If the simple steps we described in the article on initial fault finding did not get you started, here is the second round of slightly more complicated checks. Have some tools handy... Follow the steps in order, missing one detail cann cause you to start all over again.
Unscrew the spark plug. You have just made a half-inch hole into the heart of your machine. Every single grain of dust, every speck of dirt that enters that hole, will shorten the life of your engine, guaranteed. Use soft brushes to gently and very carefully dust off around the spark opening BEFORE fully extracting the plug. A twisted-up cone of clean paper shoved in carefully will stop dirt going in.
Inspect the plug. If it is fairly clean, but dripping with fuel, it may be that your engine is merely flooded. Spin the motor a few times with the plug out, to dry the piston, and try again.
The condition of your spark plug can tell a lot about the internal condition of your engine. For checking, diagnosing and setting spark plugs, see the Spark and Plug section. If it is clean and dry, test it for integrity:
Set it onto the spark wire, and test again for spark by turning over the engine with the starter cord. Spark plugs have a limited lifetime, the spark might not reach the gap.
If the spark is gone, replace the spark plug, at least with the old one you used to test with earlier.
Clean the entire spark plug of all carbon deposits, even inside the grooves, these often lead the electrix astray, making the spark appear outside the engine, where it serves no purpose.
Vinegar may be used to dissolve carbon deposits, especially inside the rubber boot of the plug cap, where you can’t see the dirt. Rinse with water and dry completely. A quick rinse with non-oily carb cleaner or alcohol may help the drying process.
Clean the thread before installing BY HAND. The first few complete rotations of the spark plug in its well must be done using your fingers. Using a spanner at this point is almost sure to lead to cross-threading sooner or later, a famous malady in engines serviced by idiots with no respect for machinery. Use the spanner only once the plug is finger tight, and tighten.
Try to start the engine. Still no luck? Next!
The bottom half of the carburettor may contain a screw. Unscrewing this little bolt at or near the bottom should let the fuel from the float chamber run out. This confirms two things: The presence of fuel, and the quality of that fuel. Look for tell-tale beading of water, or, more dangerously, a jelly or sludge.
Let the fuel run for two seconds. If it turns up only a few beads of water in clean fuel, close up and try to start again. Sometimes a drop of water is enough to prevent the thin stream of fuel through a jet hole.
If you find a jelly-like sludge, you should probably take the carb apart for some proper cleaning.
You may try a commercial carburettor cleaning fluid in an aerosol can. With the little pipe nozzle, you can attempt the blow clean the small holes around the air intake of the carb. The important ones usually have little brass inserts visible. Blowing cleaner into each, you should see fluid escaping into the carb throat, near the piston side.
If you still have no luck, it is probably best to remove the carburettor for a proper cleaning.
Note that removing the carburettor will open a much larger hole into your engine than a spark plug does, be ready to shove a clean rag or ball of paper in there to keep the dirt out.
With the aid of a small stick, hard brush, and some paraffin or petrol, try to wash all the dust on, around, under, above and below your carburettor. Do not do this on a lawn or veranda, the wife will have Things to Say.
Removed the air filter box, you should be able to see the carburettor, it is probably loose, as the air box usually bolts onto the carb’s mounting studs.
You may want to take a photo of the current setup, from as many angles as you can, because you are looking at a bewildering array of rods, springs, linkages and pipes. Getting them all back into place later might task an unaided memory to frustration, despair, even.
Note that the fuel pipe is still attached. Do not remove this without first closing the fuel tap. After removal, open the stop-cock for a second, just to make sure the fuel flows, it might be a simple blockage of the pipe.
Pull the carb away from the intake, and stop the hole with a clean rag.
Carefully remove all the linkages and levers needed to liberate the carburettor.
Set the carburettor carefully aside, with all the little springs and bolts and thingies in a small container of some sort, each one is a vital part of your engine.
Careful not to get dirt into the engine, wash the entire area with paraffin. The surfaces you are working with here, are all machined perfectly flat and straight, and a stray grain of sand can damage the perfect, I said Perfect, seal between your carburettor and engine. Refitting the carburettor requires a high level of cleanliness and hygiene you will do well to adhere to from here on.
After you have serviced the carburettor (you did find the Carburettor Service chapter, did you?) refit the entire shoot shebang, and refit the air filter box.
You have now assured there is Air, there is Fuel, and, by servicing the carburettor, you have done your best to provide the correct mixture thereof. You made sure there is Fire to ignite this all, now try to start your motor.
Purr-purr. As soon as the motors runs smoothly, open the choke. Running the motor on choke, will overheat the engine, burn the valves and cause more damage than what you just cured.