Carburetors

The original 12mm Keihin PA carburetor on the NC50 (shown on right), really was problematic for me. The pressed in main jet really limits any easy tuning. The carb didn't respond well to temperature and density changes, so much so that I zip tied a small screwdriver to the NC50 so I can readjust it on days that it didn't want to idle well.

I moved onto the Dellorto SHA 15.15, middle. It does have replaceable main jets and has been much better. The idle air/fuel mixture, you have to adjust by replacing slides, which is unique to my experience. Treatland, http://treatland.tv, has an adapter that will let you bolt this carb directly to the NC50 stock intake. This makes it a very easy swap. The large 60mm air horn on the carb limits the choices in air filters, but again, Treatland has some options and I am using a Mallosi air filter from their site.

I have purchased a Mikuni 20mm VM round slide carb and a Weak Ends intake to use with it. Again, both from Treatland. My suspicion is that I will like this carb since it is similar to many of the Motocross VM carbs that I worked on in the 80s and 90s. The Mikuni has a 43mm air horn.

Right now, I running about 10,000 rpm at 38.6 mph. If we use the Jennings formulas for carb sizing, and the K-Star 44mm cylinder I am using, safe carb bore size comes out 19.6mm.

Starting Port Molds

Starting to make some molds of the ports on the stock cylinder, so that I can take measurements.

Seal off the inside of the cylinder. This is a stock 1978 NC50 cylinder.

I am going to have to do this in two steps, since I have to set the cylinder at an angle to fill the exhaust port. The transfer ports I can lay the cylinder flat. Grease the inside of the port:

Microwave the molding compound. In this case, I am using Composimold:

Pour into the exhaust port:

Fill it up:

On the left is the Composimold exhaust port from the stock NC50 cylinder. I get Composimold from Amazon, http://smile.amazon.com/ComposiMold-FC40-Reusable-Material-40-Ounce/dp/B0096BSQ2S/ref=sr_1_1?ie=UTF8&qid=1438548890&sr=8-1&keywords=composimold. I used to use another molding material, that was a reusable molding compound available in different stiffnesses. It was like a silicon rubber, but turned to liquid in the microwave and reverted back to rubber state quickly. An example of this is the ZXI 1100 cylinder head mold on the right. I still have some, but I am using it sparingly since every time you use it, you introduce some contamination. I don't like the Composimold that much, but it is readily available.

Now the transfer ports.

From the port mapping, I get a rear transfer port width of 18.54 mm as a cylinder segment width. Converting to chord length, this is 17.88 mm. Measurement of the mold, I get 18.02 mm.

From the port mapping, I get the magneto side transfer port width of 19.20 mm as a cylinder segment width. Converting to chord length, this is 18.47 mm. Measurement of the mold, I get 16.51 mm.

From the port mapping, I get the clutch side transfer port width of 19.24 mm as a cylinder segment width. Converting to chord length, this is 18.50 mm. Measurement of the mold, I get 16.67 mm.

From the port mapping, I get an exhaust port width of 29.42 mm as a cylinder segment width. Converting to chord length, this is 26.83 mm. Measurement of the mold, I get 22.02 mm.

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.

Trying to find the correct hub for the two speed

This is the stock rear wheel: