Why does my garden tiller quit running after a few minutes?

Question

My tiller has a 6hp Tecumseh engine on it and I have had to (at times) operate it with the choke lever in the half way detented position. This suggested a fuel delivery issue to me. I changed the fuel line between fuel tank and carb and eliminated the filter, just as a test. I also tweaked the carburetor settings on both main metering jet and idle screws. But it just runs rough and then will die after 3 - 4 minutes. When it dies, there is fuel in the carb bowel when I depress the sediment drain spring loaded device. I plan to replace the spark plug next and then possibly remove and clean carburetor. I am suspicious of a possible leaking head gasket (I can see a little black exhaust goo coming out of one side) as well as a weakening / failing magneto and / or an issue with the valves.
Any one a good small engine service man?

Answer 1

check the gas cap it should have a vent hole in it next loosen the screw's on the magneto and re tighten rust here is bad don't forget to re gap best way put a business card between magneto and magnets on flywheel loosen bolts and let magnets pull magneto in than tighten bolt spin flywheel to remove card
head gasket probably isn't it but check the valve clearance i think clearance is 10 thousandths grind valves with lapping tool if they look bad

Answer 2

Check the vent in the gas cap, it may not be allowing the fuel to flow freely. The coil may be bad if you are losing ignition, when it heats up they tend to cut out. The fact that you had to run it on half choke indicates there is a problem with the carb, either sediment is being drawn up into the mixture circuits or the gasket where the carb mounts to the engine is leaking. You may have a warped carb body or an internal crack. Clean the carb first and then go from there.

Answer 3

Start by tightening the head bolts to eliminate the leak.
Next have the carburetor soaked out to remove the gum left by old gas, trapped by leaving gas undrained over winter.
A new plug is wise each year. With the engine running check spark color at plug by removing wire slowly with appropriate insulated pliers (fuse puller type) If blue it's ok, but if yellowish in color the mag is going down. Mag replacement is a bit difficult since it must be properly spaced from the flywheel.
Don't hammer on the crankshaft threads, use a puller.