Yamaha 25BMH Specifications
| Engine Model: Yamaha 25BMH |
| Engine Type: 2-stroke 2-cylinder gasoline outboard engine |
| Total Displacement: 30.2 cu.in (496 cc) |
| Rated Engine Power: 25 Hp (18.4 kW) at 5500 rpm |
| Fuel System: Carburetor |
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The Yamaha 25BMH is a 25-horsepower,
two-cylinder, two-stroke outboard engine from Yamaha's enduring Enduro
series. It is widely recognized as a lightweight, robust, and
uncomplicated power source, purpose-built for small fishing boats,
inflatables, and utility craft. This model is celebrated for its
mechanical simplicity and dependability, utilizing a manual tiller
control and a recoil starter system, making it a favored choice for
boaters who prioritize straightforward operation and ease of maintenance
in a portable package. The 25BMH is designed to deliver reliable power
without the complexity of heavier, four-stroke alternatives.
The Yamaha 25BMH is built around a fundamental, well-proven
architecture. It features a 2-stroke, L-type (inline two-cylinder)
engine configuration. The engine displacement is precisely 496 cc (30.27
cu in), achieved through a bore and stroke of 72.0 mm x 61.0 mm (2.83 in
x 2.40 in). This powerhead produces a maximum output of 18.4 kW (25 hp)
at 5,000 rpm. The engine is designed to operate within a full throttle
operating range of 4,500 to 5,500 rpm, with a specified engine idle
speed of 1,050 to 1,150 rpm. The compression ratio is stated as 6.2:1.
The power unit of the 25BMH is engineered for durability in demanding
conditions. The cylinder head has a warpage limit of 0.1 mm (0.0039 in).
The cylinder bore inside diameter is manufactured to 72.000-72.020 mm
(2.8346-2.8354 in). Correspondingly, the piston diameter measures
71.935-71.960 mm (2.8321-2.8331 in). The piston pin outside diameter is
19.895-19.900 mm (0.7833-0.7835 in).
Further internal specifications include a crankpin diameter of
23.995-24.000 mm (0.9447-0.9449 in). The connecting rod small-end inside
diameter is 23.904-23.917 mm (0.9411-0.9416 in), while its big-end
inside diameter is 32.000-32.016 mm (1.2598-1.2605 in). The connecting
rod big-end side clearance is maintained at 0.200-0.700 mm
(0.0079-0.0276 in).
The piston rings are critical for sealing and performance. The top ring
end gap is specified at 0.20-0.35 mm (0.0079-0.0138 in), with a side
clearance of 0.02-0.06 mm (0.0008-0.0024 in). The second ring end gap is
identical at 0.20-0.35 mm (0.0079-0.0138 in), and its side clearance is
slightly larger at 0.03-0.07 mm (0.0012-0.0028 in). The engine utilizes
a reed valve for the intake system and a loop charge scavenging system
to efficiently direct the fuel-air mixture and exhaust gases.
| Fuel System and Lubrication |
The 25BMH operates on a pre-mixed fuel and oil system. The recommended
fuel is regular unleaded gasoline, which must be mixed with a
high-quality 2-stroke outboard motor oil that meets the NMMA-certified
TC-W3 grade. The standard mixing ratio for this model is 50:1. The fuel
delivery is managed by a single carburetor. For cold starts, the engine
is equipped with a manual choke valve.
The cooling system is a raw-water, thermostatically controlled design,
featuring a large-capacity, high-efficiency water pump to prevent
overheating. The durability of this system is enhanced by a
hard-chrome-plated interior surface on the water pump, which resists
wear from debris in muddy water. Exhaust gases are expelled through the
propeller boss, which also helps to reduce noise.
| Mid Section and Lower Unit |
The lower unit houses the critical components for transferring power to
the water. The gear system uses a spiral bevel gear with a reduction
gear ratio of 2.08 (27/13). The gear shift positions are the standard
Forward-Neutral-Reverse (F-N-R), engaged via a dog clutch.
The propeller shaft uses a spline type connection, and the propeller
direction, when viewed from the rear, is clockwise. The correct
lubricant for the lower unit is hypoid gear oil meeting API GL-4 SAE 90
specifications, with a total gear oil capacity of 0.32 L (0.34 US qt).
This outboard is equipped with a manual trim and tilt system. The trim
angle can be adjusted to one of five preset positions: 4.0, 8.0, 12.0,
16.0, or 20.0 degrees. This allows the operator to adjust the boat's
attitude for optimal performance and fuel efficiency under different
load and water conditions.
For maneuvering in very shallow water, the engine features a shallow
water drive capability, allowing it to be tilted up further to avoid
striking the bottom. The maximum tilt-up angle is 76.0 degrees. The
steering angle is 40.0 degrees to both port and starboard.
To accommodate various boat types and applications, the Yamaha 25BMH can
be fitted with a range of aluminum propellers. The recommended propeller
sizes, all with a diameter of 9-7/8 inches, include the following pitch
options: 8, 9, 10, 10-1/2, 11-1/4, 12, 13, and 14 inches. The correct
propeller selection is crucial for allowing the engine to operate within
its recommended full-throttle RPM range.
The overall length is 843 mm (33.2 in) and the overall width is 399 mm
(15.7 in). Short Shaft Model: Overall height is 1,146 mm (45.1 in) with
a transom height of 381 mm (15.0 in). The dry weight for this model is
53.0 kg (117 lb).
Long Shaft Model: Overall height is 1,273 mm (50.1 in) with a transom
height of 508 mm (20.0 in). The dry weight for this model is 54.5 kg
(120 lb). Extra Long Shaft Model: Overall height is 1,359 mm (53.5 in)
with a transom height of 635 mm (25.0 in). The dry weight for this model
is 56.5 kg (125 lb).
Regular maintenance is essential for the longevity of the 25BMH. A
critical daily check involves ensuring the correct 50:1 fuel-oil
mixture. The gear oil in the lower unit should be checked regularly for
the presence of water or metal particles and changed according to the
manufacturer's schedule, which generally recommends an initial change
after the first 10 hours of operation and then annually or every 100
hours.
The water pump impeller is a wear item that should be inspected and
replaced periodically, typically every 1-2 years or if there are signs
of reduced cooling water flow, to prevent overheating. The spark plugs,
specified as NGK B7HS-10 or BR7HS-10, should be removed, cleaned, and
gapped annually, and replaced as needed. All moving parts, such as
steering and tilt mechanisms, require regular greasing with
water-resistant marine grease to prevent corrosion and ensure smooth
operation.
Starter nut: 10 Nm (7.2 lb-ft)
Drive plate bolt: 15 Nm (11.1 lb-ft)
Starter mount bolt: 12 Nm (8.9 lb-ft)
Power unit mount bolt: 22 Nm (16.2 lb-ft)
Flywheel magnet nut: 140 Nm (103 lb-ft)
Reed valve stopper nut: 4.5 Nm (3.3 lb-ft)
Cylinder head bolt (1st step): 15 Nm (11.1 lb-ft)
Cylinder head bolt (2nd step): 27 Nm (19.9 lb-ft)
Crankcase bolt (1st step): 15 Nm (11.1 lb-ft)
Crankcase bolt (2nd step): 27 Nm (19.9 lb-ft)
Spark plug: 25 Nm (18 lb-ft)
Lower case mount bolt (nut): 37 Nm (27.3 lb-ft)
Cooling water inlet cover screw: 4 Nm (3.0 lb-ft)
Propeller nut: 35 Nm (25.3 lb-ft)
Propeller housing bolt: 11 Nm (8.1 lb-ft)
Steering bracket bolt: 11 Nm (8.1 lb-ft)
Float pin screw: 2 Nm (1.5 lb-ft)
Float chamber screw: 2 Nm (1.5 lb-ft)
The Yamaha 25BMH is a versatile power source ideally suited for a wide
range of small watercraft. Its lightweight design and reliable power
output make it a perfect match for aluminum fishing boats, jon boats,
and utility skiffs commonly used on inland lakes and rivers.
It is also a popular choice for rigid inflatable boats (RIBs) and as a
dependable auxiliary or primary motor for small sailboats. The manual
tiller operation and simple maintenance requirements make it an
excellent engine for rental fleets, hunting and fishing cA, and any
application where ruggedness and ease of use are paramount.
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