Motobecane USA Speed Versions Table
20 mph Dimoby (1-speed) 01-1976 to 12-1978
54115 Side mount AR2-10-910 carburetor (10mm)
53496 Intake pipe (Ø10 restricted)
52646 Intake gasket (20mph)
53505 Cylinder (20mph)
22914 Dual clutch with fixed front pulley
14756 Drive belt
16653 Rear pulley with 11 tooth front sprocket
18368 Motor chain 99 links
53873 Rear sprocket 48 tooth (11/48 = 0.229 ratio)
25 mph Dimoby (1-speed) 01-1974 to 12-1980
22160 Side mount AR2-12-729 carburetor (12mm)
21201 Intake pipe (Ø12)
17073 Intake gasket
54328 Cylinder (25mph)
22914 Dual clutch with fixed front pulley
14756 Drive belt
18940 Rear pulley with 12 tooth front sprocket
17693 Motor chain 96 links
20090 Rear sprocket 44 tooth (12/44 = 0.273 ratio)
30 mph Dimoby (1-speed) 01-1974 to 12-1980
22160 Side mount AR2-12-729 carburetor (12mm)
21201 Intake pipe (Ø12)
17073 Intake gasket
22335 Cylinder (30mph Dimoby)
22914 Dual clutch with fixed front pulley
14756 Drive belt
20731 Rear pulley with 13 tooth front sprocket
18427 Motor chain 97 links
20090 Rear sprocket 44 tooth (13/44 = 0.295 ratio)
25 mph Variator (2-speed) 03-1977 to 12-1980
52175 Center mount AR2-10-991 carburetor (10mm)
16673 Short intake (Ø10 restricted)
17073 Intake gasket
54328 Cylinder (25mph)
22921 Dual clutch with variable front pulley
14756 Drive belt
16653 Rear pulley with 11 tooth front sprocket
17785 Motor chain 102 links
20093 Rear sprocket 54 tooth (11/54 = 0.20 in low, 0.28 in high)
30 mph Variator (2-speed) 01-1974 to 03-1977
20026 Center mount AR2-12-705 carburetor (12mm)
16619 Short intake (Ø12)
17073 Intake gasket
23460 Cylinder (30mph variator)
22921 Dual clutch with variable front pulley
14756 Drive belt
16653 Rear pulley with 11 tooth front sprocket
17785 Motor chain 102 links
20093 Rear sprocket 54 tooth (11/54 = 0.20 in low, 0.28 in high)
30 mph Variator (2-speed) 03-1977 to 12-1980
54973 Side mount AR2-12-873 carburetor (12mm)
22899 Intake pipe (Ø12?)
17073 Intake gasket
23460 Cylinder (30mph variator)
22921 Dual clutch with variable front pulley
14756 Drive belt
16653 Rear pulley with 11 tooth front sprocket
17785 Motor chain 102 links
20093 Rear sprocket 54 tooth (11/54 = 0.20 in low, 0.28 in high)
Discussion:
This information is mostly from the Motobecane USA Parts Catalog, a big blue plastic 3-ring notebook from 1978-79.
Porting: The deepest secret is that there are four different cylinders, all identical on the outside. The different cylinders could have different port widths and heights (duration’s). Much more needs to be learned and explained. Why does a 2 hp one-speed model have a different cylinder than a 2 hp variable speed model?
Gearing: The drive belt is the same for all versions. The 1-speed front pulley is the same for all versions. The 2-speed front pulley is the same for all versions. The rear pulley (without the front sprocket) is the same for all versions. Therefore, the pulley ratio is the same for all versions. The variator models in “low gear” have the same pulley ratio as the Dimoby models. As the rear wheel speed increases, the variator makes the front pulley bigger. The engine swings back as the front pulley grows, until it is in “high gear”. In high gear the pulley is about 40% bigger. So even though the variator models are geared low (small front and large rear) in chain sprockets, the variator makes up for it, above 18mph, by making the overall gearing about the same.