Starting Port Map

Exhaust Port Open

I get the exhaust port opening at 29.7mm from top dead center, this would put the angle at 113 ATDC. http://hondaspree.net/wiki/index.php5?title=Cylinder_Port_Timing gives the export port open at 65 degrees BBDC which would be 115 degrees ATDC. The shop manual also gives the exhaust open as 65 degrees BBDC. This seems pretty late for two stroke motors I am used to. Also note that the exhaust port on the cylinder I mapped wasn't very symmetrical.

The other problem you can see both from the port map and the above picture is that at Bottom Dead Center, the ports are not fully open in this picture of a stock cylinder. This is the original motor from my NC50.

Transfer Port Open

The transfer port opening measurements I get are 34.06, 33.94 and 34.02. Using 34mm as a rough average, this gives a transfer port opening of 130 degrees ATDC. http://hondaspree.net/wiki/index.php5?title=Cylinder_Port_Timing give the transfer port opening (scavenge) at 47 degreess BBDC or 133 degrees ATDC.  The shop manual also lists 47 degrees BBDC as the scavenge open time.

Non-symetrical small ports that open late in the stroke don't make for much power or RPM.

Measuring the crankcase volume at 290cc at TDC.

Measuring the stock head at TDC, I get 8cc. Calculating the compression ratio:

Stock bore - 40mm

Stock stroke - 39.6mm

Stock displacement - 49.76cc

Static Compression Ratio - 5.65:1

Full Stroke Compression Ratio - (49.76+8)/8 = 7.2:1

The shop manual gives the compression ratio as 6.5:1 for the 1978 NC50 (mine). Any way you look at it, the compression ratio is anemic.

First look at the K-Star 44mm bore cylinder, the ports don't look that different in timing. With the rear transfer port bottom at 46.95mm, that is 10mm further down than the stroke. To really use this cylinder, I probably need to cut 10mm off the top of the cylinder and get or make a cylinder base plate spacer.

Exhaust Port Open
28.74mm gives an exhaust port open at 109 degrees. My gut says I would like to see something more like 89 degrees.