Owners of PVC boats often face a dilemma: low power does not allow you to get on plane with full load, and the purchase of a new engine 9.9 hp punches a gap in the budget. TohatsuIt is structurally almost identical to the more powerful models. Stir the Tohatsu 5 engine - means not just remove the seals, but competently reconfigure the fuel supply and ignition system for safe productivity improvement.
This process requires technical literacy, because interference with the engine removes warranty and increases the risks of operation. However, with the right approach, you can get a full 8-9 horsepower from a 5-horsepower unit, saving the life of the nodes. In this article, we will discuss all the stages of the upgrade, from mechanical removal of the limiters to fine tuning the carburetor.
Risk assessment and legal aspects of modernization
Before picking up tools, you need to clearly realize that any interference with the design of the internal combustion engine changes its technical specifications. Tohatsu's smothering 5bhp It moves the engine from a category that does not require driving rights (up to 5 hp inclusive) to a category where having a tractor mechanic license or GIMS license becomes a legal requirement, and ignoring this fact can lead to serious fines when meeting a water inspection.
From a technical point of view, factory constraints are not set for nothing. Tohatsu intentionally limit the speed and fuel supply to ensure the maximum life of the piston group and crankshaft bearings when using fuel of not the highest quality. After removing the limiters, the temperature in the combustion chamber increases, which requires higher quality gasoline (AI-95) and the mandatory use of high-quality two-stroke oil labeled TC-W3.
It is also important to consider the condition of your boat: the increase in thrust can be excessive for the transom of a small inflatable boat, which will cause it to deform or even rupture. Engine power If the boat is designed for a maximum of 5 forces, the installation of a suffocated engine creates an emergency situation, especially when moving against the wave.
⚠️ Warning: Self-engine modification cancels the factory warranty. If the engine is new, consider whether the risk of losing the warranty is worth a few kilometers per hour.
Design features and types of limiters
Motors. Tohatsu MFS 5C And their analogues (including some Mercury models) are built on a single platform with engines of 6 and 8 hp Factory restrictions are implemented in two main ways: software (through the CDI unit) and mechanical (through the carburetor and throttle).
In most modern versions of the "five" the main "strangler" is an electronic control unit (CDI). It does not allow the crankshaft to spin above a certain frequency, usually about 4500-5000 revolutions per minute, while a full-fledged motor develops 5500-6000 rpm Mechanical limiters can be present in the form of a washer under the screw of a throttle or a special stop in the gas traction.
You need to have a visual inspection to determine your motor type. Take off the top casing and find the carburetor. If you see a screw on the throttle lever with a restrictive washer or a plastic stop that prevents you from opening the valve completely, you're lucky that part of the work is done mechanically. If the damper opens completely and the motor doesn't pull anyway, then you're lucky that the damper is mechanical. speed limiter software.
How to distinguish CDI blocks from different years of release?
Early release CDI units (before 2010-2012) often had markings to determine the possibility of soldering. Modern units are filled with compounds and require a complete replacement of the module or the use of an external controller. Visually determining the possibility of chip tuning without opening is almost impossible, so often you have to go by sampling or searching for information by engine serial number.
Mechanical improvement: removing physical constraints
You should always start with the mechanical part, because that's the base for further adjustments. Even if the main limit is set by the electronics, the mechanical obstacle will not allow the carburetor to work effectively. The first thing you need to do is dismantle the air filter and get to the carburetor.
In many models Tohatsu 5bhp There's a screw that limits the throttle, and it can be on the lever itself, or it can be on the mounting point of the gas cable, and you have to twist that screw out, or you have to sharpen the restrictive washer, to make sure that the throttle is running at full speed, and you can get the maximum amount of fuel-air mixture into the cylinder.
It's also worth checking the needle's position in the throttle. Factory settings often suggest poor mix for the environment. By raising the needle's locking ring one position higher (closer to a wide part of the needle), you'll enrich the mixture at medium speeds, which will improve acceleration dynamics. But be careful: too rich a mixture will lead to soaking on the candle and loss of power.
☑️ Checking of the mechanical part
Electronic Blowing and Flashing by CDI
If mechanical improvements did not give the desired effect (the engine rests on the "cut-off"), then interference with electronics is required. Tohatsu 5 stir software can be in two ways: replacing the CDI-block with a model of 9.9 hp (if they are compatible with connectors and coils) or soldering / reflashing the standard unit.
The most reliable but expensive way is to buy a sports CDI unit or a unit from an older model, which guarantees a stable spark and the right ignition angle (IEO) at high revs. Cheap Chinese counterparts often work unstable, which can lead to a piston burn.
The second option is to sell the resistors inside your CDI, which requires opening the sealed housing of the unit (often filled with resin, making the procedure irreversible without damage), finding the resistor responsible for limiting the speed, and replacing it with a counterpart with a different resistance value. The exact values depend on the specific scheme of your year of release and should be searched on specialized forums of motorists.
| Type of refinement | Complexity | Power build-up | Risks. |
|---|---|---|---|
| Removing the throttle puck | Low. | 5-10% | Minimum |
| Replacement of the CDI block | Medium | 40-60% | Incompatibility of connectors |
| Resistor soldering | High. | 40-60% | Complete failure of electronics |
| Integrated tuning | High. | Up to 100% (up to 9.9bhp) | Reduced engine life |
⚠️ Warning: When soldering a CDI unit, there is a high risk of damaging tracks or overheating the chips with a soldering iron. If you do not have experience with microelectronics, it is better to entrust this work to professionals or choose to replace the unit entirely.
Setting up the carburetor after swelling
Once you've removed the restrictions, the engine will start to consume more air, and if you leave the carburettor settings in the factory, the mixture will become too poor, and working on a poor mixture is the main enemy of the two-stroke engine, leading to overheating and bullying. carburettor This is the mandatory final stage.
Carburetors. Tohatsu You'll usually have two adjustable screws: an idle screw (quality) and a mix quantity screw. After suffocation, you often need to turn the mix quality screw 0.5-1 more than the factory tags, which will enrich the mixture in transition modes.
The mixture is best checked by the color of the spark plug soak. After 15-20 minutes of full gas (water, under load), twist the candle. The brick or light brown color of the insulator indicates the perfect mixture. White signals overheating and poor mixture (to be enriched), and black velvety plaque - about overenrichment (to twist the screw of quality).
Use only fresh spark plugs NGK or Denso with the potassium number recommended by the manufacturer for forced modes. An old candle can give a false impression of the color of the soda.
Testing and running of the upgraded engine
Do not immediately after assembly give "full gas". Suffocated engine The first 2-3 tanks of gasoline are recommended to roll back in a gentle mode, opening the throttle by no more than 70-80%, which will allow the piston rings to fit correctly to the walls of the cylinder in new temperature conditions.
During the test runs, listen carefully to the engine. The appearance of a ringing metal knock (knock) indicates too early ignition or an excessively poor mixture. In this case, you should immediately stop operation and adjust the settings. Also watch the temperature of the body: if it becomes hotter than usual, water cooling may be disturbed or the mixture is too poor.
Be sure to check the emergency shutdown and reverse, after tampering with traction and electrical, these systems must work flawlessly, and water safety is more important than any speed.
Successful swelling is the balance between removing constraints and maintaining reliability. Don't chase the maximum numbers unless you're willing to maintain the engine more often.
Frequently Asked Questions (FAQ)
How much horsepower can you get from Tohatsu 5 hp?
With a competent approach and replacement of the CDI-block, you can get real 8-9 hp Mechanical removal of washers without electronics will only give a small increase in dynamics, but will not remove the “ceiling” of revolutions.
Will the engine life decrease after swelling?
Yes, the life of the piston group and bearings will decrease by about 20-30% with active operation at full gas, and the use of quality oil and gasoline AI-95 will help to minimize this effect.
Can I return everything before selling or checking?
Mechanical changes (pucks, screws) are reversible. If you don't solder the CDI unit, but just replaced it, it takes 15 minutes to get back to drain. Electronics are irreversible.
Will a replacement screw be required?
Often, a regular propeller (usually 7-8 inches) does not allow for the increased power, and the engine “does not rotate.” A swelled motor often requires a screw with a smaller pitch (for example, 6 inches) to reach maximum speed.