Diamond......

"Clean" does not necessarily mean OK.....
These are 2 stroke engines. They get their Lubrication from the oil in the Fuel.
You may be on the borderline of running too lean, in which case you are burning ALL the fuel/oil mixture and leaving the engine clean and shiny.
In this sort of situation, you may not blow the engine in an Explosive sort of way, but you WILL wear out the piston/sleeve assembly and lose "Pinch", real quickly.
If you are running HOT,,,,then you ARE running lean.......

Get yourself a cheap temp gauge,,,like [link=http://www.amainhobbies.com/product_info.php/products_id/31166/n/DuraTrax-FlashPoint-Infrared-Temperature-Gauge]THIS[/link]. It is a Must Have, for your RC Tool Box.......