When my Honda CBR900’s engine is cold, it works only on three cylinders. When the engine begins to warm, all goes fine and all the cylinders begin to work. What can cause this?
Answer in progress (see comment to the question for clarification questions).
Checking spark plugs and spark
First thing I would check is condition of the spark plugs and quality of the spark on each cylinder both before and after warm-up.
Make sure to be thorough and cover all the steps so as not to miss any possibility. Check back after it is done.