Over time, carbon from running an overly-rich fuel mixture can accumulate in your head. This problem is more likely in older, carburettor-equipped engines.
Signs you need to decarbonate your engine
- Engine keeps running on after you turn it off.
- Popping, surging, spark breaking down
How to decarbonise your engine
The easy way
- Give it a good thrash for a few minutes. Often a carbed engine leans out (towards stoich) at higher rpm, and the good burn takes most of the excess coke with it. You'll no doubt hear many stories about the old pensioner who brings their spluttering old car into a mechanic, who gives it a hiding around the block a few times, and returns it to its owner purring like a kitten.
- With the engine running and air filter off, spray some misted water, or carby cleaner, into your carburettor. Be careful not to let your engine stall during this process. Hold the throttle wheel open a little bit. If you have excess spray around the carburettor, put your fingers over the opening to seal it off and create greater suction inside the carb, and give the engine a good hard rev for a few seconds.
- There are also fuel additives, called things such as Fuel System Cleaner, Carburettor and Injector cleaner, etc which can help.
The hard way
Take your head apart, and clean it. Use of soft scotch brite or similar would be alright. Do not use anything sharp or metallic. Don't touch the valve seats unless you want to have them ground again.
Until someone writes a guide specific to a Corolla engine, refer to http://www.elsham.pwp.blueyonder.co.uk/cx500/decoke/. This page refers to a motorbike engine, but the theory should be the same.