in the Moto Tabloids), and ride the bike, using closed-to-1/4 throttle transitions. Turn the air screw slightly in either direction until you find the point that gives you the best response when cracking the throttle open. Most bikes are sensitive to changes as small as 1/8 of a turn.

The air screw is not a set-it-and-leave-it adjustment. You have to constantly re-adjust the air screw to compensate for changing outdoor temps and humidity. An air screw setting that is perfect in the cool morning air will likely be too rich in the heat of the mid-day.

Now, it's time to work on the needle. Mark the throttle grip at 1/4 and 3/4 openings. Ride the bike between these two marks. If the bike bogs for a second before responding to throttle, lower the clip (raising the needle) a notch at a time until the engine picks up smoothly. If the bike sputters or sounds rough when giving it throttle, raise the clip (lowering the needle) until it runs cleanly. There isn't really any way to test the needle other than by feel, but it's usually quite obvious when it's right or wrong.

Last is the main jet. The main jet affects from 1/2 to full throttle. The easiest way to test it is to do a throttle-chop test. With the bike fully warmed up, find a long straight, and install a fresh plug. Start the engine, and do a full-throttle run down the straight, through all gears. As soon as the bike tops out, pull the clutch in, and kill the engine, coasting to a stop. Remove the plug, and look deep down inside the threads, at the base of the insulator. If it is white or gray, the main is too lean. If it is dark brown or black, the main is too rich. The correct color is a medium-dark mocha brown or tan.

Once you have a little bit of experience with jetting changes, and you start to learn the difference in feel between "rich" and "lean", you'll begine to learn, just from the sound of the exhaust and the feel of the power, not only if the bike is running rich or lean, but even which one of the carb circuits is the culprit.

The slide is also a tuning variable for jetting, but slides are very expensive, and few bikes need different slides, so we won't go into that here.

Keep in mind, even though this article is intended primarily for two-strokes, four-strokes also need proper jetting to perform right, although they are not quite as fussy as their oil-burning cousins. The only real difference in the two is with the pilot circuit.

Two-strokes have an air screw that you screw in to make the jetting richer, ansd screw out to make the jetting leaner. Four-strokes, on the other hand, have a fuel adjustment screw that you screw in to make the jetting leaner, and out to make it richer.

Great idea to keep this as a sticky! It will save a lot of space and room on this new board.

Keep up the good work editor...

kawiracer27 wrote:

Great idea to keep this as a sticky! It will save a lot of space and room on this new board.

Keep up the good work editor...

Wow thanks !!

Thank's for taking the credit for my idea Editor LMAO

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so u cant jet a fourstroke?

Yes you can. Check the end of Spank's guide.

so you dont have to buy anything? all you have to do is adjust some screws on the carb? which screws would it be on my drz125? how come they dont "jet" them as fast as they could go right out of the factory?

please some help me out here!!!!!

You might have to buy different sized jet's.

ryanr1490 wrote:

so you dont have to buy anything? all you have to do is adjust some screws on the carb? which screws would it be on my drz125? how come they dont "jet" them as fast as they could go right out of the factory?

please some help me out here!!!!!

What you have to buy is dependant on how your jetting is from the factory.

On a DRz125, the jetting will be a bit lean in stock trim. The more modifications you do to help the engine breathe better (uncork or replace the exhaust, open up the sirbox, and put a good high flow filter on, etc...) the more you will need to richen the jetting.

How much of a change will you have to make? No one can tell you that. Only YOU, riding and testing your machine can determine what jets need to be changed. Spanky's guide is just that, a guide. It is a great tool to have at your disposal that will make the job of jetting a bit easier to understand. But YOU still have to do the work and make the determination for yourself.

Best of luck!
-Steve

^^^ Yeah trust this guy...he taught me how to jet over AIM.

I ended up bringing my carburetor inside...but I forgot to drain it. When I tipped it upside down... OH $#!^ lol

I have been running 40:1 that is what the previous owner told me to do but I looked at an online manual at bikebandit.com and it said my bike was supposed to run at 20:1, do you think that the previous owner jetted it in a way so that is runs at 40:1?

probably wanna ask that question somewhwer else.

oil seeping out of the silencer is a sign of poor jetting right?

thanks

so is there a website, that go step by step on how to build new tops ends... how to jet a bike etc??? if so please pm the website. thanks..

YZ149 wrote:

oil seeping out of the silencer is a sign of poor jetting right?

thanks

Oil seeping out of the silencer (if you have a 2 stroke) usually means you're not "Piping it" or basically riding your bike without blowing out the carbon and basically it'll add up in the silencer and come out sometime. New silencer packing and a farther twist of the throttle might be necessary.

Jk