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05 yz 125 questions/problems

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hey guys,

ive had my yz 125 for about 2 years now and since i got it ive had a jetting problem. im always fouling plugs! right now im running almost stock jetting but i have vforce reeds and a full fmf system on and i dont think the previous owner did the jetting for them. so my jetting is a 400 main, 40 pilot, 2 1/4 out on air screw, stock needle, 3rd position clip. my elevation is 1200ft and average temp is about 75 degrees and very mild humidity. i also have a massive bog at about half throttle that literally "eats plugs" i think im way out on my main jet. agree? i run 32:1 with tanaka two stroke oil and use a BR9ES plug wich is recommended but they dont last 2 days in my bike! ive even went through BR7's! please help! also, who has had experience with the JD jetting kits? are they worth it?

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What kind of rider are you and what type of riding to you do.

Your current jetting should be working fine.

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I would say something else is wrong other than the jetting. How does it start? What are you using for fuel and oil? When was the last top end?

My 05 runs great on stock jetting, using race fuel and 32 to 1 mix.

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My first recomendation is do a compression test. The 05+ yz125s come jetted from the factory almost perfect (actually almost too lean for colder temps) . So the fact that you are fouling plugs so quickly makes me think you have a weak top end. You need about 150+ psi to show a healthy top end . So before you potentially waste a ton of time jetting first get a baseline compression number. Also what is you factory jetting specs (you said you currently have "almost stock" jetting?

Edited by Polar_Bus
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Like mentioned above, a weak top end will do it. In my old 02 YZ, I was having a similar problem. I wasn't fouling them as often but It was happening way too often. Once I put a new top end in, it was perfect. Do a compression check and go from there.

Also, what type of rider are you. A slower rider who never is on the pipe doesn't need 32:1 (IMO). I run 52:1 spectro synthetic and have had great experiences. I have tried dropping it down to 42:1, and I fouled a few plugs, and was having a lot of oil coming out of my pipe. If you don't ride hard, I don't think you should be running 32:1. Just my opinion though. I'm sure there are many on here who are going to argue about it with me. It's an argument that could go on forever.

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I find that measuring your premix fallout is a good way to determinate if you running a good and healthy enough premix ratio. As long as you get 2mm of premix drop out you could get away with 50:1 although I doubt that with that ratio you have any leftover premix in the crank.

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I find that measuring your premix fallout is a good way to determinate if you running a good and healthy enough premix ratio. As long as you get 2mm of premix drop out you could get away with 50:1 although I doubt that with that ratio you have any leftover premix in the crank.

My mechanic (who is a vet expert, and leqd mechanic at local shop) said its perfect. Thats what he and his son run as well. Hes been in the mx world a long time and knows his stuff, so ive got to trust him.

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Go down on the main jet and that bog will go away. my son's kx125 was doin the same thing went down on the main jet and cured the problem completely. It has fmf pipe and Boyesen reed cage and valves in it.

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Go down on the main jet and that bog will go away. my son's kx125 was doin the same thing went down on the main jet and cured the problem completely. It has fmf pipe and Boyesen reed cage and valves in it.

Bad advice. The op is claiming he has a 1/2 throttle bog. Swapping main jets will only affect 3/4 - full throttle. If in fact his issue is jetting related, adjusting the needle clip position has the greatest impact at 1/2 throttle. Systematic jetting adjustments takes experience, and a good "ear" . Simply throwing different main jets in your carb is NOT the correct method. lastly going "leaner" on your main jet is also very risky if you don't know how your engine is jetted on top end to begin with. If the op's engine is already jetted ideal on the main jet, a leaner jet could melt the piston on a hard extended WO throttle run.

Edited by Polar_Bus

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does anyone think it could be my power valves?? because the big bog comes aruond the rpm range (half throttle) that the valves would open. but what i came up with is to either put a new top end in or buy a jd jetting kit. i would much rather put money into jetting because 15 hours on a fresh top end is a bit early for a new one. agree?

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does anyone think it could be my power valves?? because the big bog comes aruond the rpm range (half throttle) that the valves would open. but what i came up with is to either put a new top end in or buy a jd jetting kit. i would much rather put money into jetting because 15 hours on a fresh top end is a bit early for a new one. agree?

Not unless its stuck in the closed position somwhow.

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I'm unsure of the 'techo' side of things but down here in NZ we are also told to run 9 heat range plugs on most 2 strokes by dealers and owners manuals but everyone I know runs an 8! The only time we run 9's is with different jetting when we take 125's into the dunes if we're going to be 'on the pipe' all day long! Easy enough to make sure you're not running too lean with a simple plug chop, again due to your altitude, humidity etc etc maybe a 9 is right? My 05 YZ125 I currently have just had the same problem with a huge bogging in the mid range and I pulled it all down, turns out that the main rod that goes up to control the power valve had snapped in half, would definitely be checking power valve by the sound of that, Jason

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I'm unsure of the 'techo' side of things but down here in NZ we are also told to run 9 heat range plugs on most 2 strokes by dealers and owners manuals but everyone I know runs an 8! The only time we run 9's is with different jetting when we take 125's into the dunes if we're going to be 'on the pipe' all day long! Easy enough to make sure you're not running too lean with a simple plug chop, again due to your altitude, humidity etc etc maybe a 9 is right? My 05 YZ125 I currently have just had the same problem with a huge bogging in the mid range and I pulled it all down, turns out that the main rod that goes up to control the power valve had snapped in half, would definitely be checking power valve by the sound of that, Jason

Swapping plug heat ranges to correct ill engine running behavior is an incorrect "band-aid" for improper jetting.

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Swapping plug heat ranges to correct ill engine running behavior is an incorrect "band-aid" for improper jetting

^^ I agree completely, but by the sound of the bike has never run right if it's 'eating plugs' as I have a mate who had a similar problem with a KX125, the only thing we changed was the plug and the bike hasn't skipped a beat since!

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I would say something else is wrong other than the jetting. How does it start? What are you using for fuel and oil? When was the last top end?

My 05 runs great on stock jetting, using race fuel and 32 to 1 mix.

What race fuel are you using and are you running it straight?

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I am running straight 110. I have a head set up for race fuel.

Ok thanks. I want to run that fuel in my 05 yz125. Just wanted to know where to start with jetting. Thanks

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