my h22a had the same problem.idid a compresion test and it revealed that i had low compresion.as in the compesion gage would only read up to 75 psi per cylinder.it ran like that for three years always dieing before i removed the head and replaced all of my bent exhaust valves.to check compresion just insert the gage into the spark plug holes,then disconect the ignition coil and crank the engine until the gage goes as far as it will go.usually hold the starter for 3-5 seconds.