The situation when boat-engine It refuses to develop its declared power and fails to reach full speed, a phenomenon familiar to many aquatic motorists, often referred to as "sub-cool," not only slows the boat's speed, but can signal serious malfunctions within the power unit, ignoring symptoms often leads to increased fuel consumption, overheating and even jamming of the piston group.
In this article, we'll go into detail about the mechanics of the process so you can diagnose the problem yourself. Yamaha, Tohatsu or Mercury It's sluggish, it's going to help you avoid expensive repairs at the service center, and we're going to look at all the steps, from the state of the propeller to the finer points of the fuel mix.
Before you take a carburetor apart or go into electronics, you need to do a first visual evaluation, and often it's because of the trivial things that you can easily overlook when you look at it, and your job is to methodically eliminate the external factors that affect the engine.
Effect of propeller on engine speed
The most common cause of undercoiling is an incorrect propeller, and if the pitch is too big for your engine or boat weight, the engine simply doesn't have the strength to turn it at the right speed, which puts an excessive load on the crankshaft, and the motor can't physically reach maximum speed.
Notice the condition of the blades: chips, holes or deformation of geometry disrupt the fluid dynamics, causing the engine to operate inefficient mode. Even a small crack at the end of the blade can cause vibrations that the ignition system will try to compensate by resetting the angle of advance.
It's also worth checking for foreign objects, because a line, algae, or plastic bag that hits the shaft creates resistance equivalent to a sharp increase in the pitch of the screw, in which case the motor starts to suffocate and does not pull.
The optimal rev range for most outboard motors is 4500-6000 rpm depending on the model, a deviation of more than 500 rpm downwards from the upper limit of this range is considered sub-core.
- 🚤 Check the marking on the screw: the number after the “P” sign indicates a step (for example, 9P is a step of 9 inches).
- 🔍 Examine the edges of the blades for chips and burrs that disrupt the flow of water.
- 🌿 Make sure there are no coiled algae or fishing nets on the shaft and lower unit.
- ⚖️ Estimate the boat’s load: Overload often requires installing a screw with a smaller pitch.
⚠️ Attention: Operating a motor with a severely damaged screw can cause the lower unit shaft bearings to collapse due to imbalance and vibration.
Problems with fuel supply and tank ventilation system
If the engine runs at exactly low speeds, but "chokes" when trying to open the throttle, the problem is almost certainly in the fuel system. Lack of gasoline pressure prevents the carburetor float chamber or injector ramp from getting the desired volume of the mixture. The engine starts working on the depleted mixture, which causes power failures.
Special attention should be paid to the vent in the fuel tank lid, and when you consume fuel intensively inside the tank, you create a vacuum that the pump simply cannot overcome, and if you open the tank lid and hear the characteristic whistling of the intake air, it means that the engine is dead because of the vacuum plug.
Fuel filters are another critical node: a clogged sediment filter or an internal carburetor filter acts like a throttle, limiting the flow. Water entering the system also causes unstable operation, because water is heavier than gasoline and settles down, getting into the engine first.
- 💧 Unscrew the plum plug on the settling filter and check for water or debris.
- 🔩 Blow the fuel hoses and check the pear for cracks and loss of elasticity.
- 🌬️ Open the tank cover while driving: if the thrust is gone, the problem is ventilation of the tank.
- 🛢️ Use only fresh fuel and high-quality additives if the engine has been idle for a long time.
Clogging of carburetor and jelly-cloth
For two-stroke engines and older four-strokes, the carburetor is the heart of the power system. Over time, resinous deposits form in the fuel that settle in the narrow channels of the jelly-cutters. Even a microscopic particle of dirt can block the fuel supply in full load mode, causing an under-coil effect.
Cleaning the carburetor requires care and understanding of the device. You need to remove the top cover, remove the float and unscrew all the jellyplates. Pulling in compressed air and using special chemistry to clean the carburetors allows you to restore the permeability of the channels. It is important not to damage the calibrated jellybar holes with metal objects.
In today's injector systems, the nozzles are the nozzles, and the clogging is treated with ultrasonic cleaning or replacement, but before you remove the power system, make sure that the problem is not with the sensors (TPS, MAP), which can send the wrong signals to the control unit.
How to clean the jellybers properly?
Never use thin wire or needles to clean calibrated jigler holes, which changes their throughput and geometry, causing misalignment. Use only aerosol cleaners and compressed air.
- 🧴 Use a carburetor cleaner with a tube to purge hard-to-reach channels.
- 🔧 Write down the position of the mix quality adjusting screws before disassembling to return the basic settings.
- 👀 Check the condition of the float needle: if it is worn out, the fuel level in the chamber will be unstable.
| A symptom. | The probable cause in the carburetor | Method of decision |
|---|---|---|
| The engine stalls with sharp gas | Clogged with top jikler | Blowing and cleaning of jeeler |
| Unstable idle turnovers | Air suction or dirt in the channel XX | Checking gaskets and cleaning the channel |
| Black smoke from the exhaust | Fuel transfusion (floating needle) | Replacement of the needle or adjustment of the level |
| Engine "troit" under load | Uneven flow to cylinders | Complete defects and cleaning of all cameras |
Ignition system: candles, coils and switches
The sub-docury can be caused by a slight spark. When the engine goes to high speeds, the time to ignite the mixture is reduced. If the spark is weak or has the wrong timing, the combustion does not occur completely and the power drops. spark-plug.
The color of the candles tells you a lot about the condition of the engine. Black velvety soda indicates a rich mixture, white or melted electrode indicates overheating or poor mixture. Oily plaque indicates that oil enters the combustion chamber, which is typical of worn piston rings or glands.
High-voltage wires and coils are also aging, and breaking down insulation at high speeds can cause the spark to go out onto the mass before reaching the candle. In wet weather, this is especially evident. Visual inspection in the dark with the engine running sometimes helps to see the "dancing" sparks of breakdown.
Always have a new ignition plug kit on board, and even if it's not the problem, replacing it with a well-functioning spark plug is the quickest way to remove the ignition system from the list of suspects.
- 🕯️ Unscrew the candles and check the gap between the electrodes: it must meet the manufacturer’s specification.
- ⚡ Check the resistance of high-voltage wires with a multimeter (usually no more than 5-10 kOhms per meter).
- 🔌 Examine candle caps for oxidation of internal contacts and corrosion.
- 📉 Measure compression: Low compression will prevent the engine from developing power, even if the spark is perfect.
⚠️ Attention: When checking for a spark, never hold a candle in your hand or touch the metal parts of the engine to avoid being electrocuted. Use special testers or press the candle to the mass through an insulator.
Mechanical malfunctions and compression
If the fuel system and the ignition are fine and the engine is still not spinning, you should think about the mechanics. Loss of compression is the main enemy of power. The wear of piston rings, a blow in the cylinder or a burnout of the gasket gasket, cause the fuel-air mixture to not compress properly and not release the necessary energy.
Another hidden problem is the sucking of excess air through the crankshaft coils. In two-stroke engines, the coils work in an aggressive environment and eventually tan or crack. The engine begins to suck air through the sock of the crankshaft, disrupting the calculated proportions of the mixture, you can determine this by the characteristic whistle at certain turns or spraying the joints of the crankcase with a carburetor cleaner when the engine is running (the turns will change).
The jamming of crankshaft or rod bearings also creates mechanical resistance, usually accompanied by a hum or knock, but in the initial stages it can manifest itself simply as the inability to unwind the engine. The glitch of the thermostat that does not open will lead to overheating and automatic resetting of the electronics (if the motor is equipped with a protection system).
☑️ Diagnostics of mechanics
Carburetor setting and electronic control
Modern motors are controlled by an ECU unit that receives data from multiple sensors. If the throttle position sensor (TPS) sends the wrong signal that the gas is not fully open, the control unit will not give a command to enrich the mixture and change the ignition angle, as a result, the engine will not enter full power mode.
For carburetor engines, it is important to adjust the quality and quantity of the mixture correctly. Factory settings can be lost due to vibration or interference of an inexperienced owner. Too poor mixture limits power and causes overheating, too rich one strangles the engine and smokes candles.
In some cases, you need to flash the control unit or diagnose it through the interface. DSM Errors can be stored in memory, and the motor will operate in emergency mode, limiting the speed (Limp Mode).
- 💻 Connect a diagnostic scanner to read error codes in the control system.
- 🔧 Adjust the gas cable: the carburetor valve should open completely with the full course of the handle.
- 🎚️ Set up the mix quality screw on the heated engine according to the instructions for your model.
How to Reset Mistakes on the outboard motor?
Often, to reset errors, you just need to turn off the motor, disconnect the battery terminal for 10-15 minutes, but if the cause of the malfunction is not eliminated, the error will appear again, and for deep diagnostics, you should use a dealer scanner.
Can poor quality gasoline cause bad quality?
Yes, low-octane fuel or gasoline with a high alcohol/water content can cause detonation. The electronics of the motor, fixing the detonation, will force retard (shift) the angle of ignition, which will lead to loss of power and inability to spin the engine.
Does the overgrowth of the bottom of the boat affect the turnover?
Strong fouling of seashells and algae on the bottom of a boat or transom plate significantly increases water resistance, and the motor is forced to expend additional energy to overcome resistance, which can reduce maximum speeds by 300-500 units.