Here's a funny thing I saw at the field today: guy trying to start an old Enya 4-stroke and the thing kept backfiring and kicking the prop and the spinner loose. He couldn't start it until he took off the spinner.
Maybe the spinner was a red herring - but what makes an engine backfire like that? There was no gas dripping from the carb and the engine ran fine when he got it started without the spinner.
Oh yeah, one other thing: when our hapless flyer *first* hooked up his glow plug, he hooked it up to a 12V source (should have been 6V). He immediately removed his glowplug and most of the platinum element was missing. Could it have fried and dropped down into the cylinder? ? ?
Probably the reason the prop didn't kick off without the spinner is that you normally get a better grip between the prop and the drive washer than you do between the prop and the spinner backplate.
Too much heat in the cylinder caused by the glow plug being too hot , either by overvoltage or the plugs heat range. On startup , too much heat to the glow plug would most likely be the cause since the engine hasn't run enough for the lean mixture to cause it to heat up.
Could possibly be incorrect valve clearance or a burnt valve but I doubt that since you said it ran fine when it did start.
The glow plug should only be connected to 1.5 volts for the average plug , and 2 volts for the high voltage plugs , GloBee being one of them. The average 1.5 v plug will burn out above 2 volts , the GloBee about
2.5 - 3.. If you heat a plug too much...such as overvoltage , this can cause detonation because the heat actually advances the timing , the engine fires way too soon and kicks it backward spinning the prop loose. 12 volts would fry the glow plug element as would 6 volts.
Are you sure he wasn't using the glow plug igniter feature on his power panel and turning it too high ? This can cause detonation from the extra heat and also smoke a plug if turned high enough.
If he had either 6 or 12 volts hooked to the plug it would fry the element instantly , but , there would be hardly anything left to fall into the cylinder. The platinum would burn completely.
Since it ran fine once started, I'll guess over-choking. Is it mounted inverted? Gas doesn't have to "drip from the carb" for the engine to be flooded.
Although the fried glow plug element could have scarred the cylinder wall, it wouldn't cause backfiring unless it lodged under one of the valves.
Glow plugs operate best at 1.2-1.5 volts. Glo-Bee plugs can take 2 volts. Anything higher will fry one quickly.
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