ORIGINAL: w8ye
Most of the popular 4 cycle plugs are very close to same as the OS "F"
These would be the H-9 Super Plug Four stroke, The Saito "SS", and the Y-S four stroke plug.
Try an Enya #3 two cycle plug?
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I was going to suggest the same thing.
Engines are funny when it comes to glow plugs and do not always run as predicted within given heat ranges. I attribute this to combustion chamber shape, the individual characteristics of the engine being tested, the handler's characteristics, fuel, prop, etc.
I would not expect an engine with an air bleed carb to idle as low and steadily as a two needle carb (depending upon the two needle carb), but I wouldn't rule it out either.
Try different glow plugs. The Fox Miracle plug would be an inexpensive plug to try, as would the Thunderbolt, K&B and McCoy plus. I used Enya #3 and McCoy four-stroke plugs in my Enya 80-4C with great success. At that time, I had not stumbled upon the OS Type-F plug. I'll bet that would have been great too. I have two used Enya 80-4C engines now, so, once bathed in antifreeze and re-ringed and re-bearinged, I'll get a chance to try the OS Type-F plug in one of them.
By the way, if someone has a used Enya 80-4C in running condition with zero defects, I would be willing to trade a NIB Magnum .91 for it, straight up.
Ed Cregger