Obviously, I'm new at this; I noticed today that the "sweet spot" for the needle was 3/4 turn different than it was the other day. I had to run 3/4 turn richer today for optimum, and it was 20 degrees colder today.

Do you see that much of a difference in tuning based on air temp?

Thanks

yes. here it goes from 40 degrees to 70 the next day and the needle needs a little tweekin. I just do it as a matter of practice anyway, like you.

Cold air is denser than warm air, so you need more fuel in the mix.

rc_sport is correct. Off subject, but you will also need to adjust mixture if you change fuel. High nitro and high oil content fuels need a richer needle setting compared to low nitro and low oil fuels.

This makes sense to me, I was just wondering if it was typical to see nearly a whole turn of the needle to get back into adjustment. I had used a previous setting (when it was 20 degrees warmer), and I nearly had a lean condition today, which surprised me.

Also, the higher the nitro content, the "touchier" the needle valve setting will be. If tuned using 5% nitro it may be fine for 20° of temperature fluctuations, while 20% nitro needs retuning with only 10° change.
*(hypothetical temperatures - individual results may vary).

Paul,

3/4 turn seems to be a lot, even considering a 20 deg temp shift. To be on the safe side, you might want to check for loose screws in back-plate, carb, etc.. Could be sucking air. What type engine is this, and is it broken in?

It is a OS LA40. I am/was in the process of breaking it in (4 tankfuls through engine)

Morning paul_c,yes that is normal,I pinch the fuel line,if the engine doesn't speed up 300/400 rpm then I reset the needle valve so it does,then I point the plane vertical if the engine holds rpm without leaning then away we go, Sometimes I've changed a couple times a day,other times nothing changes for couple of months.

Note: This (IMHO) is a good reason to find a fuel you like and stick with it! Why add one more element to the equation?

It's air density not temperature alone that does it . barometric press & humidity also play a part.

Paul,

Your engine is probably still in the breaking-in stage and it's not uncommon to have to re-adjust needle setting more than if it were well broken-in. I would still recommend looking for anything that might have worked loose. This is especially true with new engines.
It's also a good idea to note the position of the needle before and after flights to insure it's not moving on you.

Yeah, I couln't believe it could be lean, as far open as I had the needle, but sure enough, when I pinched the fuel line, it died almost immediately.

Thanks for all wisdom out there!