First post here, go easy haha. TL;DR is the bike isn't firing. It has spark and (some) compression but won't run even with start ya bastard. Valve clearances are all in spec and the timing is spot on. I'm by no means an expert with this though so I might have missed something. The bike is a 1998 yamaha fzx250 zeal with 42,000 k's on it. I had the bike die on me shortly after an oil change. Clutched in and it stalled, wouldn't crank over, turned out the battery had lost charge. Tried bump starting the bike and got nothing. Same with a jump from a car battery. Pulled the bike apart roadside to check the spark. The plugs were firing but were fouled, gave them a clean and still nothing. Wouldn't get any ignition even with start ya bastard. I had half a tank of fuel and had recently serviced the carbs. The temp light didn't come on and she didn't seem noticeably hotter than normal. Holding a finger over the spark plug hole for pistons 3 and 4 I noticed there was next to no compression. I also found coolant residue backed up in the carbs. Assumed a blown HG as I hadn't noticed any horrible mechanical-sounding noises, and the motor would still crank freely with a jump. I might mention I assumed the battery charge issue was due to a rec/regulator issue so I got a replacement. I trailered the bike back home after having no success getting her running again. Assuming I had a blown HG I dropped the motor from the bike and got the head off. Didn't notice anything odd about the oil and coolant when I drained it. Removed valve train and head bolts in proper sequence. I found some muck in pistons 3 and 4, but nothing excessive. The HG didn't seem obviously damaged either. I had a look at the head and couldn't see any obvious damage. The valves show no signs of damage and appeared to be seating fine. The pistons themselves looked fine, with no visible scoring to the bore either. I've since installed a new HG, torqued the head back to spec. I checked the valve clearances and they are all within spec. Timing chain went back in fine, all marks matching up perfectly, chain guides went back in and the tensioner is doing it's job. I've reassembled everything and chucked some oil and coolant in her. She still won't fire but managed a little cough with some start ya bastard. I've checked the compression and got about 90 across all cylinders, although these are cold readings. I still get spark, but it may be weak, or mistimed. Haven't changed the spark plugs yet but probably will. I'd also like to do a leakdown test but I don't have a tester. I've tried to include every detail I think is relevant, but there's a fair chance I've missed something important. Hopefully there is just something simple I've missed or forgotten to check, but I'm at the limit of my mechanical knowledge here. Ultimately it should probably go to a shop but I'm a poor student just trying to fix things himself.