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KTM 250sx jetting help! Fouling plugs!

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Hi I have just bought a 2006 KTM 250 sx. It's in great condition, excellent compression but I'm fouling plugs after 2-3 hours of trail riding (I'm basically putting in first 3 gears 90% of the time). The plug is a br8es and it's black and wet oily when I get it out.

So I know I'm running rich but I'm new to two strokes and although I have a vague idea I don't want to ruin my ktm. I was going to lean the needle to top clip position (leanest) but a screw on the bottom of my carby is stripped....anyway I don't know what to do next

Carby jetting is dead stock I believe. Carb is a Keihin PWK 36mm

airscrew is 1 3/4 turns out

I run 40:1 with 98 premium gas and motul 800 2 stroke

Weather is generally warm to hot around 25-35 degrees celcius

I'm 265 meters above sea level (866 ft)

Any help would be greatly appreciated

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I have an 06' 250sx. Do you have the owners manual?

This is the jetting chart from the manual. The chart is based on a ratio of 50:1 with 98 oct. pump gas and a clean air filter.

I run 52:1 with Golden Spectro and have never had any problem with any of my bikes. I have never seized a motor or had a bearing go out in my life. I have been using this ratio with Golden Spectro since my first bike in 1970.

06250sxJettingChart.jpg

Edited by Larry63r

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Thanks Larry63r, for the chart. And no I dont have an owners manual. I'm not racing so I guess 50:1 wouldnt hurt. As for the air filter I will clean(last clean 1ride ago) it today and change needle clip position.

Thanks for the help so far guys :smirk:

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Thanks Larry63r, for the chart. And no I dont have an owners manual. I'm not racing so I guess 50:1 wouldnt hurt. As for the air filter I will clean(last clean 1ride ago) it today and change needle clip position.

Thanks for the help so far guys :smirk:

Check your oil to make sure it is OK to run at 50:1. All mix oil is different. Some oil is made to run at 20:1 or 30:1 and some is made to run at 40:1- 60:1.

I would not run an oil at 50:1 if it was not formulated to run at that ratio.

I have several air filters and a few baskets. I change air filters at least every time I ride my bike. Sometimes I even change between motos. I always take a filter oil it, put a basket in it and stuff it in a zip-lock to take with me when I go ride. I don't always need it but if it is dusty I can change a filter in a matter of seconds between motos.

Edited by Larry63r

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I cleaned my air filter (getting a new one this week) and went riding on 20 degree celcius day (sunday) and I fouled after 1 hours riding, I went to start and she dindnt fire. Whacked a new plug and she went like a dream. My next step is to take the clip position up to lean it out. Would this solve the problem?

Or will it need a smaller pilot jet/main as I don't know what the stock jets are?

Anyone know the stock jets??

Thanks heaps again guys :smirk:

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I cleaned my air filter (getting a new one this week) and went riding on 20 degree celcius day (sunday) and I fouled after 1 hours riding, I went to start and she dindnt fire. Whacked a new plug and she went like a dream. My next step is to take the clip position up to lean it out. Would this solve the problem?

Or will it need a smaller pilot jet/main as I don't know what the stock jets are?

Anyone know the stock jets??

Thanks heaps again guys :smirk:

The stock jetting is shown in bold print, in the jetting chart. So for the 06' 250sx it's 1 turn out on the airscrew, a 42 pilot jet, needle N1EF in clip position 3, and a 158 main jet. The chart recomends a leaner #40 pilot for your warmer temperatures. Since the bike is new to you I would suggest a good carb cleaning and check/adjust the float height prior to any jetting changes.

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The stock jetting is shown in bold print, in the jetting chart. So for the 06' 250sx it's 1 turn out on the airscrew, a 42 pilot jet, needle N1EF in clip position 3, and a 158 main jet. The chart recomends a leaner #40 pilot for your warmer temperatures. Since the bike is new to you I would suggest a good carb cleaning and check/adjust the float height prior to any jetting changes.

Ok thanks Hallsy, yeah I tried to clean it out (carby) but a screw is stripped on the float bowl, so I gave up to ride some more. I guess my next action is a leaner 40 pilot and maybe go leaner a clip position. I will also check the reeds, they are original reeds so I don't know the condition of them.

Thanks for the tip

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Are you sure that you are not getting tranny oil in the engine?

I just re-built my engine and saw it was very oily, the crank seal was good but the previous owner had not put the o-ring in.

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I agree. I'd be looking at that crank seal. if it's leaking, it will suck tranny oil into the crankcase and foul plugs pretty quickly. I had this same issue with my CR500. All the jetting in the world didn't stop it from fouling plugs. I replaced the crank seal and never fouled another plug. My '08 300 has never fouled a plug in thousands of miles of riding.

It shouldn't foul plugs, even if the jetting is a bit off.

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today i put in a 155 main jet (158 was in) to lean the jetting out, also the reeds where in excellent condition. i changed my tranny oil and there wasn't much in the engine so could that be a crank seal.

should i take it to the mechanic??

also if it is a crank seal, is that do-able at home or has to be a mechanic

also i was told a hotter plug may cause damage???

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today i put in a 155 main jet (158 was in) to lean the jetting out, also the reeds where in excellent condition. i changed my tranny oil and there wasn't much in the engine so could that be a crank seal.

should i take it to the mechanic??

also if it is a crank seal, is that do-able at home or has to be a mechanic

also i was told a hotter plug may cause damage???

Its pretty simple, I just did it.

Pull the side cover off, remove clutch, remove the primary drive gear, under the gear is a spacer and under the spacer is a O ring (my issue was that the PO had forgotten to install this ring). Pull the old seal out (I used a exhaust pipe spring remover hook to do it), and pull out the O ring. Replace the oring and then get ready to have some fun with the seal.

the seal has a metal outside, you need a little ball peen hammer and you slowly and carefully tap around and around and around the new seal to seat it. it will try to pop up on one side, if it does you need to start over again.

Keep tapping and eventualy it will seat in all the way. I used some assembly lube to get mine in.

put some oil on the seal lip, reinstall the spacer, gear, clutch , and cover.

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