How to stir up the outboard motor 5 liters from Yamaha: video and instructions

The issue of increasing the power of the outboard motor is one of the most discussed among owners of small boats. Often fishermen and outdoor enthusiasts are faced with a situation where the available 5 horsepower does not allow you to bring the boat to plane at an acceptable speed or do not cope with wind and current. That is why many pay attention to the possibility of reworking or, as they say in the people, “smothering” of the unit up to 9.9 hp. YamahaThese are often the same base, but with different limitations.

It is important to understand that technically swell It means removing the manufacturer's fuel mix limits for environmental compliance or power classification. In the case of popular 99 cubic centimeter two-stroke engines labeled as 5 hp, the procedure often boils down to redesigning the carburetor and exhaust system, however, before any manipulations are undertaken, the risks and responsibilities must be understood.

This article is purely informative and describes the technical aspects of internal combustion engine design. Any design changes remove the product from warranty and may affect safety. We will look at the mechanical principles of throttle valves, the configuration of carburetors and how power limitation is implemented in popular models.

Design features of the engines 5 hp and 9.9 hp

Many users mistakenly believe that 5 and 9.9 horsepower engines are completely different devices. In practice, especially in the Yamaha, Tohatsu and Mercury two-stroke engine line, the same 99cc cylinder block is often used.

The main element responsible for the power is carburetorIn the 5 hp version, the jellyclear cross section or the throttle stroke is artificially reduced. This prevents the engine from consuming the maximum amount of mixture even if the throttle is open completely. Mechanical limiters can be made in the form of a washer under the mix quality screw or a special emphasis on the throttle lever.

Also worth paying attention to the exhaust system. In some versions, the exhaust window can be partially closed, or the shape of the exhaust manifold is different from the 9.9 hp version. This is done to create additional resistance to exhaust gases, which reduces the efficiency of cylinder purging and, as a result, power. Yamaha 5 hp It often has a lower and lower exhaust sound due to these design features.

⚠️ Attention: Changes to the engine design can cause the piston group to overheat if the fuel mixture is not properly tuned.

Comparative tables are often used to visualize differences, and the following are the main parameters that may differ from one model to another with different passport power.

Parameter	5 hp engine.	Engine 9.9 hp.
Workload	99 cc.	99 cc.
Diameter of the main jeckler	Reduced (e.g. 72-76)	Complete (usually 80-84)
Throttle progress	Restricted by a point.	Full speed.
The graduation system	There may be narrowing.	Full section

So the base of the motors is identical, and it's technically possible to physically convert one into the other, but it's worth remembering that the lifetime of an engine designed for certain loads can decrease when overcharged, especially with cooling systems, which in the base version of the 5 hp may be less productive than required for long-term operation at full speed.

Tools and preparation for work required

Before you start dismantling and modifying, you need to carefully prepare the workplace and tools, working with the fuel system requires cleanliness and accuracy, any dust or dirt that gets inside can cause a failure. carburetor So having a clean desk, good lighting and a set of tools is the first step to success.

You'll need a standard mechanic kit to do the deregulation work, and you'll need to pay special attention to the quality of the screwdrivers, because the carburetor screws are often soft metal and can be easily licked, and you'll need special probes to adjust the gaps and possibly a drill if you need to machin the metal stops.

• 🔧 A set of wrenches and heads (basic dimensions for Yamaha: 8, 10, 12 mm).
• 🪛 A set of cross and slash screwdrivers of high quality (JIS standard is preferred for Japanese equipment).
• 🧼 Carburettor cleaner (spray) and compressed air for channel blowing.
• 📏 A caliper for measuring the diameters of the jellybeans and drilling holes.
• 🛢️ A fuel tank and a rag that leaves no pile.

It is important to take care of safety in advance. Work should be carried out in a well-ventilated room, away from open fire sources, gasoline and its vapors are highly flammable. It is also recommended to have a fire extinguisher on hand. Before disassembling, be sure to disconnect the fuel hose and release the residue of fuel from the float chamber.

The issue of spare parts deserves special attention. In some cases, simple mechanical modification will not give the desired result, and you will need to replace the main jeeler with a more productive one. So it makes sense to find a carburetor remake for the 9.9 hp version in advance or to buy a main jeeler of the desired diameter separately. Yamaha or proven analogues ensures the stability of the work.

Process of refinement of the carburetor and throttle

The most common and efficient way to increase power is to refine the fuel supply system. It is the carburetor that is the heart of the scattering process. In Yamaha 5 hp engines, the restriction is often implemented by reducing the diameter of the main jeeler or placing a restrictive washer under the mix quality control screw. There may also be a mechanical stop that prevents the throttle from opening completely.

First, you need to remove the carburetor from the engine. Carefully disconnect the fuel hoses, gas cable and air thrust. Once the carburetor is removed, it should be disassembled. First, unscrew the main jigsaw and measure its diameter. If it is significantly smaller than the 9.9 hp version (usually a few tenths of a millimeter difference, for example, 72 vs. 80), it should be replaced or drilled, although replacement is preferable.

The next step is to test the throttle, and if you have a fully twisted throttle handle on the rumble, the damper in the carburetor should open 100 percent. If you don't, find the throttle, it could be a screw with a sleeve on it, or just a throttle in the body, and by removing that throttle, you'll allow the engine to consume more of the mixture at high speeds, which will have a direct impact on the power.

⚠️ Warning: After increasing the capacity of the carburetor, be sure to check the color of the spark plug. White plaque indicates dilution of the mixture, which leads to the burn of the piston.

And don't forget the air valve, and in some versions, it also has a speed limiter, and once you make sure that all mechanical obstacles are removed, assemble the carburetor using a new gasket. Old gaskets often lose their airtightness after disassembly, which can lead to air suction and unstable engine operation.

After the carburetor is fitted, the engine needs adjustment, the mix quality screw (usually located at the bottom of the carburetor) needs to achieve stable idling speeds, then, on the water, the final adjustment is performed, and if the engine choked when the gas is opened abruptly, the mixture is too poor, and you need to twist the screw a little or increase the jellybar diameter.

Exhaust system development and leakproofness

The power of a two-stroke engine depends not only on how much fuel we have supplied, but also on how efficiently we have removed the exhaust gases. In stock versions of the 5 hp exhaust system often has additional constrictions or resonators of another shape that create reverse pressure. This pressure prevents the cylinder from cleaning the exhaust gases, and the fresh mixture is mixed with "working out", losing its energy value.

Emission upgrades are a more complex process that requires mechanic skills. Often enthusiasts waste the exhaust window in the cylinder or replace the muffler with a 9.9 hp equivalent. However, simply removing the partitions can cause the engine to become too loud and lose traction at low revs due to a violation of the resonant exhaust frequencies.

The critical thing to do after any intervention is to check the leakage. A two-stroke motor runs on the pressure and the pressure. Sucking excess air through the gaskets, the ossels or the poorly tightened joints of the carburetor will lead to the impoverishment of the mixture. This is the fastest way to get a piston bucket. All joints must be perfectly clean and tight.

• 🌬️ Check the condition of the petal valve - it should close tightly and not have cracks.
• 🔩 Use a sealant for the crankcase gaskets when assembly, if the design involves disassembling the crankcase.
• 💨 Conduct a leakproofness test before the first launch by applying 0.6-0.8 atmosphere pressure to the crank chamber.

Pay special attention to the condition of the lower unit. If you have changed the gaskets between the cylinder block and the dead-water, make sure that the exhaust gases do not break into the cooling system or, conversely, water does not enter the cylinder. Video tightness check instructions often show the use of soap solution to search for leaks.

Why does the engine heat up after a smothering?

After the power is increased, the heat load on the engine increases, the standard cooling system may not cope if the pump is worn out or the inlet is clogged with algae, and the reason may be too poor mixture.

Setup and break-in of the upgraded engine

Once the mechanical part is done, the adjustment stage begins, and even a perfectly executed tuning won't work without proper adjustment. carburetorIt is better to start the primary out of water, on special barrels or in containers, so as not to damage the impeller pump.

Adjustment is done in three stages: First, the Low screw (the quality of the mixture at low revs) achieves stable operation without failures when the gas is opened sharply, then the High screw (the quality at high revs) adjusts the maximum power. Be careful: twisting the screw enriches the mixture, twisting it impoverishes. You need to start with a rich mixture and gradually twist the screw until the revs begin to fall, then return a little.

Running is not a myth, it's a necessity. After making changes to the design, the engine should get used to new modes of operation. First tanks (3-5 full tanks) are not recommended to give full gas all the time. Run at 80% of maximum power, taking breaks to cool down. This will allow the piston rings to sit properly in the grooves, and bearings to distribute the lubrication.

The break-in stage	Mode of work	Duration
Initial.	Idle and slow move	10-15 minutes.
Basic	Up to 50% gas, variable	First 2 tanks
Progressive.	Up to 80% gas, short accelerations	Next 3 tanks
Full power.	100% gas (short term)	After 5 tanks.

During the break-in, carefully monitor the temperature of the engine. If the shell becomes too hot to the touch, or there is a weak water (or steam) pressure from under the control hole, immediately release the gas. Overheating can deform the cylinder head, and then the tightness will be disturbed.

Risks, legal aspects and security

Before you decide to motor-sniffLegally, changes to the design of the vehicle (and the outboard motor that is registered with the GIMS is part of the vehicle) require registration. If the power is listed as 5 hp and the engine actually produces 9.9 hp, this can be considered a violation when inspected, in some cases this can be a penalty or a ban on operation.

Technically, the increase in power leads to increased loads on all components. The rod, the crankshaft, the bearings, all work in a more stressful mode. The engine resource can be reduced, especially if you do not use quality oil for two-stroke engines and do not respect the proportions of the mixture.

Water safety is priority number one. A boat with a 5 hp capacity can have load and stability limitations. Installing a more powerful engine (even if it is formally left with 5 hp according to the documents) changes the centering and behavior of the vessel on the wave. An inexperienced driver may not cope with the increased dynamics, which can lead to a rollover.

⚠️ Note: Using a boat with modified engine specifications may result in the insurance company refusing to pay compensation in the event of an accident.

And also, you know, videos online often show the perfect result on new engines, and in reality, on a worn engine with a load in the cylinders, a simple replacement of a jellyplate might not work, but only make it harder to start, so you have to take a holistic approach: diagnostics of the condition of the engine, quality tuning and proper tuning.

In conclusion, the process of remaking a 5-horsepower Yamaha engine into a 9.9-horsepower engine is technically feasible and often produces excellent results. However, it requires direct hands, understanding the principles of the engine and being willing to take responsibility for the change, if you are not confident in your abilities, you should trust this work to professionals or leave the engine in a drain state, enjoying a reliable, albeit less fast, ride.

Will the fuel consumption increase after the swelling?

Fuel consumption will increase in proportion to the increase in power and operating time at full speed. If you used to swim at 3,000 rpm and continue after tuning on them, the consumption will hardly change. If you start using full power to enter the plane, the consumption can increase by 15-20%.

Can the engine be returned to factory condition?

Yes, unless you drilled the extra holes in the body and saved all the removed parts (clickers, washers, stops) Reverse replacement of the elements will return the engine to its original state, which can be useful when selling or checking.

Should I change the spark plug after tuning?

As the power increases, the engine's thermal mode changes, a candle with a different potassium number (more "cold") may be needed to avoid the kalyl ignition and detonation, and the candle is selected by the color of the plaque on the electrode.

Will I lose my warranty if I remove the fillings?

Yes, removing the plant seals and interfering with the fuel system automatically voids the manufacturer's warranty, and any subsequent breakdown, even if not related to tuning, can be attributed to the service as the consequences of modernization.