Ok i just bought a yfz 450 yesterday...the guy fired it right up it was in the 40's. Got home last night and it was hard to start...it took a while...not today same deal...its in the 20's..but i got it started let it warm up for 15...then shut it off to see if it would fire right up and it did quicker but still took alittle time. and when i give it throttle to get it going it will backfire and keep cranking then backfire then it will go.
its got a 13.1 piston and exhaust and K&N intake. I think it might be alittle lean for this weather but these are the same syptoms of valves are they?