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125 jetting

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Hey I was wondering if you guys could help me out with jetting my bike. It's a 04 ktm 125 sx. The previous owner left gas/oil in the carb and tank for about 6 months. The bike bogs down sometimes when putting around and then opening the throttle hard. It never bogs down once I'm on the any throttle. My elevation is 3,300 feet, Temperate is 40-60 degrees Fahrenheit. I'm running yamalube at 40:1 ratio on 91 octane gas.

I've already drained the tank and carb, that helped some. Do you think I need to clean the carb or is this a jetting issue? What is the recommended jetting for my situation?

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well to jet a bike properly you have to be sure it hasn't got other problems.

so clean your carb really good as if the bikes been sitting 6 months with old premix it will prob be gummed up

get new premix oil maybe as premix goes off and will make the engine run bad

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Hey, congrats on the new bike!

Your issue sounds sounds normal, I think, if it is just a small bog down really low in the rpms...like when you are coasting in 1st and just rip the throttle. You need to feather the clutch a touch in that situation. My YZ 125 does that when it is 50 degrees or less even when I adjust my air/fuel mixture screw. I thought I had an issue but found out it was normal.

BTW: how do you like the KTM?

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yeah since the 125's are torque monsters its probably normal if its just a slight bog

didnt you used to have a yz 125

did the yz have this problem

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Thanks guys. I've never owned a yz, but after looking at many bikes it came down to a 00 yz 125 and 04 ktm 125, same price. The yz ran crisper,instant throttle response, no hesitation whatsoever. I figured the ktm just needed new premix/gas and maybe the carb cleaned or re-jetted. So far I'm really happy with my decision, people say the suspension on ktm's are pretty bad, but I'm very pleased thus far.

So you guys notice a slight bog when coasting in gear then hit the throttle hard? If I feather the clutch there is no bog at all. Does it also take a minute or two and full throttle to get your bikes running normal, when first starting out. Because when I first start out my bike is really bogging bad untill I open it up a few times then it runs normal.

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So I took my bike into the dealer today. I had the carb cleaned and he said it was running lean on top and had to change the main jet. They still haven't found the perfect jetting so they're keeping it another day. My compression test was 135psi, he said that's about middle between a new piston and time for rebuilding the top end. The carb was pretty gummed up too, so hopefully by the time I get it back it'll run like that yz125 :banana:

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Yes, it takes a minute or two for the bike to run clean and crisp. Yes, a 125 will bog at really low rpms regardless of gear or speed. Your clutch is your best friend and you will learn how to use it well on your 125.

As for it running lean...any 2-stroke will run leaner in colder weather and you need to adjust your jetting accordingly and then again when it gets warmer as it will run richer.

Your compression reading seems low to me. Since the bike is new to you, if money permits, why not have them go ahead and do the top end? It should be cheap or you could do it yourself, it's easy.

I think you would be well advised to slow down and take your time to get the bike in order. Get your manual and go over the entire bike and set it all back to the stock settings. This way, you can have a positive baseline to work off of. I promise you that it takes alot less than it appears at first glance to get the bike functioning as it was designed. Your safety and your wallet is at risk if you just gas it up and ride on the word of the seller.

Sorry if it sounds like im lecturing, i dont mean to. I just think this is part of buying a used bike, actually its a good and fun part.

Had you not mentioned the compression, i would've said don't sweat your issues...it seems fine. The jetting will probably be spot on once the temps get back to 70+ degrees.

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So they changed the main jet from 190 to 188 and idle jet to 45. Air/fuel screw is 1.5 turns. When I got the bike back it took me about 10 minutes of continues kicking to get it started. Its never done that before, its always started on the first couple kicks. Could it be they jetting is actually worse now or could it be from having hauling it in the truck?

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