Episode 44: Why does my chainsaw start then die when I give it gas?

[Music] whenever we pull the throttle and we get that bogged down sound that [Applause] [Music] there are many reasons for the cause but the first thing i do before i do anything else is i check the mixture screws this is the high and low screws the fuel air mixture screws on the side of the carburetor usually any problems with mixture screws and bogging down is due to incorrect settings now if these mixture screws were screwed too far in then of course the obvious effects of that is too little fuel coming into the inlet area here the main jet itself is still open and it's allowing some fuel to come through but it wouldn't be enough to allow the engine to run at full speed so this could be a classic example where an engine would run at low or idling speed but as soon as we pull the throttle it does the bog down so what we need is correct mixture settings to allow a good air to fuel mixture and i'd be making sure that the fuel filter is clear with no blockages so it's not restricting any fuel if the fuel filter is blocked we can see the issues it's not drawing in fuel and if it runs at all it's quite likely to bog down always have a fuel filter on standby the more fuel that leaves the fuel tank the more air it draws in above it it's usually a special one-way valve in the fuel tank cap that allows the air to come in now when these valves fail to work which is something i have come across a few times in the past the fuel can't leave the fuel tank because there's lack of vacuum lock there so the obvious effect of this is less fuel getting into the carburetor that of course could contribute to bogging down but eventually of course it would stop the engine completely so what i've done in the past if i've suspected that the fuel tank cap is at fault i've just undone the fuel cap slightly to allow air to come through and then run the machine again if it runs okay then it could be your cap i would just check the external parts i could see because they're going to be the parts that would as well as losing fuel out of the pipes which you'd see obviously because the engine's drawing in fuel it'll also suck air in through that rupture and that will affect the correct field-to-air ratio and so can cause the engine to bog down now the inlet manifold sits between the carburetor here and the engine and on this side there's generally a gasket between it and the carburetor and it's the same story on the other side and they're all held tightly together with a special bolt there's air coming into the carburetor this end through the air filter and the fuel is added to the air before coming out into the engine now one problem that can arise is when the retaining bolt is loose and we get gaps between the gaskets and because of that there's too much air here now it's a weak fuel mixture so this is when bog down is likely to occur just make sure these bolts are nice and tight the next thing i'd check are the manifold gaskets if this gasket's damaged then air can be drawn in once again either gasket could fail by the way the manifold sometimes themselves can be damaged and draw in air and the problems of that are the same as what we've already seen what can happen is is that cap can become loose now if this happens there'll be excess fuel spilling out but what will also be happening is air will be drawn in and again we've got a fuel-to-air ratio here that's weak in fuel so at this point the engine would most likely bog down but the fuel pump diaphragm itself can also cause problems with bogging down when these diaphragms do age or they're made from a substandard material they go stiff and rigid and of course when the diaphragm goes rigid so does the valve because that's part of the diaphragm another area is this area here this is the actual fuel pump area of the fuel pump diaphragm and it relies on a good range of movement up and down to allow for good pumping action so when this area ages and becomes more rigid it reduces the pumping efficiency of the fuel so now we're left with a restriction of flow of fuel going through the carburetor and then bogged down the only remedy really is to replace this diaphragm we've got some dirt here that's come out of the fuel tank and somehow come past the fuel filter a tiny piece of debris can block it so this area is now blocked but the engine still wanting fuel now we've got a problem a lack of fuel going into the engine the engine would have bogged down and stopped if you do suspect this kind of blockage then the best thing to do is strip down the carburetor and blow down the fuel veins with an airline a common thing nowadays is to use an ultrasonic cleaner that can loosen the dirt first we can even get a blockage here at this point and in my experience this is one of the more common areas for blockages this is a metal gauze strainer filter that the fuel passes through and it's the buildup of particles that cause this blockage but these can be easily replaced and now we've got a main jet blockage and as you can see there there's no fuel coming out of the main jet at all the other two jets the high and low jet can't compensate for the main jet which is where the main of the fuel comes from so this amount of fuel is totally insufficient to support the workings of an engine and the engine would bog down and stop now let's have a look at blockages in the high jet we've got a piece of debris stuck and you can see a different air to fuel ratio now so if there's less fuel going in the engine revs will die down and as you can see now we've got less air going in as well and although the main jet on its left is still supplying fuel and so is the low jet on its right because all three aren't working in sync there's not a sufficient supply of fuel for the engine which is going to get bogged down the best thing to do then is when you strip down the carburetors remove the high screw and the low screw completely get an airline squirt the airline right down these jets and we'll get the same problem by the way if the high screw is screwed right in again the main jet's working okay and that's adding fuel and so is the high jet but because we've got no fuel supply commitment from the load yet we're left again with an inferior ratio of field to air mixture it's gonna bog down and although the low screw is associated with fine tuning low engine revs when it's screwed in like this it does cause enough of a problem to cause bog down the metering diaphragm shown here in green is responsible for regulating the correct amount of fuel coming into the inlet area of the carburetor and when fuel is used out of the fuel reservoir beneath it it draws the diaphragm down and the metering needle is lifted off its seat and fuel can freely enter the metering area at the top one issue i've come across in the past are these screws slightly loose and it can just break that seal what you'll probably notice is a gap there you'll see fuel spilling out but at the same time there'll be air being drawn into the carburetor because we're drawing in air we've actually lost the vacuum there and it's the vacuum that we need in order for the metering diaphragm to be drawn down so all that will happen now as the metering needle will stay fast on its seat it won't allow any fuel coming upward so we'll have bog down just make sure these screws are nice and tight another problem however we can be faced with is problems with the diaphragm itself this diaphragm is made from special material to allow it to move up and down nice and freely covering a good distance sometimes these can become less flexible and more rigid and that's normally due to age and the problem with it being more rigid is that the diaphragm can no longer have the same range of motion it's not moving up and down as far as it did now we're not opening the valve as much as we did it's not going to be the same amount of fuel available in the inlet eventually there's just going to be bog down now another problem that can occur and certainly one i've had in the past with these metering diaphragms is with this area here this is the plunger that's part of the diaphragm or the dowel that activates the metering lever so let's take these two diaphragms for example they both look identical from the outset they're exactly the same size the same thickness and they have the same holes as each other and so to the untrained eye you'd think they were identical and you'd fit either to your machine but there is a difference this is that plunger the one on the left-hand side is longer than the one on the right hand side and it's vital we choose the right one and that's because when the engine is running and the diaphragms come down to allow fuel to come out we need that plunger to travel down just enough in order to push down on the back of that metering lever just enough so that it tilts that lever up just enough in order to open up that needle valve at the front just enough for the correct flow okay now we've got the shorter plunger and now when everything's working and the diaphragm comes down it doesn't push down as much on the back of that metering lever and that means the front under the needle valve isn't open quite as much allowing as much flow through and unfortunately it's something that's overlooked just make sure that this diaphragm is identical in every way to the one you're taking off because the lever itself is made of metal the back of this lever can be set so that the metal can be bent in order to adjust the back of this lever and sometimes this can be set too low if that's the case then this is what happens it's a similar scenario to having a short plunger the needle valve is now not being lifted high enough again not allowing enough fuel through so eventually we've got bogged down so if you do suspect that your metering lever is over adjusted like this then it's easy enough to bend back into the right area the correct setting for these do vary between different types of carburetor so here's the metering spring and the trouble we normally have with metering springs or when we replace them i have fitted springs that look identical to the one i've taken out but they're a little stronger take a look at this scenario for example the diaphragm has come down to the back of the lever and because that spring is too hard it won't push the back of the lever down and what we're ending up with is this the diaphragm is moving up and down but we're not getting movement here on this needle valve to allow fuel to come up through and be sufficient enough to run the engine we've got air and fuel mixture that comes in through the inlet this is after the carburetor and as the piston goes up it draws the air fuel mixture underneath it and then without going into too much detail that mixture is then taken to the top of the piston and used for combustion and in order to achieve that all of these areas here have to be totally airtight to keep that mixture in with no leaks whatsoever but the barrel gasket sits somewhere here and that's at the split junction between the barrel and the crankcase beneath it these can be made from like a papery cardboardy type material sometimes a metal and it's vital that these gaskets are in good order because if they're not then each time the piston goes up to draw an air fuel mixture it can also draw in air if there's any damage around that gasket and any extra air as you can see coming in there upsets that air fuel mixture and we've got too much air in now making the fuel mixture weak that of course isn't going to combust efficiently so it would create bog down we'd get the same problem by the way if the bolts were loose that held the crank case to the barrel there is a lot more work involved in replacing these gaskets so before you go stripping the engine right down it's best to be sure that it is this gasket and the main seals of course are here and it's where the crankshaft sticks out of the crankcase so i'll turn this engine round so we can see this face got the crankshaft here sticking out the crankcase the job of the crankseal is to keep an airtight seal there on the shaft and it prevents this vital fuel air mixture from escaping but also it prevents any air being drawn in in that area and if those seals aren't working correctly or they're damaged in any way it can draw into the crankcase we know what that's going to do it's going to end up being too much air there to fuel it will lead to bog down and i have found this particular issue a little more common in the past i've had this quite a few times to diagnose and be sure if you suspect crank seals are leaking is to use a pressure tester now the whole issue of stale fuel can be somewhat controversial some people say that fuel can go stale in a short amount of time some people say it lasts much longer but let's just have a look at why we need to keep the fuel fresh so we've got the fuel at the bottom here in red and what tends to happen is from the minute we go and fill the fuel can up from the fuel station the reactive parts start to evaporate and over time the fuel it starts to go thick something that's not as combustible as it was when we first bought it and so combustion won't take place as efficiently there are ways to combat this like keep the fuel tank cap nice and tight to make it last as long as possible that's something that i hear some people have had success with and when we fill the cans full of fuel it's apparently best to fill them to the top so there's no space here at the top because if there's space here then there's space for the reactive substance to evaporate into and of course we don't want that but filling this gap is supposed to reduce the evaporation so that's why i've put that down as a cause for bog down because in the past it has caused that for me this relates mainly to chainsaws because if we take a closer look now at the chainsaw exhaust what we'll see on the exit is this structure this is the exhaust spark arrestor and it's a metal screen type structure made in a criss-cross effect like this and so the exhaust fumes pass through this on the way out of the exhaust but as they do so they obviously filter out the sparks coming from the exhaust which is a good thing the problem is though as the exhaust fumes are passing through this all the time over time this can clog up with carbon but these can generally be removed and replaced but i do know some people in the trade have had some success cleaning these using a blow torch to burn away the carbon they're not all flat screens like this there are some types like this which are a bit more of a barrel shape and of course when they clog up they look more like this so we've got these main two types here but the reason that they relate to the engine bogging down is that when these screens get clogged up the exhaust fumes can't get out of the exhaust properly and that allows a buildup of exhaust fumes inside the engine and of course if we've got a buildup of exhaust fumes in there combustion cannot take place properly it interferes with that air-to-fuel mixture and the whole combustion process and this is a problem that's quite common in these types of exhausts i really hope you've benefited from this video and you feel like you've learned something if you have please can i ask that you give me a thumbs up and comment to let me know what you think exactly of the video and in the meantime i'll be back soon thank you so much for watching [Music] you

Channel: TheRepairSpecialist

Views: 182,687

Keywords: craig kirkman, the repair specialist, how to tune a chainsaw, 2 stroke bogging at full throttle, 2 stroke bogging, engine bogging down on acceleration, engine bogging, engine bogs when given throttle, two stroke bogging at full throttle, two stroke bogging, two stroke bogs on acceleration, chainsaw bogs down when i give it gas, chainsaw bogs down when cutting, chainsaw bogs down when trigger is pulled, Chainsaw won't start, echo chainsaw, hand held chain saw

Id: _wI-X7aot8Y

Length: 17min 3sec (1023 seconds)

Published: Thu Nov 11 2021