I had same prob and found this answer on other site, mines in shop now and they said valves doing this are common, hope it helps


"If the valves aren't adjusted just so... won't work. Too much compression for the starter to overcome - try takin spark plug out and it it cranks over better it probly valves"

Yes the valves being out of adjustment will cause the engine to not spin as easily as it should, leading you to believe the battery is weak, when the battery may be fine.