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Ktm sx 125 spluttering/ backfiring.

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Need help with my ktm sx125.

Basically every time i ride slow with about 1/4 throttle for about 5 mins then go to full it will start spluttering and backfiring. When it does this if i give it loads of revs it clears and runs fine again.

If i dont ride flat out it just runs crap.

188 main jet.

Clip on 3rd position.

1.5 turns on air screw.

Not sure about the other jet but there all standard.

Running 40;1 mix with fresh fuel and motul 2 stroke.

Fresh top end and good reeds.

Air filter is clean.

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Sounds like your running rich. What does the plug look like? 1/4 throttle would be a combination of your idle jet and needle. Try lowering your needle one position or 1 size smaller idle jet. 2 strokes are meant to be run hard and putting around at 1/4 throttle for a long time with the pv closed might even gum up a decently jetted bike. But it sounds like your jetting is a little rich on idle jet and needle circuits.

Whats the compression like?

When was the last time your topend was done? power valve cleaned?

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I agree with maxamillion125. Did this start happening recently? KTM's are very sensitive to temp changes, and do require jetting changes. As the temps drop, on my 300, I have to drop the needle (raise the clip one position) and readjust the air screw.

Usually changing the clip and an air screw adjustment will correct it.

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It was fine in the summer. It just doesnt seem to snap on power as quickly either. Im gonna drop the needle tomorrow and see if that helps but i have rounded off the bolt so gonna be a pain to sort out.

Its running on a fresh top end and its just had a full rebuild everything is in good condition.

Im hoping to use the bike for enduro, can it be used for enduro once jetted right or am i gonna have to trade it in for a exc

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If it was running fine in the summer and now is sputtering in the fall/winter its definitely your jetting. Lean it up a bit. You rounded the bolt that holds the needle? The needle and the jets don't need to be very tight at all. Just snug them down and your good.

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Im gonna adjust the needle and air screw im worried about going to lean though. I know its not ment to be tight but i always end up overtightening lol. Cheers for the info.

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Im on the second clip position now, 1 turn out on air scew. Found out my main problem was i had put to much oil on air filter.

Its running alot better but it seems to be spluttering top of revs now.

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Im on the second clip position now, 1 turn out on air scew. Found out my main problem was i had put to much oil on air filter.

Its running alot better but it seems to be spluttering top of revs now.

Ahh, bingo!

I run a 190 main jet on my 125 during the summer and its fine, so I knew it wasn't your jetting unless you ride at really high altitudes.

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Ahh, bingo!

I run a 190 main jet on my 125 during the summer and its fine, so I knew it wasn't your jetting unless you ride at really high altitudes.

Im running a 188 at the moment. Need to get it jetted for winter as its always crap weather around here.

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Ahh, bingo!

I run a 190 main jet on my 125 during the summer and its fine, so I knew it wasn't your jetting unless you ride at really high altitudes.

The main jet has absolutely nothing to do with the problem he was describing. At 1/4 throttle your on the idle jet and needle circuits, the main jet is not involved. He was running rich like I figured but it wasn't because of jetting, the excess oil in the air filter was decreasing air flow.

You should get a couple paper towels to soak up the excess oil in the oil filter. Very little oil is actually needed to coat the filter sufficiently. A lot of guys will overdue it. I always pat my oil filter with paper towls and apply grease around the contacting surface.

In the summer I run 190 main, 48 idle, 1469G (200sx needle) 3rd clip

position

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The main jet has absolutely nothing to do with the problem he was describing. At 1/4 throttle your on the idle jet and needle circuits, the main jet is not involved. He was running rich like I figured but it wasn't because of jetting, the excess oil in the air filter was decreasing air flow.

Woops, I read his description backwards! :lol:

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Spent some time tweeking the carb today. Got it running near spot on it just splutters at 1/2 rev now. Ive set the air screw on 1.5 turns out but if i turn it 1/4 turn out more the splutter moves to 3/4 revs. Its starting to get frustrating but its getting there slowly but surely.

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