Model: Y4, Y4 MK,2 250 Stormer
1968 - 1974

Standard Technical Data for MX use, without "Works Tuning"

Engine
Bore: 68mm
Stroke: 68mm
Capacity: 247cc
Compression Ratio: 11:1
Port Timings: Exhaust: 180 deg, Transfer: 135 deg, Inlet: 152 deg.
Ignition Timing At Piston: 2.5mm (.1 ins.) At Crankshaft: 20 - 22 deg.
Contact Breaker Gap: .38mm (0.015 ins.)
Spark Plug NGK B8ES - Champion N3 Plug
Gap: .635mm (.025 ins.)

Transmission
Engine Sprocket: 20T
Clutch Sprocket: 43T
Gearbox Sprocket: (520) 13T
Rear Sprocket: (520) 61T
Gear Ratios (Close Ratio): 1st - 2.00 2nd - 1.51 3rd - 1.25 Top - 1.00

Carburetter
The Amal Mk1 32mm "932" was fitted as standard, however we recommend fitting the Amal Mk2, 34mm for standard Stormer 250 engines and Mk2 38mm for "Works Tuned" engines.
(Check for eligibility with your club).

Amal 932 Settings:
Throttle valve: 3 Main Jet:
280 Needle Jet: 107 Needle position:
Middle Pilot Jet: 20

Capacities
Fuel Tank: 1Imp Gallon, 2.4 US Gallons, 9L.
Gearbox: 1pint, 20 fl oz,
Primary Chaincase: 0.57 pint, 7.5floz, 210cc
Front forks (each leg): 0.57 pint, 7.5floz, 210cc

Lubricant Types (recommended)
Fuel Mix: Silkolene Casterene R40 @ 32:1
Gearbox: Silkolene Medium Weight Gear Oil,
E.P.90 Primary
Chaincase: Silkolene Light Weight Gear Oil sae 10/40 75w or Silkolene 10w Front Fork Oil If using our red dot primary chains.
Rear Chain: Silkolene Chain Lube.

Other data
Steering Head Angle: 30 deg.
Fork Trail: 5.15ins.
Pistons Piston Skirt Diameters: 67.913mm/67.887mm (2.6738/2.6727 ins.)
Piston Clearance in bore: .076mm/.89mm (.0025/.003 ins.)
Ring Gap: .178mm/0.305mm (.007/.012 ins.)
Con-Rod length, centers: 126.97mm/127.025mm (4.999/5.001 ins.)
Small End Bush Bore:17.47mm (.675 ins.)
Torque Settings See Workshop Manual.

Model Y5, Y50 370, 370 Stormer
1970 - 1974

Standard Technical Data for MX use without "Works Tuning"

Engine
Bore: 83mm
Stroke: 68mm
Capacity: 368cc
Compression Ratio: 11:1
Port Timings Exhaust: 180 deg, Transfer: 130 deg, Inlet: 170 deg.
Ignition Timing At Piston: 2.5mm (.1 ins.) At Crankshaft: 20 - 22 deg.
Contact Breaker Gap: .38mm (0.015 ins.)
Spark Plug NGK B8ES - Champion N3 Plug Gap: .635mm (.025 ins.)

Transmission
Factory set-up. Engine Sprocket: 24T
Clutch Sprocket: 40T
(we use Engine Sprocket: 20T
Clutch Sprocket: 43T)
Gearbox Sprocket: (520) 12T
Rear Sprocket: (520) 65T
Gear Ratios - Wide Ratio. (We use the 250 Transmission set-up and a C.R. gearbox) 1st - 2.52 2nd - 1.66 3rd - 1.25 Top - 1.00

Carburetter
The Amal Mk1 34mm "1034" was fitted as standard, however we recommend fitting the Amal Mk2, 36mm for standard Stormer 370 engines and Mk2 38mm for "Works Tuned" engines. (Check for eligibility with your club).

Amal 1034 Settings:
Throttle valve: 2.5
Main Jet: 370 - 410
Needle Jet: 108
Needle position: middle
Pilot Jet: 25

Capacities
Fuel Tank: 1Imp Gallon, 2.4 US Gallons, 9L.
Gearbox: 1pint, 20 fl oz,
Primary Chaincase: 0.57 pint, 7.5floz, 210cc
Front forks (each leg): 0.57 pint, 7.5floz,

Lubricant Types (recommended)
Fuel Mix: Silkolene Casterene R40 @ 32:1
Gearbox: Silkolene Medium Weight Gear Oil, E.P.90
Primary Chaincase: Silkolene Light Weight Gear Oil sae 10/40 75w or Silkolene 10w Front Fork Oil If using our red dot primary chains.
Rear Chain: Silkolene Chain Lube.

Other data
Steering Head Angle: 30 deg.
Fork Trail: 5.15ins.
Pistons Piston Clearance in bore: Check with us as different type pistons require different clearances.
Ring Gap: .178mm/0.305mm (.007/.012 ins.)
Con-Rod length, centers: 26.97mm/127.025mm (4.999/5.001 ins.)
Small End Bush Bore:17.47mm (.675 ins.)
Torque Settings See Workshop Manual.

Model Y51, 410, 410 GP, 410 Stormer
1971-1974

Standard Technical Data for MX use without "Works Tuning"

Engine
Bore: 83mm
Stroke: 74mm
Capacity: 400cc
Compression Ratio: 11:1
Port Timings: Exhaust: 180 deg, Transfer: 130 deg, Inlet: 154 deg.
Ignition Timing At Piston: .125 ins/3.2mm At crankshaft: 20-22 deg.
Contact Breaker Gap: .38mm (0.015 ins.)
Spark Plug NGK B8ES - Champion N3
Plug Gap: .635mm (.025 ins.)

Transmission
Engine Sprocket: 24T
Clutch Sprocket: 40T
Gearbox Sprocket: (520) 12T
Rear Sprocket: (520) 65T
Gear Ratios-Wide Ratio. (The C.R. gearbox may be used. See 250 data.) 1st - 2.52 2nd - 1.66 3rd - 1.25 Top - 1.00

Carburetter
The Amal Mk1 34mm "1034" was fitted as standard, however we recommend fitting the Amal Mk2, 38mm for standard Stormer 410 engines and "Works Tuned" engines.

Amal 1034 Settings:
Throttle valve: 2.5
Main Jet: 370 - 410
Needle Jet: 108
Needle position: middle
Pilot Jet: 25

Capacities
Fuel Tank: 1Imp Gallon, 2.4 US Gallons, 9L.
Gearbox: 1pint, 20 fl oz, 210cc
Primary Chaincase: 0.57 pint, 7.5floz, 210cc
Front forks (each leg): 0.57 pint, 7.5floz, 210cc

Lubricant Types (recommended)
Fuel Mix: Silkolene Casterene R40 @ 32:1
Gearbox: Silkolene Medium Weight Gear Oil, E.P.90
Primary Chaincase: Silkolene Light Weight Gear Oil sae 10/40 75w or Silkolene 10w Front Fork Oil If using our red dot primary chains.
Rear Chain: Silkolene Chain Lube.

Other data
Steering Head Angle: 30 deg.
Fork Trail: 5.15ins.
Pistons Piston Clearance in bore: Check with us as different type pistons require different clearances.
Ring Gap: .178mm/0.305mm (.007/.012 ins.)
Con-Rod length, centers: 139.67mm/139.725mm (5.499/5.501 ins.)
Small End Bush Bore:17.47mm (.675 ins.)
Torque Settings See Workshop Manual.

Gear Ratio Chart

250
370
410