If you have good compression, a good glow plug, and good fuel, a model engine WILL fire. If it won''t fire, then you have to do some checking. First off, glow plugs are considered consumable items. They do wear out. In harsher environments, such as helis, they don''t last as long as in most airplane models. Try a new plug first.

If the engine still won''t fire, and you have a good glow plug battery, then make sure the engine is getting the proper amount of fuel. Too much or too little will prevent firing. One way to check is to start out with the engine dry of fuel. Remove the glow plug and put in a few drops of fuel. Spin the engine over a bit and then attach the starter battery to the plug and see if the engine will fire. If it still won''t fire, then compression is the likely culprit.

Ringed engines will lose compression if they''ve had an overheat, if they''ve had the ring break, or if foreign matter has severely scored the piston, ring, and liner. Only disassembly will help you find that.

An engine can have a good "feel" when turned over by hand, but still lack enough compression to fire a proper fuel mixture.

If you put the engine on a test stand with an airplane propeller, you should be able to diagnose what may be wrong.