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520 MXC engine missing at mid-high RPMs

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I purchased the bike in june and it was running terrific. I ran maybe 1 or 2 tanks of gas through it while I was making it street legal. Ran strong every time. I took it out for a full day of riding in the hills and about half way through my day it started sputtering at high RPMs. I thought it may be a clogged jet so I pulled the carb and cleaned the jets. After, it seems marginally better. It starts missing at about 2700 RPM and as it goes up it gets worse. I would say I need to jet it, but it WAS running perfect. Is there something else I should check first? The fuel mix screw is set at 3 1/4 turns out, which seems like way too much, but figured this shouldn't affect the high RPMs anyway... thoughts?

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Sounds very much like a stator going out, which is very common in earlier RFS engines.

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+1 on stator, should read i believe 14ohms at wire. if less its tired. might check your ignition box on steering stem as alot have been cracked by improper lock udjustment

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3 1/4 TURNS???!!!!????  Stock setting is 2 1/2 turns....that's definitely somewhere to start looking into carb issues....just sayin...

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Clogged petcock screen (it'll look perfectly fine), clogged air jet behind the bell on the carb. Those were the only two things that made my 520 run like that in the 15 years I had it. More recently my friend's 400 was tight valves. Stators weren't a real problem on them. Now the 90s KTMs, yeah.

Edited by DR.billZ

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Faulty stators are (were) a big problem with the earlier RFS bikes. Several years back, the stator in my '04 540 EXC gradually gave up the ghost and finally died. The stators in three RFS bikes, '02 -'05, of riding buddies also went out. After '06-'07 OEM replacement stators were installed there have not been any further issues.

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I'd be willing to bet they changed something else when they replaced the stator.

Not really. One of the guys replaced his bad stator with a Trail Tech unit. I simply replaced my stator with a newer OEM unit and it has worked perfectly ever since. I sold the bike to a friend several years ago and the engine is still running great today.

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3 1/4 TURNS???!!!!????  Stock setting is 2 1/2 turns....that's definitely somewhere to start looking into carb issues....just sayin...

 

Fuel screw is just for idle and idle + so that can't be it. 

 

I agree, bad plug, plug wire, plug cap, bad ground (like at the battery or in the harness), corroded stator connector, or dying stator.

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this is what fresh fuel looks like in my area after a couple of months, I use it to break down air filter oil.

 

0418142209.jpg

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