TE410E valve problems

Bought a 1999 TE410E (6,600km on clock) last summer and come autumn thought I'd better do a full service. Adjusted valves to the factory 0.05 (they all needed closing up a little), but when it came to start up it turns over easily but never quite manages to spark into life.
I suspect I'm not getting compression as the starter has little trouble turning over the engine (previously the battery used to die from exhaustion after turning over a few times). I re-checked the clearances but seemed fine (this is a pain to do as I see no way of checking valves without dismantling the damn cooling system each time). Before you tell me I'm overtightning I am very careful with this. The valves should be sealing fine and I'm just wondering if the auto decompressor may be at fault here. I'm thinking of maybe just opening up the exhaust valve clearance a bit to see what happens (but reluctant as I'm fed up with draining the cooling system etc.). Anybody else have this problem? I must also mention that the bike has lain dormant for 4 months, sparking fine (carbs shouldn't be causing a prob should they?).
Any suggestions welcome.

Can you get a compression tester? Then you'll know if the valves are open a little. If it's 4 month old gas it could be a problem. Won't hurt to drain it and get new but i'd check the compression first. disconnect the auto compression release and try to kick it after easing past TDC.