Jetting to the wet oil line dimension


die you sycophant !
May 22, 2000
Throttle position jetting utilizing wet oil line dimension

“Give a man a fish and he eats for a day. Teach him to fish and he’s likely to get into the fishing business and feed hundreds.” It’s apparent that there is a need for some basic info to get started jetting bikes based on the number of comments that start with “How do I?”

This is a technique for optimizing jetting at various throttle positions utilizing the wet oil line boundary measurement in the exhaust port of 2 stroke engines. Since fuels, oils and riding techniques and conditions vary widely throughout the world a method of optimizing jetting for specific conditions would be useful. Plug reading has benefit primarily at wide open throttle positions and readings can be skewed by oil choice, fuel additives and plug heat range. In addition a reliable way to evaluate jetting at part throttle positions is helpful. Ideally data acquisition equipment would be best but it may beyond the resources of the enthusiast. It also may be too fragile to be used in an off road environment. Additionally the lag time between rider noticing changes and reacting may be too slow to prevent serious engine damage. The ride and feel method is widely used and those who are adept at that type of jetting may find this technique will allow a more accurate assessment of the jetting package. With a minimum amount of fuss it is possible to pull the pipe after riding a particular section to evaluate your jetting under those specific conditions.

The wet oil line is the boundary between the hot dry exhaust gas and the fresh charge in the exhaust port. Since two stroke engines utilize exhaust port supercharging with expansion chambers the fresh charge is drawn through the cylinder into the pipe and stuffed back into the exhaust port by the return pressure wave. The speed of the pressure wave is affected by the temperature of the exhaust gas. When the mixture is too lean the hot gas enters the combustion chamber and huge pressure rises or detonation occurs. If the mixture is too rich the return pulse is weak and lost power results. Fuel and oil choice as well as timing affect speed of the pressure rise during combustion so those variables should not be changed until an adequate baseline is established.

The guide is an extremely valuable tool to aid in making adjustments to the fuel curve. James provided invaluable insight into the nuances of altering the fuel curve. HGNR.Com enthusiastically provided the basic information and is a great resource for those wishing to delve further into the subject. Oil choice is a key component to the success of this procedure so you don’t want to overlook the value of clean burning oil.

Getting started

Fresh fuel, oil and silencer packing are a must. Enough room to accelerate up to 3rd or 4th gear at ¼ throttle increments and hold till speed stabilizes is necessary as well. The engine should be brought up to temperature and several hard acceleration runs made to duplicate racing conditions.

Step 1) Quarter throttle

With the engine hot accelerate hard up to 3rd or 4 gear and hold the throttle at the ¼ throttle position till rpms stabilize for several seconds. Pull in the clutch as you hit the kill switch and hold the throttle at ¼ till the engine dies. After stopping pull the pipe and look at the port and head pipe. You should see a dry area in the head pipe and a wet area closer to the piston. The safe dimension is 2-1/2 to 3 inches from the piston skirt. If it is closer than 2-1/2 inches you will want to richen the jetting, more than 3 inches you may want to lean it slightly. Pilot jet, needle straight section diameter and clip position all have an effect in this area. If you are using multi taper needles you may want to test the next increment before making a change. With single taper needles clip position has the greatest effect so you may want make that adjustment first. With ¼ throttle sorted out which may take several runs move to step 2

Step 2) Half throttle

Get the engine hot again and repeat the test in the same gear as step1 holding throttle at ½ open. Check the wet oil line dimension and make adjustments to the clip position as necessary to get the wet oil line to 2-1/2 to 3 inches. If your ideal clip position is different from the ¼ throttle you will have to make adjustments to straight section diameter and pilot to get ¼ throttle in line. If you are using multi taper needles it may be necessary to change pilot and or throttle slide cutaway to adjust ¼ throttle readings.

Step 3) ¾ Throttle

Repeat you basic procedures but this time using ¾ throttle. Needle taper is the main variable but main jet will have an effect. You may change taper or get the main optimized and return to this section

Step 4) Full throttle

This test is about the main jet. With the main jet right you will be surprised at the overev hiding in your engine. You will want this test to closely resemble your riding conditions as much as possible. If you are riding in the woods you are likely to be on the main for a few seconds at a time or longer if you are climbing a hill. Desert riders may be on the main for much longer periods so your test should be as close to real riding conditions as possible. Sand riders will use larger mains still.

Danger signs

Knocking, pinging, rattling, overheating are signs that you are lean, too lean, and damage is occurring rapidly and in a big way. Signs of scuffing on the exhaust side of the piston or a bright stripe from the crown of the piston to below the ring lands or a horizontal smile on the skirt are signs that you are lean somewhere. You may find this occurring as you make changes to your fuel curve going thru the steps so you will have to back up and correct your jetting before you go further.

Sound advice and other considerations

Like all empirical methods this techniques has limitations. In conjunction with good notes and observations it will make a higher level of performance possible. Ignore temperature, barometric pressure, altitude or make fuel, oil and timing changes and your results may be expensive failure. It is prescriptive in that it measures what is happening and tells how to improve your state of tune. Proceed cautiously and you will reap the reward of better performance. The increments used are arbitrary so you may use any increment that makes sense to you and can give repeatable readings. Once you are correctly jetted you may find yourself riding a gear higher and getting better fuel milage as the result of improve efficiency.

James Dean of, Stuart and Andy of and Rod Faulkner of Dumonde Tech oils have provided an invaluable amount of encouragement and help. Their help has made this bit of information possible and we all benefit.

Good Jetting!



Jan 7, 2003

Thank you for the fishing lesson. :thumb: Yet another usefull method to get closer to those magic numbers. I appreciate your post and will definately be rereading it in the near future.