I have an Enya .15 that fires up just fine, but after its second flight, the power starts to noticably fade. Is that a symptom of a bad glow plug, or too lean a mixture? Is an OS #8 ok for that engine?

Thanks!

Either of your suggestions are viable and easy to correct.

Change the Plug and retune for a lower RPM, then go flying. Cost one Plug and ten minutes.

However; both problems should have been evident in the FIRST flight, I would also start looking at things like crud in the fuel tank that gets stirred up or loose fittings. An Air leak around the Carburator can cause no end of problems.

Basic fault finding, change one thing at a time and try to duplicate the fault. When the problem disappears recall what the last change was and do it first the next time.

G'day Mate,
Sounds lean, be aware, early in the morning, the air is cooler & so it is more dense, so the motor can be run slightly leaner, as the day warms up, your mixture may need to be richened, a couple of clicks, to allow for the air becoming less dense.
The best test is, run your motor up to WOT, & hold the plane nose up at 45 Degrees, the engine should pick up revs slightly, if it doesn't, it is too lean, & needs a couple of clicks richer, & try again.
Don't worry about it not sounding like it is screaming its **** off, if it won't fly at this setting, you need a bigger motor.

Do you have an explanation for this? Full size aircraft have to lean the mixture in less dense air, so it seems intuitive that when model engines encounter cold dense air we need more (not leaner) fuel to mix with the extra air. Is the model carb. different in the way it sucks fuel when the air is dense?

G'day Mate,
I dunno about full size aircraft, they don't use the same fuel, but i do know model aircraft motors, more air means more available oxygen, & so it can run with less fuel mix, as the day warms up, it will run leaner, & so needs a richer mixture.
Try it sometime, it is true.

I'm almost certain your Enya has a cast iron piston in which case I'd say you're using the wrong fuel for it and it's overheating. With a cast iron piston the bare minimum oil would be 20% but only if it's all castor. Personally I'd use 25% all castor.

This is precisely backwards. I'm certain physics doesn't work backwards south of the equator.

Cooler air is more dense. Therefore, your engine is drawing more air in cooler air. More air to burn with the fuel means the mixture is LEAN. Therefore in cooler air, you need to run at a richer needle setting to prevent your engine from overheating. If the day warms up significantly (20-30 degrees F), you may want to recheck the high speed needle, leaning it out a couple clicks.

When I'm flying in winter, (30 degrees F) there is between 1/4 and 1/2 turn difference in the high speed needle setting.
Brad

Brad, you are right, you beat me to it.

Jim

Most gas engines take 14.7 parts air to 1 part gas to burn proper. The denser the air is, the more fuel you need to keep that same happy mixture going to the engine. I'm sure glow engines work on the same principle, maybe just a little diff in the ratios though.


Ronnie