Mower Carburetor Adjustment

Adjustment and repair of the carburetor lawn mowers Instructions for adjusting the carburetor lawn mowers.

Principle of operation

The carburetor is a power system module in which liquid fuel and air are mixed before being fed into the engine cylinders. Mixing and feeding are adjusted accordingly depending on the tool and its mode of operation.

The carburetor device for lawn mowers is the same for almost all cases. It belongs to the category of float devices. The latter provide stable parameters of ready-made mixtures.

mower carburetor adjustment

The principle of the module is as follows.

  • Air enters the body of the air damper tube. The latter increases or decreases the air flow depending on its position.
  • In the area where the gasoline feed hole is formed, the tube has a narrowing diffuser. Here the flow rate increases.
  • Fuel from the float chamber is fed through the nozzle into the tube. The float level determines the amount of fuel supplied. Since the pressure in the chamber is normal, and in the tube it is low due to the greater rarefaction of the air, gasoline is sucked in through the nozzle.
  • The accelerated air flow picks up the fuel and sprays it, forming an air-fuel mixture of the desired density.
  • The mixture is sucked into the cylinder through the pipeline.

The more open the air damper in the tube, the higher the density of the air flow, which means more fuel enters the cylinder. Adjusting the carburetor of lawn mowers is reduced to setting such a ratio of flaps, at which the density of the fuel mixture becomes optimal.

Adjusting and repairing the carburetor lawn mowers

The trimmer does not belong to complex household appliances, therefore most of its breakdowns are mechanical in nature and are quite successfully eliminated with their own hands. Even if the engine fails for serious reasons, damage to the piston ring, for example, repairs are reduced to replacing a part.

But even with a relatively simple design, such a part of the device as the carburetor needs to be adjusted.

Signs of imbalance

Debugging the carburetor must be done during the manufacture of the device. As a rule, the owners will learn about the need to adjust the fuel supply with their own hands only after serious enough breakdowns, in the process of correcting which, the module was removed and, for example, cleaned.

Signs of a configuration failure are the following:

  • Starts, but the engine immediately stalls because the engine can only work when a certain amount of gasoline is supplied, with too lean fuel, that is, blocking the supply channel, the engine will stop;
  • Too high fuel consumption, a lot of exhaust gases are the result of incomplete combustion of the mixture. In this case, on the contrary, there is too much fuel and the mixture is too dense.

Adjusting the motorcycle carburetor

Adjustment of the carburetor of a motorcycle of any model may be required after a certain mileage or after their repair, when the normal quality and quantity of the combustible mixture is no longer the same, and the engine does not develop full power. In this article we will look at how to adjust the carburetor of most motorcycles with one or more cylinders and what you need to do.

In addition to the loss of power and deterioration in the dynamics of the bike, the fact that it is time to adjust the carburetor (or carburetors) can be prompted, for example, by a changed color of the exhaust, or the color of carbon deposits on the spark plugs.

Adjusting the carburetor of a single cylinder motorcycle engine is fairly straightforward, but adjusting the carburetor of a multi-cylinder bike may seem daunting to many. In addition, to fine tune the carburetors of a multi-cylinder engine, you will need some tools, which will be described below. But I will nonetheless start with the simplest carburetor adjustment for a single-cylinder engine, and after that we’ll look at adjusting the carbs of modern multi-cylinder engines.

Single cylinder engine carburetor adjustment.

The design, operation and adjustment of most single-cylinder motorcycle carburetors is almost the same. To begin with, I advise you to read about the device and about the malfunctions of a modern motorcycle carburetor in this article, which will help beginners draw the right conclusions before adjusting. And I advise you to read about malfunctions and restoration of vacuum carburetors here.

Vacuum carburetor for a four-stroke motorcycle engine.
1 diffuser cavity, 2 rubber diaphragm, 3 throttle valve spring, 4 throttle valve, 5 needle, 6 butterfly valve, 7 atomizer tube, 8 main jet, 9 float, 10 needle valve.

And as will become clear from the above article, any carburetor has a device that allows you to change the position of the fuel needle, the position of the air damper, and the patency of the air channel. And these are the basics that you should know before adjusting and I will remind them below:

  • The needle position is adjusted by moving the stopper along the grooves of the needle and thus the needle is set higher or lower relative to the calibrated hole in the spray gun. When moving the stopper on the needle up, the needle goes down and the mixture becomes leaner, and when the stopper is moved one groove down, the needle rises and the combustible mixture is enriched. I hope this is clear, let’s move on.
  • The air damper position is adjusted with a special screw, and on most carburetors, when the screw is tightened, it presses on the damper and raises it a little higher. When the screw is unscrewed (this screw is called the screw for the amount of mixture), the flap is lowered. However, on some carburetors, for example, on the St. Petersburg K65, the screw number 1 (see photo below) is installed at the very top of the carburetor (and not on the side) and works the other way around, that is, when it is unscrewed, the damper rises and the speed increases (the adjusting screws of vacuum carburetors will be described in a separate section below).
  • Well, the adjustment of the patency of the air channel is changed by a quality screw with a cone at the end, and this screw works according to the principle of the simplest tap, that is, when the screw is unscrewed, the cone at its tip moves back and the cross-section of the air channel increases, while more air enters and the combustible mixture becomes leaner And when the quality screw is screwed in, the cross section of the air channel decreases, less air enters (in relation to the fuel) and the mixture is enriched. I hope this is clear and it’s time to move on to adjusting the carburetor.
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Before starting the adjustment and doing what I describe below, if you are a completely newbie, then first check with the manual of your engine, what markings your carburetor has and where exactly the quality screw and the quantity screw are on your carburetor. If you already know where which screw is, then go ahead.

The correct ratio of air to fuel, that is, obtaining a normal combustible mixture, is the ultimate goal of correct carburetor adjustment. A normal combustible mixture consists of 1 kilogram of gasoline and 15 kg of air, theoretically required for complete combustion of gasoline. I wrote about this in great detail in an article about the device and malfunctions of a motorcycle carburetor (link above in the text), as well as what combustible mixture is required for each operating mode of the engine.

Here I will describe a practical guide for most modern motorcycle carburetors. And so, in order to adjust the carburetor of a single-cylinder motorcycle engine so that it prepares a normal combustible mixture, you first need to determine what combustible mixture it already prepares and feeds into the engine lean or rich.

You can determine this by the signs that I will describe below.

The most common way is to unscrew the spark plug and look at the color of its carbon deposits. Carbon deposits on the electrodes and spark plug insulator can tell a lot, not only about whether the carburetor is properly adjusted, but also about the technical condition of the engine (for more details on the color of carbon deposits, see the link above in the text).

If everything is in order with the engine and it does not have an increased oil consumption, and the color of the soot on the plug is black, then the mixture is too rich. To make the working mixture lean, unscrew the quality screw by 1/4 of a turn and after replacing or cleaning the candle, drive several tens of kilometers along the highway and re-inspect the candle. If the carbon is still black, then unscrew the quality screw another 1/4.

But if after that, when gas is supplied, you hear shots in the carburetor (inlet pipe), then you are too lean in the mixture and you should return the quality screw a little back. It should be achieved by adjusting the screw and the correct carbon deposits on the spark plug (light brown) and a good response to the throttle, without shots in the inlet. When the quality screw is unscrewed, the engine speed will increase, which means we reduce their quantity with the screw, lowering (closing) the air damper with the screw.

However, one can judge the quality of the combustible mixture not only by the soot on the candle, but also by other signs that should be remembered, namely:

Signs of a lean mixture pops (shots) in the intake manifold, engine overheating, loss of power (poor traction). But keep in mind that such signs can also be with too early ignition, so first check the ignition adjustment. As I already said, if you find that your mixture is poor, then screw in the quality screw by 1/4, and if the signs of a poor mixture are not eliminated, then screw the screw a little more.

It so happens that it is impossible to enrich the mixture with twisted quality (the engine does not react), which means that the mixture is depleted due to clogging of the fuel supply channel (needle valve), clogging of the nozzle of the main dosing system, or the nozzle of idle (if the engine does not work well at low speeds ), sticking of the float and lowering the fuel level in the carburetor float chamber.

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Of course, you should check the correct fuel level and blow out the gasoline supply channel with compressed air. Depletion also occurs from the appearance of a gap between the carburetor and the air filter (or when the rubber pipe breaks). Do not forget to check that all connections are tight and free of gaps, breaks, clogging of the main metering system jet, or idle jet (if the motor does not work well at low speeds). Another sign of a too lean mixture is the difficult start of a cold engine.

Signs of a rich mixture are shots in the muffler and black smoke coming out of it, loss of engine power (traction), engine overheating, increased fuel consumption and oil dilution in the engine crankcase, and the formation of black carbon deposits in the combustion chamber, on the piston and on the spark plug. Difficulty starting a hot engine is also a sign of mixture enrichment.

A rich mixture can be caused by wear and loose closure of the needle valve in the float chamber and the weakening of the seat of its seat, the fuel level in the float chamber is increased. Also, an increase in the level of gasoline (and, accordingly, enrichment) may be due to damage to the float and filling it with fuel, due to jamming of the float behind the walls of the float chamber.

Enrichment can be on old carburetors due to the development of a calibration hole in the jets, due to incomplete opening of the air damper, or simply from a violation of the fuel level due to improper bending of the float tongue (check in the engine manual what the fuel level should be for the carburetor of your model ). Well, a common reason for enriching the working mixture is a too dirty air filter, when you replace it with a clean one, everything will return to normal.

After eliminating all the above faults, if there are still signs of enrichment, then the quality screw should be unscrewed by half a turn, but if signs of depletion appear when turning the throttle knob (pops in the carburetor), then the quality screw should be screwed back a little (1/4 turn)

Well, that seems to be all for the adjustment, and as for the installation of the needle, it is best to set it to the middle position, in which on most motorcycles the dynamics are good and the fuel consumption is moderate. But for someone who is not satisfied with something, then you can experiment with setting the needle, one division up or down. By raising the needle, you can slightly increase the engine power, but the fuel consumption will increase.

Adjusting the carburettors of a modern motorcycle multi-cylinder engine.

Most modern motorcycle multi-cylinder engines have vacuum carburettors and I will describe their adjustments later. But if your motorcycle is quite old or with high mileage, or you just bought a used motorcycle (or just bought used carburetors and want to install them on some motorcycle), then before you start installing and adjusting them, I advise you to put the carburetors in order.

How to do it correctly and not spoil anything, I advise you to read this article, which is called How to restore normal operation of vacuum carburetors. And after you restore the old carbs, they still serve and you can start adjusting them. Before making adjustments, inspect the carburettors on your bike and find out where the quality adjustment screw and mixture adjustment screw are located, and then you can start adjusting.

The main difficulty in adjusting several carburetors, each preparing a combustible mixture for its own cylinder, is that it is stupid to set the adjusting screws in the same way throughout the factory (counting the revolutions when tightening). After all, each carburetor is installed separately on its own cylinder, and the parameters of each cylinder, after a certain mileage of the bike, change in different ways.

Carbon deposits are deposited in each cylinder in different ways, compression changes in different ways, resinous deposits are deposited in the channels of each carburetor and in the jets, all channels, jets and needles of each carb also wear out differently. This means that the work of each carburetor and the power output of each cylinder is gradually mismatched and must be periodically restored.

For this, there is an adjustment operation called synchronization. But synchronization is not a complete adjustment and adjustment of carburetors, because by connecting vacuum gauges and performing synchronization with their help, many craftsmen adjust only the amount of fuel with the quantity screws. But what about the quality of the mixture, the adjustment of which is very important.

I have already written in detail about how to make the correct synchronization and adjustment of not only the amount of fuel, but also the quality, and I advise you to read it here. It also describes how to make a vacuum gauge (synchronizer) with your own hands.

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After using vacuum gauges and screws to achieve the same amount of vacuum in the inlet pipes, you should use quality screws to ensure that the CO indicator in the exhaust gases is within the toxicity standard and is absolutely the same on all cylinders.

Of course, most motorcyclists don’t have a gas detector in their garage. But you can go to a car service and use the CO check service. And in the same place, adjust the quality of the mixture on the carburetors. Most multi-cylinder motorcycles have only one muffler, and in order to check and adjust the quality of the mixture in each carburetor, on multi-cylinder motorcycles there are special holes in the exhaust pipes for this, into which the plug screws are screwed.

Before adjusting these screws, moisten them with a card (WD-40) and unscrew them. Next, we start the engine and substituting the gas analyzer probe to the hole in the pipe of the first cylinder, turn the quality screw on the carburetor of the first cylinder until the CO indicator becomes within the normal range (the operating personnel know the norm for a four-stroke engine).

Then we do the same operation for each cylinder, rotating the quality screws on each carburetor and make sure that the CO indicator is within the normal range and the same on all four cylinders. After adjustment, grease the screws with graphite (or copper) grease and screw them into each outlet.

If, for example, you have a two-cylinder motorcycle with two mufflers, then there are no screws in the exhaust pipes and the probe of the gas analyzer, when adjusting, we simply substitute it to the outlet of each muffler.

Having achieved with the help of vacuum meters the same amount of mixture, and with the help of a gas analyzer of the same quality of the mixture on all carbines, only in this way the adjustment of the motorcycle carburetors will be considered complete, and the response of the engine to the throttle handle will pleasantly surprise you, success to everyone.

Features and best models

The Stihl mower deserves a special mention, which is distinguished by its versatility, reliability and ease of use. The company’s specialists have developed many types of braids, with different characteristics and applications.

For instance, the range of lawn mowers is designed for mowing grass in large areas. For this, a self-propelled wheel system was developed, with the help of which it became possible to take care of a large area without spending a lot of time and effort. A lawn mower is perfect for decorating shrubs, giving them a certain artistic image and texture. And if you need a lawn mower for tall grass and uneven areas, then follow this link and get complete information on the selection of this type of technology.

If you need a more subtle approach to trimming grass, for example, near flower beds or between trees, then the gasoline trimmer, designed for accuracy and precision, is perfect for this task. The company’s specialists have foreseen all the nuances and possible difficulties when working with braids, so they equipped them with nylon strings that prevent mechanical damage to trees.

A special AutoCut trimmer head is also used to ensure the normal functionality of the petrol trimmer string knives. This article will tell you about the prices of petrol hand-held lawn mowers and trimmers, it will also help you make the right purchase that you will not regret.

The professional trimmers include Stihl FS 130 and FS 250. Let’s consider the characteristics and properties on the example of several mowers, in particular FS 38, FS 70 and FS 130.

DIY Stihl trimmer repair and operating instructions FS-38 and 55 (Stihl FS), carburetor adjustment

It is inherent in man to ennoble the place in which he spends the lion’s share of his time. Residents of cities resort to the services of designers, or they themselves try to change something in their home, buy new furniture, wallpaper, so that when they come home after a hard day, they can mentally rest and relax in a favorable environment. Residents of their own homes outside the city have completely different concerns and requirements. It is necessary to mow shrubs, grass, various dead wood in a timely manner, carefully monitor the condition of the personal plot and garden. It is clear that this requires slightly different means of achieving comfort. And here the new brainchild of Stihl mower engineering comes to the fore.

Recommendations for use

It is necessary to take temporary breaks from work. This is necessary so that the engine of the device does not overheat and fail prematurely. For amateur models, there is a break every 20 minutes for the same time. For professional mowers, work should be interrupted every 50 minutes, in which case the downtime should be at least 10 minutes.