GENERAL SYMPTOMS WHEN PLUG IS...

TOO COLD

1-Engine pops weakly each time it is cranked, but will not start. Also weak batteries.

2-Engine will not lean out into a smooth peaked run, but always sounds rich. Exhaust sound is extremely ragged.

3-Engine may quit running when the battery is removed, even though it may peak okay with the battery connected.

4-ln flight the engine will become progressively richer until it bogs down completely. This is due to the additional cooling in flight of an already borderline combination.

5-A mild situation will show up with the engine slowing down slightly when the battery is removed. This should serve as a warning that the plug has deteriorated and is due for replacement.

TOO HOT

1-Engine backfires when cranked and kicks back against rotation resulting in bruised fingers. Often results in reverse running. Also caused by using 2 Volts on some plugs.

2-Engine cannot be richened smoothly to break out of peaked sound. Generally occurs with highly nitrated fuel in lo-compression engines.

3-If you backed the needle valve up under (2), in flight it would go rich suddenly. Look for dirt in the fuel line, but remember this trick.

4-The engine will be more likely to overheat and sag. Always launch on the rich side of peak. Sag is characterized by a thin smooth exhaust sound. It simply sounds pooped, and it is!

5-If the engine exhaust sound has a sharp cracking note super-imposed over the steady tone, the fuel is pre-igniting, or detonating. This condition causes lost power, increases the wear on the engine,
and can lead into the overheat circle that can destroy your engine.

Note: The TOO HOT conditions can all lead to the destruction of your pet engine and they can do it in one flight! So, be particularly on the lookout for them. The TOO COLD won't generally hurt anything more than your pride.