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86 YZ 125 cobbing out at high rpm

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Hey all, let me get right to the point, basically for the past couple years I've had this bike, it hasn't been running right. It tends to start well, and has a decent amount of power for a 200+ lb kid on it, but once I attempt to hit the top of the power band, it tends to cough and sputter like it's choking. When I recently checked to carb to jet it, I realized ALL but one of the Jets are missing. I've checked the reed valves, they seem to be in great shape... However, today (it is a warm day) it ran great for a while, after topping off the oil and running with the choke half on, this bike also leaks oil, but I'm almost convinced that this is a jetting issue (being that they're missing)  at this point, but I've also never put a top end in this bike. Thanks for any help, I'd just like to get an opinion on what you guys think.   I know I'm all over the place btw, sorry for that 

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First rebuild the carb, set float and get the correct jets in it. Check compression and do a leak down test. Re-pack the silencer. and come back on with results and questions. Keep it to this thread, and just add "bump" if it falls down the page without comments. 

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And don't forget to thank Gary for his great advice.  :thumbsup: I didn't understand the "all but one of the jets are missing" but you are in good hands.

(Hoping Gary will move to Flagstaff) 

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11 minutes ago, Spooge said:

And don't forget to thank Gary for his great advice.  :thumbsup: I didn't understand the "all but one of the jets are missing" but you are in good hands.

(Hoping Gary will move to Flagstaff) 

Have the house in Prescott now for 3.5 years. Retiring early as in 3/30/17 not 6/17. Not to far of a drive to ride Flag, as it's not to far the other way either. :smirk:

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Just now, Gary YZDOC said:

Have the house in Prescott now for 3.5 years. Retiring early as in 3/30/17 not 6/17. Not to far of a drive to ride Flag, as it's not to far the other way either. :smirk:

We have great trails up here :banana: yet, I keep trying to get the wife to move to Prescott, rocky but good down there.

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1 minute ago, Spooge said:

We have great trails up here :banana: yet, I keep trying to get the wife to move to Prescott, rocky but good down there.

Those are rocks? I thought that was the trail? :cool:

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I can remember from the 1986 125 2-stroke MX shootout that the '86 YZ125 was a midrange engine,

in stock form it lacked bottom end and, had very little top end overev versus all the other 125's

meaning it needed more clutching at low speeds and to be short shifted before it fell off the midrange pull.

Since it's state of tune seems less than ideal (owned for a couple of years / never refurbed) some engine component perhaps being 31 years old doesn't help.

 

Edited by mlatour

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