Morning all, took my hyper 21 engine apart today as a matter of maintenance and found the piston to be covered in black, as was the bottom of the cylinder head. Wondering what does this meen? Also noticed a small leak when the piston goes up air is going past the piston but there's still plenty of compression, I did the break in correctly and never ran lean, im worried something has arsed up.

I think if it's black it means you've been running lean
Normally it should be a golden brown colour if you're using good quality fuel/oil

Im using Byron 20percent the engine isnt stalling or anything and the performance is great but im just concerned

I would richen it out a tiny bit
My understanding is the oil should "burn" onto the piston head etc making it look golden/brown if it's tuned well

Oh okay, just to ad that byron 20percent has 16percent oil content which seems pretty high

There's your prob, high oil fuels require you to lean the engine alot....... just a bad thing overall. Drop to something betwen 8-12. I run sidewinder 20% nitro, 10% oil so it is 'midrange' for oil, between the max and min of oil content for good performance. Another way to tell if the motor's to lean is look at the plug, if it comes out wet its to rich. If it comes out with a bent coil or two, its to lean. You want slightly damp to a greyish coil color. Thats optimal tune [8D]

OKAY THEN but what about the leak, i think i might have to get a new piston and liner[:@]

If you have an air leak
#1 the engine should be next to IMPOSSIBLE to tune
#2 it will idle like CRAP (or won't even idle at all)
#3 may have a hard time trying to stay running
#4 is sucking fuel up at an extremely fast rate but still isn't tuning right.
I remember I created a "giant" air leak on my old worn out TTR motor and leme say forget atempting to tune it, it wouldn't even tune due to the massive amounts of air being pumped into it [&:]

Calvino the leak he was talking about is in the piston/sleeve assembly, not external

J4K3SHNEG , if it runs fine, I wouldn't change the p/s. You can only get rid of that leak by replacing the p/s, not repairing it.
You probably got it from dirt coming in the engine etc

Yea Ive just had a look in the exhaust hole, there is a scratch right down the side of the sleeve and piston[:@] The only way i can think of it happening is through dirt getting in through changing the glowplug, but i always made sure to do it carefully. It still runs and starts ill just replace the piston and liner when it gets to the point where it wont start.

Well the scratches can be from dirt coming in from the carb (either air or fuel, make sure you have good filters on both) also maybe metal pieces from bearings and/or other rotating parts of the engine, and even from the plug filament itself

But yeah, like you said, only change it once its totally shot

Ive got a built in fuel filter in the fuel tank and an in-line one through the tubing, the airfilter is dirty but just before it needs cleaning so its unlikely but could have caused it. I had just changed glowplug so it might be the filament but its from changing it nonetheless, anyway thanks for helping guys