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My 2007 500 Foreman was working fine at the end of last season, it was in my garage all winter and it never got used. I may have forgot to add fuel stabilizer at the end of the season. This year it is missing badly at higher RPMs.
I drained thew gas, I added some water treatment to the fuel tank & filled it with high test gas. (No change)
I pulled the air filter and I found it to have a lot of oil on it, I pulled the main air filter & the other 2 smaller type filters, one that goes on the end of a hose & the other is rectangular & fits into a rectangular slot at the bottom of the filter housing, I found it strange to see oil on my air filter, filters are all clean & back in service. (N0 change)
Next, I'm planning to pull the spark plugs but if it was good at the end of the season &* works well at low RPMs, I'm thinking it's not likely the plugs.
Anybody have any thoughts on what I should do next.

 

[shadetree]

check the oil for gas !!!!. my money says your lower engine case has gas in it !. float needle stuck, flooded the crank case, too much lower engine pressure from being over filled ( fuel does this ! ), and its blowing back into your air box.

 

[LedFTed]

were you using ethanol gas? before it sat. stranger problems begin with ethanol gas, even if it is premium gas.

 

[ATC4ever]

Any chance theres a loose ground or wire somewhere, & its vibrating at higher RPMs?

 

[Goober]

Sounds like bad fuel and fouled plug. Drain fuel and add new with some Berryman Chemtool. Run it a bit if it will start. And then change plug.

Also change oil and filter; smell for gas.