Notes on the Stock Motor

1978 HONDA NC50'78 EXPRESS

NC50

A 49 cc scooter which was manufactured from 1977 until 1983, it featured an air-cooled 2-stroke engine. The versions from 1977 until 1980 had a manual choke, points ignition, and a spring starter. Versions from 1980 onwards included a kick start, capacitor discharge ignition, and automatic choke as well as an improved compression ratio. Most versions include an automatic one speed transmission with a centrifugal clutch, double suspension, and oil injection. Two speed automatic transmissions were introduced towards the end of their production run.

• ENGINE
  • Engine: 49cc 2-stroke single cylinder
  • Bore 40mm
  • Stroke 39.6mm
  • Compression Ratio 6.5:1
  • The engine was lubricated by oil injection.
  • Spark plug: BP4HS
  • Plug gap: .028
  • The ignition was controlled by breaker points.
  • It also had a manual choke.
  • The starting system used a wind-up spring.
  • Battery: 6N4-2A-6 (see also 6N4-2A-8)
• DRIVETRAIN
  • Transmission: only one speed
• CHASSIS / SUSPENSION / BRAKES
  • N/A
• DIMENSION
  • N/A
• OTHER
  • Colors:
    • Parakeet Yellow
    • Pal Green
    • Mars Orange
  • It had a two-tone black and white solo seat.
  • The downtube featured a logo change with the name "EXPRESS"
  • Serial number began NC50-2000013.

Rear Wheel Diameter, approximate 18.5”

Rear Wheel Circumference 58.2” (pi * diameter = pi * 18.5)

I measure a little over 58.25

The stock tires are 2.25 - 14. The 14 gives us the rim diameter, 14 inches. The 2.25, the way I understand it is that this gives an imaginary circle sitting on the rim, representing the tire:

And we know the tires are not ideal circles. In the case of the stock tires, 2.25 - 14, we would expect the outside diameter of the tire to be 14 inches + 2 x 2.25 = 18.5 inches. Then the circumference would be 18.5 inches x pi = 58.12 inches. In the ballpark of what we measured.

Gear Ratio 14.220/1

(from http://endless-sphere.com/forums/viewtopic.php?f=10&t=59952 )

Service manual also gives the Gear Ratio as 14.220 : 1

MPH = (RPM * WheelCirc/GearRatio * ft / 12inches * mile / 5280 ft * 60 mins/hour)

38.7 MPH = 10000rpm * 18.5 * pi  ÷ 14.220 ÷ 12 ÷ 5280 * 60

Gear Ratio that SIP wants is the Gear Ratio (14.220) divided by the tire circumference in meters (18.5” is 0.4699) which is 30.2617

Old engine 100psi of compression

Seals leak down within a minute

Some additional information from https://sites.google.com/site/vpcrabtree/home/honda-nc50-tuning

Measured Parameter	Value
Bore	40
Stroke	39.6
Con Rod Length	80mm (est)
Number of Transfer ports	2 plus 1 boost
Main transfer Height	6mm
Main transfer chord width	17mm
Boost port height	6mm
Boost port chord width	17mm
Exhaust port Height	9mm
Exhaust port chord width	19mm
Reeds Cage	2 petals, 10 by 20mm windows
Carb dia	12mm
Exhaust Type	Plain Muffler
Combustion chamber volume	7cc
Squish Clearance	~1.5mm
Compression Ratio	~6.6