[SamUK]

I'm guessing by your name it's a 300 fourtrax you have.

I've recently fitted new aggressive tyres and found I can only spin them in deep mud when in first/low gear. Knowing what I know now I'd have opted for slightly less aggressive so I could spin them in 2nd gear. See my signature for tyre type and size.
So, in my opinion, the bigger you go the less aggressive you want to go unless you plan on a BBK or some kind of power upgrade.

 

[SamUK]

I certainly wouldn't go any bigger than 25" with an aggressive mud tread, I'd probably go for 8" wide too.

 

[SamUK]

Hearing all this talk of running 25" & 26" tyres is making me think that my motor is well due a top end rebuild (I'm due to do that anyway) sooner rather than later. I can't spin 2nd at all wether it's in really water swampy bogs or the thick clay mud holes. Compared to some of the tyres you guys run I'd call mine a semi aggressive mud tyre.
I'm also thinking I may have gone to wide, currently running 10" in width front and rear, maybe 8's or 9's would have been more beneficial.

 

[SamUK]

My bike can still pop a wheelie so I'm assuming that my motor is fine for now


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Mine lifts the front wheels about an inch, it won't wheelie on power alone, I have to pull up and drop down in the seat.

 

[SamUK]

Compression tester was in the garage I believe dad, I'm sure I only ever used the diesel tester so not 100% sure. It doesn't feel as lively as your 300 though and does have a slight tap/tick, nothing to be overly concerned about though.

I'm slowly gathering the parts to do a top end rebuild fish, dropped lucky with an OEM gasket set for £12 off eBay. Have to spend a little each week you see so the Mrs doesn't go crazy at me haha!