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MECHANICAL
Husqvarna 40 Maintenance
Complexity: Low
The Original Swedish
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If a real example should be given of the principles of reliability, durability, and quality, I would immediately say that Husqvarna 40. ​This original Swedish technique was made before 1990, so more than 30 years, but I think this machine would be able to define its own World Brand even completely alone too.
It is so simple, easy, and durable. Perhaps that's why it's no wonder that in its own time this chainsaw was very popular around the world. Moreover, I still remember those times as well, when the brand new was really expensive and people treated it - if someone managed to get it somehow - almost as a family member at home.
Yes, this is not a slip of my pen, but this model was first introduced to the market in 1978, and despite the fact that technically there is no special equipment inside or outside, it is still a highly competent tool in the used market as well, because it simply works well in all conditions.
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To say a few words at the beginning specifically about this device as well, unfortunately I have to mention that this was not mine, but I really liked this machine during the whole maintenance process. All its plastics components are original Swedish, and were produced of course only from high quality materials. I know its background very well, and I have to say that it worked really a lot in many forests, because it was the only chainsaw for the owner in the last three decades, but there was neither breakage nor other injuries on its components during this long period of usage. It was brought to me for a short maintenance, because it was a little dirty inside and its engine didn't want to start at the 20th pull either.
Misleading Appearance?
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When I first picked it up I didn't even try to start its engine, because I was quite sure that there will be at least piston change in parallel with crankshaft bearing replacement, and I was thinking minimum of a complete engine overhaul as well. ​This was my very first impression based on its age and appearance, and of course also because of what I have already said a little above, that its owner could not start it even after 20 attempts.
In addition, I should mention that a similar machine, but 10 years younger, required a complete engine overhaul a month before this project.
Just briefly, in terms of its appearance at first glance it wasn't too confidence building machine.
Burned 30-year-old black tar was in a thick layer on the exhaust muffler, and dark oily sawdust everywhere. This machine operated mostly with mineral-based two-stroke oil because that time there was nothing else. Of course, that was enough for this engine as well, as it was designed specifically for that type of oil.
So at first it didn’t look so nice because it was quite oily, tarry and dirty, but it is also true that the oil can preserve things nicely and does not give way to the harmful corrosion processes.
Nonetheless, the general saying here was especially true that appearances sometimes deceive the worker, because in fact - as it turned out later - this machine was in really good condition and all I had to do was mainly - along with other little things - the carburetor maintenance.
The sequence of quick Checks
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If a petrol engine does not want to start, I always check for proper ignition first. I was satisfied with the ignition system itself because the spark plug gave a nice thick, and stable signal. It is just a two-stroke gap driven engine, so we can say that half of the system works properly. :)
The next step is to pour out the old gasoline content because, after a year of standing, the water separates nicely from the petrol, and settles in a coherent mass at the bottom of the fuel tank, while the lighter mineral oil content floats on top of the water. So, I like to flush the tank with fresh petrol and after that, the complete fuel supply system has to be checked, whether it is nicely permeable and free of blockages.
Of course, the air filter should not be forgotten either. The filter was made from a durable nylon mesh cartridge and it was pretty much clean, so it can most certainly get enough air.
In summary, let's not forget that this is a forestry machine and not a pharmacy scale, so some dirt is completely allowed.
What's going on inside the Carburetor?
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Going through the above checkpoints, I was able to narrow the circle to the fuel injection unit, which is also called - due to its purely mechanical structure and operation - as carburetor, and not fuel injector.
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Its name is certainly known to everyone, but its varied design and construction already requires more serious expertise. I, too, only came into closer contact with the carburetors through my previous motorcycles. However, if I have already mentioned it, it may be worth noting here that there are also serious and precise - especially Japanese - carburetors on the commercial market, which are already equipped with various sensors - such as the TPS - and thus they are already connected to the electric controller unit (ECU) of the engine as well. It is true that, compared to the much more accurate and purely electronically controlled injectors, the larger carburetors are already quite obsolete, but they are still in production on some models - even on more serious 400-650 cc motorcycles - to this day.
In addition, carburetors can be found on almost all 2 and 4-stroke petrol engines as well, so I think it is worth to be aware of their operation on some level. So, let's start at the beginning.
The fuel enters the carburetor through the rubber fuel line. Very briefly, the fuel path in the carburetor looks like as follows. At first, the fuel flows through the fuel pump membrane then through the needle valve to the dosing membrane.
The black rubber membrane - see pictures below - with a small round metal plate is called the dosing membrane, and the other transparent membrane is the fuel pump membrane with small semicircular valves. These small valves provide the one-way fuel delivery.
The fuel pump membrane is driven by the pressure pulse which comes from the crankcase, and
the dosing membrane drives the needle valve lifting arm, but the needle valve is closed in the default position, so it cannot pass fuel through itself. More precisely, if the dosing membrane rises it moves away from the needle valve lifting arm, and this way the needle valve also closes. Thus, it also means that no more fuel can flow into space under the dosing membrane.
When the vacuum is generated - thanks, of course, to the piston - the dosing membrane moves downwards and pushes the needle valve lifting arm, so the needle valve opens. As a result of this simple step the space below the rubber membrane will be filled with fuel, and the dosing membrane expands again due to its flexibility, and the low and high-speed nozzles receive the fuel from the space below the rubber membrane due to its elastic force. Then the step comes when the nozzles blow the gasoline into the air which comes from the direction of the butterfly valve and the air-fuel mixture enters the combustion chamber, due to the strong suction effect of the piston.
Carburetor Troubleshooting
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So, if the operation of the membrane-type carburetors is pretty much understandable, let’s start the troubleshooting process, which is also not a too complicated task, because it is possible to check quite simply with pressure tests, that how much gasoline the needle valve is able to pass through on itself, and how well the seals close the compressed air. So in other words, the first step is to check the gas tightness of the carburetor.
Hopefully, everything becomes clear what needs to be replaced after this step. Anyway, during this inspection, putting the carburetor in gasoline, the leakage will be visible, due to the bubbles. However, I used to do it - so this sinking test in a small gasoline container - only at the end of the whole assembly process if the carburetor is not enough gas-tight, but it is worth examining this thing at the beginning as well.
After all this, the precise and careful disassembly process of the carburetor can be started. The seals are usually very difficult to separate, thanks the constant compression and heat load, but nevertheless nothing should be over-forced. After disassembling the carburetor, it’s no surprise that we can find all kinds of dirt almost everywhere.
As it can be seen, the main task is to remove the dry gasoline layer from all the components.
I think so, it is important to note that the little storage where the dried green gasoline can be seen is completely removable, and you can easily replace this little container from the Walbro carburetor repair kit, but I always like to clean thoroughly and gently the original factory parts.
After all this I like to clean all the elements of the disassembled carburetor multiple times in very hot dishwashing water. If possible, I use carbon-filtered water to remove mineral salts and other additives from the tap water which could cause harmful corrosion. My favorite detergent gel is the classic lemon Frosch, which is completely environment friendly and very efficient. Anyway, I always used to wash the primary pump and the plastic parts together with the carburetor body.
Don’t worry about hot water because after washing, I blow the whole carburetor through with compressed air. After that, all parts are deeply cleaned with EGR cleaner as well, and dried with compressed air, and finally comes the complete drying in warm air.
I disassembled the small spring arm system which moves the needle by swinging of the dosing membrane, but to my greatest surprise, I found everything to be perfectly fine.
Don't forget, these components are about 30 years old.
I decided that I won't replace anything here because it is absolutely unnecessary.
After thoroughly cleaning all the nozzles, I started the reassembling very carefully.
The old needle was beautiful with a perfect seal. This was the main reason for the excellent gas tightness of the carburetor during the first pressure control test. I have already talked about this perfect quality.
The next step was to replace the fuel pump membrane and dosing membrane together with the seals.
At the end of the complete assembly, I checked the gas tightness again. It was beautifully strong because the carburetor was permanently able to keep the pressure of around 1 bar.
The reassembly of the carburetor and air filter went smoothly, but it took some patience.
All threads of the screws were completely fine, and I also took care of the perfect assembly.
Its carburetor is original Walbro, and it was never been disassembled, and so it has never been cleaned in the past 30 years. It basically means two things to me. The whole construction is made of very high quality materials with excellent precision, and the other that it was used with great care.
In other words, only the carburetor cleaning - along with replacing its seals and membranes - and setting the carburetor adjustment bolts correctly caused basically the main challenge. The final settings - which required a great deal of expertise - was necessary because the carburetor adjustment bolts have already been turned back and forth by the owner. My advice is to keep the factory settings because the problem is usually not with these screws.
Anyway, three letters, namely T, H, L are written on the right side of the gray round handle, which means the following:
T: throttle valve control screw
H: high engine speed fuel controller screw
L: low and medium engine speed fuel control screw
These screws are responsible for the very fine adjustment of the carburetor.
One more time, my short piece of advice is not to touch these screws without deep experience.
It takes a lot of knowledge to adjust them, and it is very difficult to explain this know-how here.
It also takes me a while to set them properly, but hopefully, I can describe it in a separate topic.
Cleaning the Muffler
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With gas flame and wire brush I carefully removed the 30-year-old thick layer of black tar from the muffler as well. Of course, I also burned out the large coke deposition from the muffler to achieve a result close to 100% in the end, whereas the increased resistance of the clogged exhaust hinders the optimal exhaust gas flow and healthy gas oscillations.
As You can see in the picture below I also removed the old coke from the exhaust stump with chemicals. Don't forget that this can cause serious damages to the piston, because the glowing coke - since during suction the piston can easily suck back the coke particles as well - can ignite the gasoline-air mixture in the cylinder sooner than the spark plug, and this time the engine will knock back while running.
Furthermore, the seal under the exhaust muffler can be easily replaced, because the engine may blow out due to the improper sealing, but it can be noticed in no time because it is characterized by loud crackling, and this time the chainsaw works uncomfortably noisy.
Still good Compression?
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The secret to preserving the high engine compression for high-speed two-stroke engines is the correct mixture of the two-stroke fuel.
This thing starts there that at first, choosing the right quality (E5) fuel and OEM Husqvarna or nearly equivalent - such as McCulloch - two-stroke oil.
In my opinion, the proper fuel / oil ratio is 40:1, however, the recommended is 50:1, so 5-liter fuel and 100 ml two-stroke oil.
Don't forget that the better the lubrication, the longer the engine lasts.
What about the Clutch?
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During this maintenance, it was worth to check the clutch system as well.
I checked the needle roller bearing, the clutch spring and the sprocket, but everything was fine.
Here have been previously exchanged things by someone.
So after profound cleaning and lubrication, I assembled everything back.
The Result speaks for Itself
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The old Swedish became quite biting because there is again a lot of power and dynamism in the engine.
According to its owner, it was like as in its new age.
Well yes, it is a work tool and it is not famous for pharmacy cleanliness, but I always work with clean tools, and I also like to clean all the workpieces profoundly.
I never had a chainsaw, but after this service, I looked for a Husqvarna 40 in the used market for weeks. I made a lot of phone calls within a 200 km circle to get somehow something similar old school chainsaw.
By the way, I still don’t have one and it seems that I have already recovered from this period. :)
Thank You for reading this article!
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