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Old 26 Jun 2004
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Join Date: Jun 2004
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High Altitude Carb Jetting

Jetting a 2-stroke motorcycle is somewhat difficult to do. There are lots of variables that must be observed before a bike can be jetted properly. Most professional tuners keep a detailed log of their jetting changes, and plot those with respect to altitude, temperature and air density changes. Here are some guidelines to keep in mind before attempting to jet your bike:
At higher altitudes, the bike runs richer (so you must jet leaner). The air is less dense at higher altitudes (so you're essentially getting a smaller volume of air than you do near sea level). At warmer temperatures, the bike runs richer (so you must jet leaner). During the summer, you typically want to use a smaller main jet, like a 310, for example. During the winter, you may have to go up to a 320, for example, and you may want to raise the jet needle (lower the clip). At high humidity or when it's wet, the bike also runs richer (so you must jet leaner). The most sound advice is this: If it is too rich, you foul the plug, but if it is too lean, you'll eat the engine. Also remember that 20% of the work will correct 80% of the jetting and get you 90% of the engine's power. That last 10% of power requires four times as much work.

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Name It Affects
1) air screw Idle to just off of idle (not the idle screw)

2) pilot jet 1/16 to 1/4 throttle

3) jet needle 0-1/2 throttle, work with the needle shape first, then the clip
position

4) needle jet 1/8-3/4 throttle, also called nozzle, most Keihins don't interchange

5) main jet 1/2-full throttle

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Every component overlaps the throttle range of one or more of the other components. Most people only change needle clip position, and pilot/main jets. Some older Mikuni carburetors also have a power jet (1/8-1/2 throttle). Most people check jetting by looking at the plug or the exhaust spooge. We will attempt to jet by how the bike sounds and feels in response to throttle input at different rpm starting points. We won't really address the cutaway because of price. I've never known anyone who can jet a bike right the first time; all jetting is trial and error, because every bike is different.
To develop your skill at jetting, you need to experience too rich and too lean with every jet (pilot, needle diameter, needle clip, and main jet). Only after you feel what too rich and too lean is will you be able to jet your bike perfectly. One size smaller in the main or dropping the needle a position or two can often make a huge difference in performance. Keep in mind that one area (i.e. throttle opening) can be rich while another is lean. For example, the midrange can be lean, which will cause missing, but the top end can be too rich (which would cause bogging).


Step 1: Basic Assumptions

Okay. We will assume that your air cleaner is clean and that you are using the fuel / oil mixture you plan to stay with. We will also assume that the float level is set to the manufacturer's specifications. Most manufacturers recommend using a 32:1 mixture. I personally recommend this mixture as well, as 50:1 doesn't help that much in terms of extra power, and it will definitely help to keep your engine running longer. Timing needs to be set to factory specs. Muffler and spark arrestor should be relatively clean. Use the recommended spark plug at the correct gap.


Step 2: Main Jet

The main jet controls the mixture at full throttle. It is possible to foul your plug if the main jet is too rich (but only if you're running at or almost at full throttle). Notice that we are talking about throttle openings here, not RPM. Other jets have negligible effect at full throttle.
Your objective is to get an understanding of the mixture at full throttle (wide open) operation. You need a long up-hill straight-away for this test so you can be in the top gear with the engine under load and running up in the upper RPM range.
If you hear pinging or missing, it is running lean; go larger on the main. If full throttle causes gasping and poor pulling at mid RPMs, it is again an indication of running lean, so go larger on the main. If the bike runs clean, select a larger main jet until you find the jet that causes a blurbbering (four-cycling) sound. When you experience that sound, you have found the jet that causes you to run too rich. So back off one size to a smaller jet. This is the safe main jet to use. You could go another size leaner; but you need to be careful to avoid running too lean which causes the engine to run hot and could seize the piston. It's better to jet on the rich side.


Step 3: Jet Needle Clip Position and Needle Selection

The needle jet controls the mixture from 1/8 to 3/4 throttle. At the upper end of this range the main plays a part too, but you have already selected the main jet. The slide cutaway and the needle diameter are very similar in their actual effect on jetting. It's usually easier to leave the slide stock and try to get jetting to an optimum with the needle diameter. It is also possible to foul your plug if the jet needle setting causes the engine to run too rich. Raising the clip a notch or two on most bikes will help substantially. Most bikes come from the factory jetted too rich, especially in the mid-range.
Again you need an up-hill straight away to test the jet needle setting. This time use 1/2 throttle and allow the RPMs to reach the upper RPM range and leave the throttle there. Now do your listening. Any pinging means too lean. Blurbbering means too rich. Excessive smoking is also an indication of running too rich. To adjust to a richer mixture, take out the needle and place the clip one notch lower (nearer to the sharp end of the needle). This raises the needle out of the main jet a little higher, thus allowing more gas/oil mix to pass.
To adjust to a leaner mixture, take out the needle and place the clip one notch higher (further away from to the sharp end of the needle). This drops the needle to restrict the gas/oil mixture from flowing through the main jet. Be careful when you remove the clip. It can flick away and become lost quite easily. I use a small flat screw driver or a pair of needle-nose pliers and pry the clip away from the needle. I hold the needle and clip between my thumb and finger while I do this. If you find that you are up at the top notch of the needle (there are usually 5 notches) you should get a needle with the next lower number (leaner) and place the clip in the center notch to give the equivalent mixture as the richer needle with the clip at the top. Similarly if you find that you are up at the bottom notch of the needle you should get a needle with the next higher number (richer) and place the clip in the center notch to give the equivalent mixture as the leaner needle with the clip at the bottom.
You should not have to go to any more than one size leaner needle than stock. And, I would not expect you to need to go richer than the stock needle. Next, go along slowly in one of the lower gears at less than 1/8 throttle and move the throttle quickly to 1/2 throttle. If the engine does not pull strongly, it just sort of gasps for breath and only runs well after the RPMs build up, it is an indication that the needle is too lean. (I had a 175 Penton that came
jetted that way. I thought it was characteristic of the porting and the pipe. The dealer helped by telling me that the needle was too lean and sure enough I found nice mid RPM / MID throttle power after dropping the clip a few notches.)


Step 4: Pilot Jet Selection and Air Correction Screw Adjustment

When you are at 1/4 throttle and more, the pilot jet has virtually NO effect. The pilot jet affects cranking, idle, and particularly acceleration from idle. If the pilot is too lean, the motor will hesitate when accelerating (in neutral) off idle. If it's too rich, the motor will smoke excessively, foul plugs, and be unresponsive off idle. After you get the needle right, get the bike idling by adjusting the idle stop up where it will idle slowly on its own. The air correction screw should be at 1.5 turns out to begin with. The air screw affects the pilot somewhat; it's about like changing the pilot 1/2 step (if that were possible).
Okay, now slowly screw in the air correction screw 1/4 turn. Try to find the air screw adjustment where you get maximum idle RPM. You may have to open the screw to 1.75 or 2.0 turns to get the right setting. As the idle RPM increases turn down the idle stop to return the idle RPM to a slow, correct idle. Your objective is to find the pilot jet that will give you maximum idle with the air screw set at 1.5 turns out. Remember that opening the air correction screw admits more air during almost-closed throttle making a leaner idle mixture.
Now, when you find the correct jet size you will want to do some final air screw adjusting to improve throttle response, assuming that the needle jet is properly adjusted. Let the engine idle for 5 seconds then open the throttle abruptly and be aware of how the engine responds. If it almost dies, then you need a slightly richer idle mixture so that as that mixture is gulped at abrupt throttle openings it will be just a little rich and therefore give good response. The bottom end should be a little on the rich side, while the mid and upper end (the needle and main jet) should be leaner to give you the best throttle response.
You can test this as you ride, say slowly in 2nd gear, with the throttle closed so that the engine is drawing only from the pilot. Wick it open quickly. If you find yourself doing a wheelie, your air correction screw is set right. If instead the engine bogs down and you find your nose a few inches from your triple clamp, then your idle mixture is too lean and you need to close the screw a little.
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