In the world of small mechanization and boating technology, there is a special category of devices that, despite their venerable age, continue to arouse respect and interest among boat owners. outboard motor Johnson 4For years, the archetype of light inflatable and small rigid hulls was a symbol of an era when simplicity was more important than sustainability and maintainability was valued above electronics.
Today, when the market is filled with Chinese analogues and complex four-strokes, many boaters recall with nostalgia. Johnson 4bhp.It's looking for ways to extend the life of existing units or buy a used one in good condition, and understanding how this motor works allows you not only to operate it efficiently, but also to carry out quality maintenance on your own.
In this article, we will discuss in detail the technical nuances, configuration features and secrets of durability of this legendary engine. two-stroke This performance was so successful, and how to properly care for the engine, so that it served faithfully for decades. A unique feature of this model is the modular design of the lower unit, which allows you to disconnect the lower part without completely dismantling the engine.
Technical specifications and design features
Engine design Johnson 4 It's based on a time-tested two-stroke, crank-chamber purging scheme, and the 4 horsepower rating is achieved at a crankshaft speed of about 5,000-5,500 per minute, which is the best indicator for planing light boats, the cylinder capacity is 72 cubic centimeters, and the cooling system is based on the principle of taking off-board water through the intake in Dadwood.
The weight of the unit in dry form is about 16-18 kilograms, which makes it easy enough to install on PVC transom boats, but at the same time massive for confident overcoming the excitement. Ignition system here the classic - contact-transistor (magdino), which, on the one hand, requires periodic cleaning of contacts, and on the other - guarantees spark formation even with partial discharge of the battery or its complete absence in versions with a manual starter.
The most important element of the design is the carburetor, which in most versions has a float chamber and a petal valve at the inlet. petal-valve The design of the lower unit provides stable operation of the engine at low revs and a quick response to the gas handle. The design of the lower unit provides for the presence of a lower unit transmission with a gear ratio, usually 2.08 or 2.15, which allows you to efficiently transfer torque to the propeller.
⚠️ Warning: When operating a motor in salt water, be sure to use fresh water to flush the cooling system after each exit. Corrosion of lower unit aluminum alloys at cooling water outlets is the main cause of premature pump failure.
For visual control and understanding of the scale of the unit, below is a table with the main technical data:
| Parameter | Meaning |
|---|---|
| Type of engine | two-stroke, 1 cylinder |
| Workload | 72 cm |
| Maximum power | 4 hp (2.9 kW) |
| Ignition system | Magdino (CDI or contact) |
| Weight (dry) | ~17 kg |
Preparation and breaking in
Proper preparation Johnson 4bhp. If you buy a new engine or refurbish an old one, it's mandatory to run it in a new season, and unlike today's four-stroke engines, two-stroke engines require a separate fuel mix, strictly observing the proportions of oil and gasoline.
It is usually recommended to mix AI-92 or AI-95 gasoline with a quality two-stroke oil in a ratio of 1:50 (for modern oils) or 1:25 (for old-type mineral oils) for running and further use. Careful mixing of the mixture is a critical step, since uneven distribution of oil can lead to piston group bullies.
The process of running should be in a gentle mode. It is not recommended to immediately give full gas. The first fuel tanks are better developed at speeds not exceeding 70-80% of the maximum power, periodically changing the modes of operation from idle to medium, This allows the piston rings to properly gray to the walls of the cylinder, providing the necessary compression.
Before each launch, you need to perform a visual inspection, check the condition of the fuel hoses for cracks, make sure that the connections are tight and there is no air sucker. Fuel filterIf it is installed in the system, it should be clean, and the paging pear should be whole and elastic.
☑️ Checklist before Johnson 4 launch
Maintenance and adjustment of the carburetor
The heart of any two-stroke motor is the carburetor, and Johnson 4 It's not an exception. Over time, the settings can get lost due to vibrations or changes in fuel quality. Adjustment is done using screws of quality and quantity of mixture. carburetor The process is subtle, requiring an understanding of the engine operation in different modes.
The casing may be required to access the adjusting screws. The Low Speed adjusts idling and transition modes, and the High Speed controls maximum power. Factory setting is usually optimal, but when using ethanol fuel or after long parking, correction may be required.
Carburetor cleaning is a must if the motor has started to stall or has lost its throttle. Small-section jiclairs are clogged with resinous deposits that form when gasoline evaporates. Compressed air purges and the use of special carburetor cleaners help restore the permeability of the channels.
⚠️ Warning: Never adjust the High Speed to the point of depleting the mixture (twisting the screw) for fuel economy, which causes the piston to overheat, detonate and quickly fail.
Symptoms of improper tuning of the carburetor
If the engine dies when the throttle is opened abruptly, the mixture is too poor at low revs. If the candle is black and wet, and the fuel consumption is high, the mixture is overenriched. Smoke exhaust and the soak on the candle also indicate an excess of oil or a rich mixture.
Ignition system and electrician
Electrical circuitry Johnson 4 is relatively simple, which is its advantage: the main elements are a magneto (magnet flywheel), a ignition coil, a breaker (in contact versions) or a switch (in contactless versions), a capacitor and a spark plug. A stable spark is the key to a confident start and smooth operation.
The gap between the contacts of the interrupter (if any) should be 0.3-0.4 mm. The angle of ignition advance is also regulated by the position of the contacts or the sensor. Incorrectly exposed ignition can lead to overheating (too early) or popping into the carburetor (too late). Ignition candle It must be selected according to the specification, usually NGK BR7ES type or analogues.
When troubleshooting, the first thing to check is for a spark. Twisted the candle, press it with your skirt against the engine mass, and pull the starter abruptly. The spark should be bright blue and punch the gap. If the spark is red or weak, check the state of the high-voltage cap and tip.
When checking the spark, do not hold the candle in your hands without isolated passages, even through gloves - the voltage in the ignition system can reach 20-30 kV, which is dangerous to health.
Repair of lower unit and Cooling System
The bottom of the engine, or lower unit, takes all the blows of the elements. The rubber blubbers over time and loses elasticity, especially if the engine has been idle for a long time without water. The impeller replacement is a standard procedure that should be performed every 2-3 seasons or when symptoms of overheating appear.
Overheating of the engine often results in loss of power and steam, if the water stream from the control hole is weak or absent, the engine must be immediately shut down, the causes may be clogging of the water intake with grass, destruction of the impeller or the formation of an air plug in the pipes.
Repairs are less often required, but oil replacement is a mandatory annual event. Johnson If you see an emulsion in the leaked oil, it signals water entering through the osteoils, which requires replacing them to avoid corrosion of the gears.
To dismantle the lower unit to replace the ossels or the gear shift rod, the switch thrust and fuel/water pipes must be disconnected, and the design allows the lower part to be separated, gaining access to the vertical shaft and water pump.