I obtained recently an Avastar 61 ABC engine from Brodak. Seems finely made , good quality. What i disliked :
1) At BDC (bottom dead center), the piston skirt almost closes the side transfer channels, This has negligible effect in case the cylinder sleeve has thick wall. Unfortunately, this is not the case. The piston diameter (dia) is 24 mm and the cylinder  sleeve mounting dia is 26 mm. So it seems to me making ports in the piston skirt would be far better. Just like the Enya 61 CXS engine piston has, provided by the manufacturer . As well as  many  other schneurle ported engines.
2) The craburettor ventura is not tight in the crank case, the play seems to me a bit too much. Else, the ventura's side facing the crankshaft   ends inside with a cone opening. Better no cone , straight cylindrical with sharp edges . Having in mind the crankcase intake opening shape.
There is some more to say but   seems debatable.
I would like to see opinions on the said up to here.
Ognyan.
 

Yes, it will add a good bit of power. Before you cut the skirt put the piston to TDC and lightly scratch the outline of the exhaust port on the piston and use it as a guide. You don't want to relieve the piston only to find it opens up to the exhaust. Also leave the skirt alone on the boost port side so the piston doesn't rock.

Motorman

Reasonable, of course. Only that putting the piston to TDC is problematic in case the engine is brand new. Because , as said in the manufacturer's paper, the cylinder is tapered in the internal wall . Normal for the ABC construction, aluminium piston and chrome plated brass cylinder, no piston ring. Strongly recommended, when first time crankshaft rotated, to heat the engine a bit , enough castor oil applied , then  TDC passed fast. 
Else, after break-in, disassembling the engine not recommended except indispensably necessary.
Ognyan.

I would like to add something else about my Aviastar 61 ABC engine. It turned out this engine comes unbalanced, too much vibration when operating.    Rich setting not possible because all the fuel in the fuel tank turns into foam state, no liquid. Thus no matter how much you open the needle valve, full speed available only .   
After reading in google about balancing a two stroke single cylinder engine , measurements and calculations lead to a tungsten rod 4.8 milimeter (mm) diameter pressed opposite to crankpin at 11th mm radially  from the center of rotation.  Tungsten rods are available from welding equipment shops. Pure tungsten rods  have a green mark. Other marks are also usable as long as additived in tungsten welding rods do not exceed 2%. This percentage does not affect specific gravity, nearly three times the specific gravity of steel.
In case some material of the crankshaft from the back of the crankpin has already been erased away, then it is possible the tungsten rod to be 4.0 mm instead of 4.8 mm diameter.
The crankshaft is hardened; I would recommend to bore the hard counter weight because making it soft may  make soft the crank pin too. Carbide tipped tool works good enough. The final bore size needed achived by use of a diamond powder coated circular file.

Ognyan,

Did you bond the tungsten slug in place?

Dave

Well, some details about  pressing the tungsten slug in place. I had already removed some material from behind the crankpin; this turned out  not enough to balance the engine, (measurements, calculations , etc.), so a 4.0 mm  thick tungsten rod obtained, green marked. A hole / orifice was drilled on a coordinate milling machine with a carbide tipped drill bit. At slow speed of rotation. At the desired place. The crankshaft counter weight about 8 mm thick as far as I remember, so this was the length of the orifice. A bit longer piece cut off from the tungsten rod obtained , using an angle grinder.  The tungsten piece became red hot while cutting. The two ends ground  both  to  parallel. Chamfers provided. Then precise measuring tools at a friend of mine  showed orifice diameter obtained 3.92 mm; the tungsten slug 4.00 mm. Same friend provided a thin enough round file covered with fine diamond powder. By filing and frequent measrements the orifice in the hard counterweight,  the orifice diameter obtained  3.98 mm . At this point , the crankshaft heated a bit, about 60 degrees centigrade - this is what a human hand may handle most - the tungsten bit/slug pressed in the orifice, using a vise. The exessive length of the tungsten slug/bit, what was exceeding the 8 mm  length, sanded away. Both sides of the counterweight. Until no tungsten protruding. No glue used. I hope the said/stated  up to now to be enough. About the engine, it seem to run OK, rich settings available.
In case any other/further  question, I reply  to the best of my knowledge. Else, I have got another same engine that needs balancing too. Broken in but not in use. Aviastar 61 ABC engine seems to provide a bit more power than an Enya 61 CX (ringed.).  I have got this too. Only that the Enya comes balanced, ready to break in and use.

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