A few months back I tried to start the CB500 in the cold, and stupidly switched on the choke before switching in the engine on. This little mistake turned into a big nuisance as the engine started, coughed and stopped and left me standing outside my office, repeatedly trying to start the bike again. However as the engine had got flooded with excess petrol, nothing happened. I ended up parking the bike overnight, in order to get it fixed the following day.
As I told the mechanic what had happened he explained the following: Once the carbs flood with fuel, the spark plugs will get wet and basically become useless. After that, the points of the spark plugs (even if you do take them out, wipe them dry, and sand off the dirt from them) will never be clean enough to give a reliable spark.
It’s not all that bad though. Replacement spark plugs come for around £6, and if you have the original tools that come with the bike, you will already have all required tools to replace the spark plugs yourself.
Here’s what you do. Once you get two replacement spark plugs, take the side covers (shrouds) off the tank. On the top of the engine you will see black rubber plugs which go all the way inside. This is a little fiddly, especially if you have big hands, but it’s just about possible to pull out the plugs from the engine and take them out entirely, put the spark plug socket inside the cavity you just removed the rubber plugs from. Open the spark plug by twisting on the tool with a spanner going anti clockwise. This may require quite a bit of force!
Once the spark plugs are unscrewed, just take them out, if you can reach make sure the area round the spark plugs is clean and dry, and put the new one in. Repeat this process on the other side of the engine. Once you’re done, simply push the plugs back in, make sure they are properly tightly connected – you might feel a subtle click when they push in, after which they won’t move around anymore. Then put the shrouds back on and you should be up and running!
[...] this other mechanic close to where I work who helped me out on previous occasions (see: Replacement Spark Plugs) I gave the bike to him, explaining what the other mechanic had said (blown headgasket – repair: [...]