THE DARK ART OF ENGINE TUNING

Winter and early spring conditions probably cause more engine problems than any other time, mainly due to temperature and humidity fluctuations, one outing it can be nice clear and frosty and hovering around zero C and then next time out it could be a breezy damp 14C day, this will have a big impact on the correct needle valve settings.

A common misconception is you have to richen the mixture in hot weather to help cool the engine and you can lean it out in cooler weather but the reverse is closer to fact.

Cold air is more dense and will hold less moisture, whereas the warmer the air is, as a rule, it will hold more moisture.

Warm air is Less Dense than Cold Air

Moist air is Less Dense then Dry Air

So Cold, Dry Air will effectively have more Oxygen than Warm Moist Air.

A Hot humid day means Less Oxygen - So you can’t burn much fuel and have to LEAN the mixture or it will be too rich and have little power.

This is why on cold days you have a lot more power available than on humid summer days!

The Biggest Danger is taking the model out in much colder weather than you last flew it in, as the mixture will be set for warmer weather and probably way to lean for the cold air, 30 seconds of flight with a very lean mixture is enough to blow a hole in the piston on a modern engine. You have been warned!

The obvious solution is to fly every day!

How do you know if you engine mixture is right?

Firstly your engine will just sound just right (OK I know that is not very helpful but when it is there you WILL know) it will be producing the most power and this is something you wil learn from experience.

Setting your throttle hold position to give a nice steady idle is a good start and then, if you are comfortable to do them, try autorotating down and listen to the engine - it should come down to the preset steady idle within a second or two, if it does what’s called “hanging on the pipe” ( then is means it’s to lean so richen up the main needle a couple of clicks and try again, if you are not confident with auto’s then simply land and either throttle down or hit the hold as quickly as possible and listen to the engine for the same signs.

Conversely, if comes straight down to an idle and you suspect you are not getting the best from your engine go in a click or two at a time and repeat until it just starts to hang and then go a couple of clicks richer to be on the safe side.

Another way of telling is to again land as quickly as possible and put your fingers on the engines back plate, this should be the coolest part of the engine and you should be able to keep your finger there comfortably for a few seconds, if it’s too hot to do this then again it’s too lean, do not touch anywhere else on the engine or muffler or your likely to lose some skin!

When setting up the engine it’s a good idea to disable the engine governor as this will tend to hide some of the changes you are expecting to see when you change the mixer, once your happy with it then enable the governor again.

Some engines don’t tolerate high nitro fuel without extra cylinder head shims, these are mainly some of the older OS 91 engines but the classic symptoms of being over compressed is:

You find the engine is just a little rich, lacking power and RPM so you lean it out just a couple of clicks and all seems better for a few seconds but then the engine starts to sound harsh and lean, you land richen it back up a couple of clicks and you are back to too rich and lacking power again.

In this case you either need to run lower nitro fuel or add a shim or two.

Lastly, always run the engine dry at the end of the flying session by clamping the fuel off while the engines idling, once it stops reattach the glow and start to start it until it stops firing at all, this will make your engine bearings last much longer.

Never run it dry in the air as it will go very lean before it stops!