Motom 48 – Repair Manual

This is the Motom 48 repair manual, translated into Italian from the original German "Reparatur-Hinweise".

A previously unpublished and likely unique Motom 48 repair manual published for the Swiss market.

Manufacturer: Motom Italiana S.A., Milan Distributor: Motom AG, Zurich, Pelikanstraße 6

Translation from the original German "Reparatur-Hinweise".
Note for German-speaking readers: We recommend referring directly to the original German manual. This translation was done from Italian and may contain inaccuracies in terminology.

Document courtesy of Massimo Prata of the Motom Club Zurich

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ENGINE

Piston Alignment in the Cylinder: Every time the engine is reassembled, check that the piston is centered in the cylinder. If it is not, insert spacer rings with a thickness of 0.1 to 0.2 mm behind the ball bearing. If the piston is not properly centered, excessive oil consumption may occur. The crankshaft must be well centered (max. runout 0.025 mm).

Tightening the gearshift lever shaft retaining nut: Extreme care must be taken when tightening this screw with a screwdriver. It must not be overtightened, so as not to damage the safety pin on the gearshift lever shaft. On the other hand, it must not be too loose, as otherwise, with the engine running, the pin would have play in its seat. Tightening must therefore be performed precisely to prevent either of these problems from occurring.

Checking the oil pump: If the pump fails to operate, and during every engine overhaul, verify that the threaded stud in the pump’s internal channel is properly screwed into the pump housing. To prevent the pin from loosening, chamfer the threaded edges: this improves the seal.

Installing the side cover on the engine block: During this procedure, ensure that the camshaft pin is seated in the designated bearing located in the cover, and under no circumstances resting on the cover itself. If this were to occur, the safety pin would be damaged: the camshaft would press inward toward the engine, and the drive shaft flange would prevent its rotation.

Checking valve clearance: The intake and exhaust valves should have virtually no clearance, but they must be able to rotate freely. If the seat has become oval-shaped due to wear, or if excessive clearance has developed, this must be eliminated. There must be no play between the valve spring seats and the half-cones mounted on the stem.

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CHANGE

Installing the retaining ball on the drive shaft: When installing the drive shaft, do not forget the retaining ball and its spring (parts 41–44), which are housed inside the central pin (14). It is the selector that, by sliding inside the pin, pushes the balls outward, causing them to engage with the gear wheels. If the retaining ball is missing, the mechanism cannot hold the selector in position, and the gears will not engage properly.

Installing the oil seals: When installing the oil seals on the drive shaft after mounting the gearbox gears, make sure that the lip of the oil seal faces toward the inside of the engine. You must also ensure that the teeth of the first engagement gear do not rub against the inner surface of the oil seals. Finally, ensure that the gears are properly lubricated. Even the slightest misalignment can cause the clutch to overheat.

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CLUTCH

Clutch adjustment: If the clutch operated by the handlebar lever does not disengage sufficiently, adjust it using the adjustment screw. However, if the adjustment screw has already been fully tightened during previous adjustments and further adjustment is not possible, the new adjustment is made by moving the clutch cable to a different hole in the mounting plates.

Clutch bushing seizure: It may happen that the engine continues to turn even when the clutch lever is engaged. The problem is caused by the sprocket on the drive shaft being seized. To fix this, remove the sprocket and the bronze bushing and grind them down by a few hundredths of a millimeter, smoothing out any irregularities on the drive shaft if necessary.

Adjusting the selector fork rod: If you are unable to engage a specific gear while the engine is running, check that the sprocket mounted on the fork rod is precisely centered within its housing on the drive shaft. To do this, open the engine casing and remove the drive shaft sprockets along with their respective selector cams. Ensure that a wedge is inserted beneath the drive chain sprocket to press the crankshaft bearing firmly into its seat, eliminating any axial play. Then, by operating the external gearshift lever, engage the three gears one after the other. The exact position of the gear ring relative to the housings can be checked using a small mirror inserted into the casing opening. If the sprocket is not properly centered, bend the drive shaft plate horizontally until it fits into all three positions. Be careful that the bending does not detach the weld point of the plate from the fork arm.

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TECHNICAL SPECIFICATIONS

Engine

Entry	Value
Type	4-stroke
Cylinders	1
Bore	39 mm
Running	40 mm
Displacement	48 cc
Maximum power at 4,500 rpm	1.7 hp
Compression ratio	7:1
Cooling	Air-cooled

Broadcast

Entry	Value
Gear ratio	Straight-toothed cylindrical gears
Speeds	2,63:1
2nd gear	1,69:1
3rd gear	1:1
Clutch	Single-disc dry clutch

General Information

Entry	Value
Electrical system	6V, 15W
Shock absorbers	In the rear wheel hub
Hand brake	2 oversized cables
Tire size	2,00×18
Step	1.13 m
Width	0.70 m
Height	0.95 m
Dry weight	41.5 kg
Fuel consumption	1.3 L/100 km
Maximum speed	40 mph

Workshop data — Piston

Entry	Value
Original size	39 mm
Oversized	39.4 and 39.8 mm
Piston-cylinder clearance	0.05 mm

Plug

Entry	Value
Diameter	11 mm
Length	32 mm
Axial play	0.015 mm

Segments

Entry	Compression	Oil scraper
Number	2	1
Width	2 mm	2.5 mm
Service game	0.2 mm	0.2 mm
Side play	0.02 mm	0.02 mm

Crankshaft

Entry	Value
1st main bearing journal Ø	15 mm
Length	13 mm
2nd main bearing journal Ø	15 mm
Length	10 mm
Crankpin Ø	20 mm
Length	9 mm
Axial play	0.1–0.2 mm

Camshaft

Entry	Value
Bearing seat diameter	10 mm
Building game	0.03 mm
Total length	56 mm
Camshaft profile angle	45°

Valves

Entry	Value
Head diameter	15 mm
Shaft diameter	5 mm
Total length	56 mm
Office Corner	45°
Guide Rod Game	0.025 mm
Cold valve play	0.15 mm

Timing

Entry	Value
Suction opens	16 days before PMS
Suction shuts off	44th after the PMI
Drain opens	44th before the PMI
Drain plug	16 days after PMS

Power On

Entry	Value
System	Flywheel magnet generator
Voltage	6V, 15W
Champion Spark Plug	10 hours to go
Bosch Spark Plug	W 175 T1
AC Candle	45-L
Auto-Lite Spark Plug	AL-5
Candela Marelli	CW 175 A
Electrode spacing	0.5 mm
Maximum advance	23° = 22 mm at the flywheel
Pin spacing	0.5 mm
Capacitor capacity	0.12–0.15 μF

Carburetor

Entry	Value
Type	Weber 14 MFM
Venturi diameter	9 mm
Main jet	No. 60
Idle jet	No. 40
Floating weight	5 g

Brakes

Entry	Value
Front type	Drum-type, internal brake shoes
Front drum diameter	107 mm
Rear type	Drum-type, internal brake shoes
Rear drum diameter	107 mm

Capacity

Entry	Value
Fuel tank	3.5 L
Reserve	0.3 L
Motor oil	0.4 L
Winter oil	SAE 30
Summer Oil	SAE 40

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For more information on this article, please see:

All Guides Guides

Reboring the Motom Crankshaft — What You Need to Know

Which Spark Plug to Use on a Motom — The Definitive Guide

What Oil to Use in Your Motom — The Definitive Guide