RE: engine operating temperature
When you remove the glow igniter from the glow plug, the rpm of the engine should not change much at all. perhaps 100 rpm. this is true at high rpm and even at idle. So if yours drops 1500 rpm something is wrong. Could be a bad glow plug. these are relatively cheap and do wear out. Unlike a light bulb, they sometimes fail "partially". If the needle valve is set too rich this can also be a problem, but it sounds like you have been adjusting this a lot already. your temperatures seem reasonable. I'd get a new glow plug and try it (under $10).