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Published: Monday, 20 February 2012 13:20
Written by PB van Geelen




Spark plug Carburetor
Adjusting fuel level Ignition and Electrical
 Valve system Lubrication
Suspension Tightening torques
Engine removal  Cylinder head & valves

Spark plug electrode gap should be 0.024 - 0.028 inch on all models. Recommended spark plug is NGK type C6HB. The tightening torque can be found here.

The variable venturi type carburetor is shown in Fig. HN1-1. The idle mixture is adjusted at needle (11). Idle speed should be 1500RPM and is changed by turning stop screw (10). Normal setting for needle (11) is 1 1/8 turns open. Refer to Fig. HN1-1 and the following specifications:

NOTE: Type A & B carburetors are used on P50 only.

Type 50A Carburetor
Before frame serial number A148608

Type 50B Carburetor
AFTER frame serial number A148607

Part   Setting/Size   Part.no.   Part Setting/Size     Part.no.
Main jet (18) 55   00000-000-000 Main jet (18) 62   00000-000-000
Idle Jet (21) 35   00000-000-000 Idle Jet (21) 35   00000-000-000
Needle Jet (24) 2.6 x 2   00000-000-000 Needle Jet (24) 2.6 x 1.55
Circlip (6) 3rd groove from
top of needle
(7) 00000-000-000 Circlip (6) 2nd groove from
top of needle
(7) 00000-000-000


1 Cable adjuster 15 Float cover
2 Cover 16 "O" Ring
3 Washer 17 Float
4 Spring 18 Main Jet
5 Retainer 19 Gasket
6 Clip 20 Cover
7 Valve needle 21 Idle Jet
8 Throttle slide 22 Plug
9 "O" Ring 23 "O" Ring
10 Idle stop screw 24 Needle Jet
11 Idle mixture needle 25 Fuel inlet needle
12 Choke plate 26 Fiber washer
13 Choke shaft 27 "O" Ring
14 Choke spring 28 Inlet valve seat

Fuel level on early models with type  A carburetor should be set as follows: Remove carburetor and cover (15). Invert the carburetor. Clearance (Fig. HN1-2) between fuel bowl and  float with the inlet valve closed should  be 3.4-3.6mm (0.1378 inch). Adjustment is accomplished by bending float arm.


On P50 models with type B carburetor, float setting can be checked after  removing the float valve cover. Fuel inlet valve and float are inside cover as shown in Fig. HN1-3. With inlet valve  closed (cover inverted), distance (H) between float and bowl should be 4.9- 5.1mm (0.1969 inch). If incorrect, bend  the float arm as required.


The ignition system is of the energy transfer type with the low tension (generating) coil and breaker points located under the flywheel. The high tension coil and condenser are located inside the frame above the engine. The breaker cam is inside the flywheel. An automatic ignition retard is used on P50 models to keep the maximum engine rpm at approximately 4500. Breaker point gap should be adjusted so that points just open when the "F" mark on flywheel is aligned with the mark on crankcase as shown at (Fig. HN1-4). Breaker point maximum gap should be within limits of 0.012 - 0.016 inch when timing is correctly set. The ignition low tension coil (Fig. HN1-5) is located toward rear and lighting coil (3) toward front. Flywheel retaining nut should be tightened to 13 -1 8 Ft./Lbs. torque.

HN 1-4

HN 1-5

The valves are actuated by a chain driven camshaft located in the cylinder head via rocker arms (cam followers). Valve clearance should be set when engine is cold and piston is at TDC on compression stroke. The flywheel has a mark "T" indicating top dead center when aligned with crankcase mark "M" (Fig. HN1-5). Clearance should be 0.05mm (0.002 inch) for both valves. Make certain that the end of decompression lever has 0.2 - 0.4 inch free play. Adjustment is accomplished at rear of cable housing. If the lever does not have free play, the exhaust valve may be held open.

The engine is lubricated by 1½ pints of high detergent motor oil. SAE 30 motor oil should be used above 60 degrees F., SAE 20 or 20W from 32 to 60 F., SAE 10W below 32 degrees F., multi-grade 10W-30 oil can be used for all seasons. Oil should be maintained at upper mark on filler plug dipstick. NOTE: Do not screw filler plug into crankcase when checking oil level. Engine oil should be changed every 2000 miles or every three months. Front suspension should be greased at fittings (12-Fig. HN1-7) every 1000 miles.

HN 1-7

The front suspension is shown in Fig. HN1-7. Twenty-six bearing balls (26) are used in lower race of steering head and twenty-one in the top race (21). When servicing the front suspension, renew rubber parts if deteriorated. Check all parts for wear or damage and renew as necessary. Fittings (12) should be greased every 1000 miles.
The following lists the recommended tightening torques.
Part name Imperial torques Metric torques
Clutch nut 27.5 - 32 Ft./Lbs.  
Crankcase 5.8 - 8.0 Ft./Lbs.  
Cylinder head.
( 4 nuts & 1 screw)
6.5 - 8.7 Ft./Lbs.  
Cylinder head cover
(4 screws)
5.8 - 8.1 Ft./Lbs.  
Flywheel 13 - 18 Ft./Lbs.  
Rear axle 29 - 43 Ft./Lbs.  
Spark plug 8.0 - 10.8 Ft./Lbs.  

Most service to the engine and drive system requires removal of the engine. Disconnect spark plug wire and other (four) electrical wires. Remove the carburetor and disconnect the decompression cable, rear brake cable and the exhaust pipe. On P50 models, remove the torque arm screw (2-Fig. HN1-8) and nut (3). Loosen the rear axle nut and raise the frame until the engine and rear wheel are clear. Be careful to prevent oil from running onto the brake shoes.

HN 1-8

Remove the cy Iinder head cover (with rocker arms). Remove dowel pin (1-Fig. HN1-9), withdraw the camshaft center shaft (3) and remove camshaft. Remove the four stud nuts and one screw then lift off the cylinder head. Cylinder head gasket surfaces must be perfectly flat. Refer to the following specifications for cylinder head and associated parts.
Part name Imperial sizes Metric sizes
Combustion chamber volume   5cc
Valve face and seat angle    
Both valves   45 degrees
Valve stem diameter    
Both valves 0.188 - 0.189 inch  
Wear limit-minimum 0.187 inch  
Valve guide inside diameter    
Inlet 0.189 - 0.1894 inch  
Exhaust 0.1898 - 0.1902 inch  
Valve seat width    
Both valves 0.028 - 0.039 inch  
Valve head minimum thickness    
Both valves 0.008 inch  
Valve spring free length    
Inlet & Exhaust 0.764 inch  
Wear limit 0.708 inch 18mm

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