Run the engine without the fuel tanks cap on. If the engine runs normally and continuously, the vent hole on the gas cap is restricted or plugged.
Change the spark plug. Ensure it is the correctg plug and the Heat Range. Possible it is getting grounded out internally. A new plug may solve the problem.
I found this mentioned on a website, I know it is lengthy but good informaion:
One first things to check is whether plug correct type for engine. Then, check condition of electrodes. On new plug, bottom surface center electrode flat, surfaces lower electrode squared. Used plug in normal condition will look the same, but electrodes will be colored ashy grey or light tan from carbon deposits.
An oil fouled plug will have its end saturated with wet, sooty, black oil deposits. In four stroke engine, oil fouled plug may indicate rings aren稚 sealing cylinder properly. Or oil may be passing through valve stem area. Sometimes, clogged breather can cause oil fouled plug (remember that breather is vent in crankcase). Clogged breather will prevent crankcase from venting properly, and pressure will build up in crankcase. Pressure will cause oil to be pushed past piston rings, into combustion chamber where will foul plug.
On two stroke engines, oil fouling common. Is normal by product of engine operation when fuel and oil are burned together. Oil fouling in two stroke may also be caused by too much oil in fuel and oil mixture. In either two or four stroke engine, oil fouling may also be displayed as excessive smoke in exhaust.
Spark plug fouled by excessive fuel will have dry, black, fluffy deposits on plug electrodes. Deposits won稚 have caked or lumpy deposits of oil fouled plug. Fuel fouling most often caused by extended operation with air and fuel mixture that is too rich. Usually is carburetor problem; could also be blocked exhaust or faulty valve. If fuel fouling present, probably will be able to smell fuel on plug electrodes, especially if problem is severe.
Another possible cause fuel fouling is weak ignition. If high tension cable, points, condenser, electronic module, or coil faulty and spark weak, plug can be fuel fouled. Fuel fouling can also be caused by use of a too cold plug in engine.
Oil and fuel fouling can both cause condition known as bridged gap, where carbon or oil deposits build up in gap until gap completely blocked.
Deposits caused by fuel and oil fouling can usually be cleaned from plug and plug reinstalled in engine. However, since plugs inexpensive, should usually simply be replaced during tune up.
After many hours use, plug electrodes begin to erode. Eroded center electrode will appear rounded, while eroded side electrode will have curve on inside surface. Plugs with eroded electrodes should be replaced.
May find plug痴 electrode or insulator damaged. Electrodes may be heavily pitted, insulator broken or cracked. This usually caused by too hot a plug being used in engine. Physical impact can also damage plug. If piston or ring part breaks and hits plug, may find damaged or bent electrodes or cracked and broken insulators. If plug reach too long, piston head may strike electrodes. Most common cause damage is debris or foreign objects in cylinder. A bolt or washer may loosen and be sucked into cylinder. Will then strike plug electrodes when piston rises.
Plug heat ranges may be changed depending on condition of plug that痴 removed from cylinder head. Hotter plug generally installed if plug looks dirty. Cooler plug installed if plug displays heat damage such as cracking or chipping of insulator. Manufacturer痴 manual will provide recommendations about plug type. Should always follow these.
Never sand, sandblast, or file plug and then replace in engine. Sandpaper or file will leave tiny grooves on electrodes. Grooves will either burn off or collect deposits as engine operates. Sandblasting and filing will leave tiny particles sand or metal behind on electrodes; these will get into cylinder and cause damage.