I have an os 46ax which i am very happy with on all fronts but i have a question about tuneing that i hope someone can help me wiith.Why does the engine have to be run leaner in the morning before work and richer in the afternoon to get the same performance and a slight smoke trail running slightly rich. Is it something to do with atmospheric pressure or relative humidity.Its not life or death i know but i was just wondering why ?????. I am going through the same start up procedure now every fly after reading another post on RCU and i have noticed it needs a couple of clicks to run sweet,if its not right after one circuit i bring it in and adjust it.

There are so many factors that can effect tuning. Humidity/temperature and altitude for a few. I see you live in Australia so... and I must admit I'm not sure about your climate. If the temperature variations are fairly large between morning and afternoon this could cause a click or two of tuning adjustment. Cool air is more dense then warmer air. Thus when the air is cooler you need less volume to get the same amount of Oxygen molicules required for combustion so you can run a little richer. More air means more fuel to mix with it. At higher altitudes and hot air conditions when the air is less dence you will need a leaner needle setting.

As Mr67stang said, there are a lot of factors that are involved. But since you are dealing with a morning and afternoon difference I would venture a guess that it's because of the temperature and humidity differences between when you tune it in the morning and when you run in the afternoon.

Ken

Ever notice the weather maps they show you on TV? They show fronts as lines and they put them into motion sometimes. And darned if the line will be a state or two away in the morning and over you town in the afternoon.

Those lines actually represent atmospheric pressure. And that's one of the primary things that affect an engine's performance. And that pressure can change as fast as those lines on TV move.

And mornings can have the highest humidity of the day. And afternoons can have the highest temperatures of the day.

All three of those things have an effect on our engines. The effects all work to influence how many air molecules the atmospheric pressure can push into the carb at any given time.

Temperature, humidity, pressure. One can change fairly quickly, and the other two are very probably going to change for sure.

G'day Presso,
It's perfectly normal, but if you run it a bit richer in the morning, you won't need to change it in the afternoon.
I find that here in NSW, on the north coast, I can set my engines, a click richer, & not have to retune later in the day.

Good thing about retuning is it takes about 15seconds. So why not?

I used to fly a lot of practice flights. Usually 5 flights an evening, at least 3 days a week. Competition will make you do things like that. And I got to know my engine well. And got a feel for the weather. When you get used to your model, if you pay attention to how the engine takes the needle setting, you will too. And most times can feel if the needle needs a click richer or leaner and how much. If you can't fly that much, or fly more than a couple of engines, adjusting the first flight of each session takes almost no time at all. So why not?

There's one thing about tuning an engine in Queensland...it can be beautiful one day and perfect the next.

sorry guys, bit of an in joke there

Yep. If you're not messing with the low idle setting the main needle is no pain to tune. I back mine off a quarter-turn to start on any given day and then set it for max RPM after a few seconds, then back off a little towards rich. Starting rich has the same effect as starting a gas engine with the choke closed. And, with an engine lubricated by the fuel, a rich start gives a longer lifetime to the engine. 'Course, mine all seem to die of other causes than a natural death of wearing out.

Yeh i thought it may have been something to do with the moisture in the air diluting the mix in the combustion chamber when the humidity was up,or something like that.But i thought i would ask the question and get some ideas from around the globe. thanks for the ideas fella's, this hobby is more complex than it seems.My poor brain has to get used to accepting information again not easy when its trying to get used to hand eye co-ordination again as well.""Hey Downunder i agree""nearly perfect all year.