reincarnate

I have an Evolution 26GT that has 1 gallon of fuel ran through, at the 32:1 mix. Using Yamalube synthetic racing oil (formulated for air cooled dirt bikes). Engine is running great on the ground, around 8700 RPM, but on maiden I had problems attaining full RPM in flight. Bent axle on landing prohibited further experiment, but I am assuming I was rich in the air.

Anyway... I pulled the cowl to inspect possible air leaks adjust needles, etc. When I pulled the plug, while the porcelain was a nice medium brown, the bottom of the threads of the plug showed a great deal of buildup...almost looked like baked oil. Wouldn't come off with fingernail. So I pulled the head off the jug, and the top of the piston, in the area of the plug, has a large black stain similar to the debris on the plug. A rag soaked in gas showed that some of it would come off. The cylinder walls look good, it's just the top of the plug.

Is my oil content too high? Poor performing oil? Normal occurrence? Changed the plug and engine was revived, reaching a higher RPM (+200) without touching the needles. Going to adjust the needles, as I am obviously too rich, but is the stain I see an indicator of something? Thanks.

pe reivers

The stain is an indication of a fire going on inside the cylinder.
Racing oils normally have slightly less cleaning dopes than other oils, so a slight carbon build up will not be cleaned again. This is normal.
When you look at the inside of the piston you will see a brown discoloration. If that turns to black it is a sign of very high operating temperatures. If it turns greyish you are on the verge of burning a piston (death ash). In that case, the black on top of the piston also has turned grey.

Tired Old Man

My Feedback: (1)

If the carbon build up is pretty thick it's a fair sign of running too much oil and poor tuning. Since this is likely your first gasser 40-1 would be a good mix until you gain a bit more understanding of the engine. If you change your oil mix you will need to re-tune the engine. Just don't run it hot. The plugs will take care of themselves once you have the mixture properly dialed in. So will the top of the piston. The tan-brown color is a good thing on the porcelain, so probably a bit too much oil.

Pe,

Good info on piston colors. Why didn't any of us pass that on before?[]

pe reivers

Pat, (I like your new avatar and having a name to speak with).
I don't know why. Maybe just because we never thought that someone else would'nt know about the things that seem obvious? Reading an engine is not difficult IF you know how. If you don't know it is much more like a mistery.
At some point in life you kind of slide into the leaving behind ignorance and then try to satisfy the hunger to learn more. You then also tend to forget what it was before all those things fell into place.

the Wasp

My Feedback: (1)

""Engine is running great on the ground, around 8700 RPM""

it's just M.O., but I would think if you went up in prop diameter and get that RPM down a bit/ around 8000 you will get more out of your motor,, if you don't have room for a larger diameter go up in pitch..

Jim

reincarnate

Running a 15x12 APC. The plane is a racing platform, and this prop/engine has shown to give the best perfomance. I plan on trying some different set-ups.

Not my first Pat, (I own 3 gassers) but I by no means consider myself anything other than a novice. I am a little rich, as I am still in the break in stage. The 32:1 is the suggested break in mix for this engine. I'm on my second gallon now and have it at 40:1, per manufacturer recommendation.

Pe... Would you suggest a mix any leaner (oil-wise) than the 40:1? Since the Evo is essentially an MVVS, what would you recommend?

the Wasp

My Feedback: (1)

""the plane is a racing platform"" got-ch-yeea

Jim

pe reivers

with the 3% oil mix the engine has best power. Less oil means a less slippery engine, so more heat goes to waste in friction instead of going to the prop.
Search the racing forums, and you will find no lean oil mixes there. These guys know a thing or two, and most of them learned the hard way.

Flypaper 2

Don't forget, The only cooling the piston gets is the heat transfer the oil conducts between the piston and cyl. wall.

pe reivers

which would be better with less oil.
Piston cooling is largely defined by fit in the bore, with a narrow limit between too tight a fit, and too loose a fit.
it also is influenced by gas passage TROUGH the piston inner side AND the squish zone at the end of the compression stroke where the piston lingers before it's on the way down. The latter is not to be neglected because of the extremely high gas velocities that occur here, and radiation that is the inverse of the distance to the third power.

reincarnate

So the 30:1 mix gives the best power?

But the piston cooling would be better with the 40:1?

Holy crap. I'm gonna quit askin' because now I don't even know enough to know what I don't know!
I know...it's not rocket science. Suck, squish, bang, blow. I'm running the 40:1 and won't go any less. Just wondered if
I needed to be concerned about the carbon on top of piston. Working on refining the needle settings, just as soon as my
ignition battery charges.

Tired Old Man

My Feedback: (1)

When you said racing engine you made the point of using more oil. The engine will absolutely produce more power with a little more oil that what is normally used for regular RC. Thanks for the clarification.