Another Jetting thread....

[cheese]

Ok so I got around to checking my bikes settings and everything is factory spec apart form the slow jet which should be 32.5 and mine is 27.5.

I'm finding that its very oily around the pipe at the end of a ride. Should I try moving the clip up one?

specs are

Main 420
Needle 6cHY12-B2
Clip 2nd down from the top
Slow 27.5
Air screw 1.5 turns out (not checked mine)

Also I noticed that the slider thingy (not sure of the proper name) has a big wear spot on it and has lost its shiney surface. I'm a bit concerned that flakes of crap could go into my cylinder!!! I'll take a photo later (when I pull it to bits again!)

[Rupe]

a bit lengthy, but have a read through this mate. It might get you in the right direction. http://www.mxboards.com/forums/viewtopic.php?f=6&t=122


The slide doesn't sound good, no idea why it would do that. Someone else prob will though.

[cheese]

Yeah I've got the feeling it will be expenive...... Bah!!

[Reckless]

sorry cheese but tuning a 2 smoker is about reading the plug. Not what setting everyone else has. Then deciding where it needs leaning or richening. That depends on your bike and where your riding it ie mostly 1/2 or full throttle etc. Thunder is a good place as you can go for a short lap come in reasonably fast so the plug reading doesn't get altered by a low speed run back to the car.
Just as a rough guide the outside ring indicates low/mid = rich/lean mix controled by your Idle jet and needle thickness and height
and the inner indicates main jet size and needle height.

[cheese]

outside ring?

[Reckless]

bloody on here at 2am bloody hell workin late again!!

Cheese much better to be shown. Remind me to give you a lesson at Ardmore or Kimis. I'll take your plug out after you've given it a good hard run and show you ok.

[Danger]

Cheese in your case the first thing you should tune is your air screw seeing as you don't even know where it is set. This could be adjusted from day to day depending on conditions.

Hey Rick, ever used the wet oil line dimension in tuning? I may have mentioned this to you at some stage and although I've never used it I find it is of some interest and here are some notes related to the method that I have saved. I find a plug reading for jetting is really only of use after fitting a fresh plug and doing a full throttle run up a hill for 15-20 seconds, pulling the clutch in and hitting the kill switch and then looking down deep into the plug with a light. This gets your main jet close.


Throttle position jetting utilizing wet oil line dimension

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.

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.

Good luck!

[jimmy 2006]

holy cow danger!, thanks ! BLING

[cheese]

Mods, can we make a sticky on a jetting thread?

[Reckless]

Danger you are 100% correct about the kill switch thing that's why I was saying you need to come back to the car fast to give you a better reading. If your racing you need to do the kill switch method but I don't think me or cheese would need that finer tuning.

Wet oil line dimension.
Interesting!
I can completely see how the theory works. We where doing a similar thing when Karting by running Exhaust gas temp and cylinder head temp sensors on our digitron gauges. We could tell how hot the gasses where referenced to how hot the cylinder/plug was at any point in the tuning. A similar thing without having to take the pipe off all the time for readings. We had our motors apart so often we could read the top of the piston and check for burn patterns and detonation etc. One thing the article doesn't go into that effects detonation and heat is, timing, and the large difference you can make on different tracks with advance and retard. But in karting you are mainly on the Main jet. You are much more over the rest of the spectrum in MX.

I don't think the average joe MXer needs to delve that deeply into the above though. Anyone that looked at my Cr250 plug would say its far to rich, black and wetish on the outer and black and sooty in the inner. But I have deliberately tuned it this way to give me good low and take the nastiness out of the bike for x country. Still has all the grunt but it doesn't rip your arms out. And it still has the same plug in, I bought it with, so can't be to far off with the tuning for my usage. Now the Ktm is burning a nicer dry copper colour but I've only had one ride on that so have to get to know it a bit better yet.
Very interesting.