The Main Jet: Understanding its Function and Adjustment

We can see the side of this main jet from this angle, we'd see a structure there that's significant to the way this Main Jet Works. I've actually removed this part of the main jet so we can have a closer look and when we do what we see on the side of it, the very thing I was talking about is this cutout structure here in the shape of an elongated triangle.

Within the carburetor, when the needle screw is fitted in place in here in a controlled manner, the needle can move up and down thus opening and closing this gap. So how does this allow the carburetor to function and the engine to run well? During the carburetor's normal functioning, fuel is of course brought up the main jet. But at the moment the needle is low and closing off the gap, if the needle was to raise slightly opening up some of that gap, it would let some of that fuel out into the Venturi of the carburetor and to the engine.

Because of the very nature of this gap from bottom to top, it gets wider as the needle raises, naturally going to get an increasing volume of fuel out from the gap. And of course, as we lower the needle, we're going to start to restrict some of that fuel coming out of the main jet, thus limiting the amount available for the engine. If the needle raises and lowers to govern the amount of fuel coming out of the main jet, what exactly is operating the needle? We've already seen the needle will protrude into the main jet at such a level to allow just enough fuel out to hopefully regulate a decent idling speed.

Let's move on to Gathering an understanding of the low screw adjustments themselves. Because we're talking about the needle screw being the low screw that adjusts the air-to-fuel ratio at low speeds, we'll start by explaining how it works. In order to explain that, what we must make clear is that this needle is directly connected to the screw on the tip and so of course when it's all fitted in place this screw is turned to fit the needle into its correct position. So if we screw the screw in clockwise, the needle will lower, and if we screw it out anticlockwise, the needle will raise.

So when the engine is idling, screwing the screw out anticlockwise will lift the needle and allow a little more fuel out, adding more fuel to the air going into the engine and thus making the mixture richer. On the other hand, if we've got a situation where the engine is idling like this, that's obviously not running at its best because this sound is a symptom that the engine is running too lean. So screwing the screw anticlockwise outwards raises the needle out of the main jet slightly, allowing more fuel out of the main jet thus richening up the fuel-to-air mixture at low revs.

So when we feel that we've adjusted this low screw to a point where the engine seems to be running well, we can then put a further tweak onto the adjustments with the idling screw either to speed up the low RPM slightly if it needs that or to slow it down slightly if it needs that. Either way, we need to bring the idling RPM to a reasonable level on this weedy to strimer. I want to get that nice sweet spot where the head of the strimer is no longer turning but not too low that it's going to stall.

So we're back to Gathering an understanding of the low screw adjustments themselves because we're talking about the needle screw being the low screw that adjusts the air-to-fuel ratio at low speeds. We've explained how the engine idling works, so now let's move on to adjusting the high screw and final tweaking of the idling RPM.

The High Screw Adjustment: Understanding its Function

To fully understand the function of the high screw adjustment, we need to first explain what it does. The high screw adjusts the air-to-fuel ratio at high engine speeds. So, how does this work? When the engine is running at higher speeds, such as when revving up or driving with a heavy load, more fuel is needed to maintain proper combustion.

The high screw operates by controlling the airflow through the carburetor and thereby adjusting the amount of air that enters the combustion chamber. By turning the high screw clockwise, the airflow increases, which allows more air to enter the combustion chamber and results in a leaner mixture. Conversely, when the engine is revving at high speeds, it's recommended to turn the high screw counterclockwise to increase airflow and provide a richer mixture.

The relationship between the idling and high screws is crucial for achieving optimal engine performance. When adjusting both screws, it's essential to balance the air-to-fuel ratio at low and high engine speeds. This ensures that the engine receives the correct amount of fuel, whether running at idle or under heavy load.

To adjust the high screw, we need to consider the engine's specific requirements for high-speed operation. Turning the high screw clockwise increases airflow and results in a leaner mixture, while counterclockwise turns increase air density and provide a richer mixture. By making these adjustments, you can fine-tune your engine's performance and optimize fuel efficiency.

Tweaking the Idling RPM: Fine-Tuning Engine Performance

Now that we've covered the high screw adjustment, let's move on to tweaking the idling RPM. This involves adjusting both screws to achieve an optimal balance between air-to-fuel ratio at low speeds and high-speed operation.

The goal is to find a sweet spot where the engine is running smoothly and efficiently at various speeds. To achieve this, we need to fine-tune the idling RPM by making adjustments to the high and idling screws. This requires careful observation of the engine's performance under different conditions and adjustments to ensure optimal fuel efficiency.

By tweaking the idling RPM, you can fine-tune your engine's performance, improve fuel efficiency, and enhance overall driving experience. Remember, precise adjustment is key to achieving optimal results, so don't be afraid to experiment and adjust until you find the perfect balance for your engine.

"WEBVTTKind: captionsLanguage: enokay so this video is intendedly focused on the hidden low screw on this type of carburetor and if you'd like to see the full version of this video where I explain about the low screw and the high screw and how to set both on your machine then click through the link at the end of this video otherwise sit back and learn all about how the idden low screw works and how to go ahead and adjust this on your machine so here's the carburetor we have the main large adjuster screw here which adjusts the fuel air ratio to richen or lean out the fuel reaching the engine at high RPM in other words when the throttle is fully open but we also need to be able to adjust the fuel air ratio to rich or lean out the fuel reaching the engine at low RPM looking at this carburetor how do we do that we don't have two screws here so with this in mind enter the tiny hidden screw here at the Top This is indeed the adjuster screw that adjusts the air fuel mix at low or idling speed and otherwise known as the L screw or low screw now it's important to remember that we must not get this screw which is the low speeed screw mixed up with the idling screw this doesn't set the pace of the idling speed this screw the low screw sets the air to fuel ratio mix going into the engine the the idling screw here operates the throttle so clearly this screw is not as accessible as the high screw which I'm sure the manufacturer did for a very good reason this was most probably to prevent the over usage of this screw that would result in the detriment of the carb settings some of these screws have a cap covering them and I'm told that some are actually covered in a resin to prevent any adjustments outside of a dealership maybe that is but because these scre screws are indeed hidden away it may well be because they don't need adjusting anywhere near as much as the H screw does but if you do feel that you need to adjust these and you want to save time and money by adjusting it yourself or you are just interesting in how this screw works and Gathering a better understanding of it or both then I will not only show you how to do this I will also show how this screw Alters the air to fuel mix and the effects that has on the engine to provide you with a better understanding of this system so that when you adjust these screws it's clear to you what you are actually doing to the engine and why okay then so as we've established this is the low screw and if we really need to adjust this carburetor then we will start with this screw so I have this type of carburetor on my weed eater and so we'll go ahead and start the engine and make the adjustments we can note here that from the engine idling if we turn the screw out anticlockwise then the engine will eventually sound like this it becomes lumpy and struggles to idle and from here if we screw the idling screw inwards clockwise then you'll see that the engine revs will start to increase and increase more the more we turn it so what just happened there what did this do well with these carburetors there is a needle that protrudes into the main jet and it's the low screw that we've been adjusting at the top that is directly connected to this needle and so when the needle screw is fitted in its correct place within the carburetor it's sheathed within the protruding plastic part of the main jet and if we could see the side of this main jet from this angle we'd see a structure there that's significant to the way this Main Jet Works I've actually removed this part of the main jet so we can have a closer look and when we do what we see on the side of it the very thing I was talking about is this cutout structure here in the shape of an elongated triangle So within the carburetor when the needle screw is fitted in place in here in a controlled manner the needle can move up and down thus opening and closing this Gap so how does this allow the carburetor to fall function and the engine to run well during the carburetor normal functioning fuel is of course brought up the main jet but at the moment the needle is low and closing off the Gap but if the needle was to raise slightly opening up some of that Gap then it would let some of that fuel out into the Venturi of the carburetor and to the engine and because of the very nature of this Gap from bottom to top it gets wider as the needle raises we're naturally going to get an increasing volume of fuel out from the Gap and of course as we lower the needle we're going to start to restrict some of that fuel so there's less available for the engine so if the needle raises and lowers to govern the amount of fuel coming out of the main jet then what exactly is operating the needle as we've already seen the needle will protrude into the main jet at such a level to allow just enough fuel out to hopefully regulate a decent idling speed and just so we know the way we raise the engine revs is to pull the throttle which pulls the throttle cable which rotates this throttle lever and as the lever rotates the whole throttle Barrel mechanism raises and the needle raises with it and that exposes more of the Gap in the main jet to allow more fuel out to allow for higher engine revs so let's now get back to Gathering an understanding of the low screw adjustments themselves because we're talking about the needle screw being the low screw that adjusts the air to fuel ratio at low speed we'll start the explanation with the engine idling okay so in order to explain that what we must make clear is that this needle is directly connected to the screw on the tip and so of course when it's all fitted in place this screw is turned to fit the needle into its correct position so if we screw the screw in clockwise the needle will lower and if we screw it out anticlockwise the needle will raise so when the engine is idling screwing the screw out anticlockwise will lift the needle and allow a little more fuel out this will be adding more fuel to the air going into the engine and so rich up the mixture and of course if we were to screw this screw in clockwise it would lower the needle restricting some of that fuel going into the engine and so the fuel to mixture would be leaner so if we've got a situation where the engine is idling like this that's obviously not running at its best because this sound is a symptom that the engine is running too lean so screwing the screw anticlockwise outwards raises the needle out of the main jet slightly this allows more fuel out of the main jet thus riching up the fuel to- a mix at low revs and so of course we adjust this screw slowly while we're listening to the engine where the engine sounds like it's running much much better on the other hand if the engine is idling like this then this is a symptom of too much fuel reaching the engine at low revs and so in this case we take the opposite stance we screw the screw in slowly lower ing the needle into the jet restricting some of that excess fuel coming out of the main jet thus leaning out the fuel slightly so that the engine is receiving the right amount of fuel and therefore runs much better so when we feel that we've adjusted this low screw to a point where the engine seems to be running well we can then put a further tweak onto the adjustments with the idling screw either to speed up the low RPM slightly if it needs that or to slow it down slightly if it needs that either way we need to bring the idling RPM to a reasonable level on this weedy to strimer I need to bring the idling RPM to a point where the head of the strimer is no longer turning but not too low that it's going to stall so I get that nice sweet spot after adjusting the low screw so that's the hidden L screw explained and that's the low RPM adjusted if you want to go ahead and watch the full video where I explain how to adjust the L screw the idling and the high screw then please click through this link here in the meantime I want to thank you so much for watching and I'll be back soonokay so this video is intendedly focused on the hidden low screw on this type of carburetor and if you'd like to see the full version of this video where I explain about the low screw and the high screw and how to set both on your machine then click through the link at the end of this video otherwise sit back and learn all about how the idden low screw works and how to go ahead and adjust this on your machine so here's the carburetor we have the main large adjuster screw here which adjusts the fuel air ratio to richen or lean out the fuel reaching the engine at high RPM in other words when the throttle is fully open but we also need to be able to adjust the fuel air ratio to rich or lean out the fuel reaching the engine at low RPM looking at this carburetor how do we do that we don't have two screws here so with this in mind enter the tiny hidden screw here at the Top This is indeed the adjuster screw that adjusts the air fuel mix at low or idling speed and otherwise known as the L screw or low screw now it's important to remember that we must not get this screw which is the low speeed screw mixed up with the idling screw this doesn't set the pace of the idling speed this screw the low screw sets the air to fuel ratio mix going into the engine the the idling screw here operates the throttle so clearly this screw is not as accessible as the high screw which I'm sure the manufacturer did for a very good reason this was most probably to prevent the over usage of this screw that would result in the detriment of the carb settings some of these screws have a cap covering them and I'm told that some are actually covered in a resin to prevent any adjustments outside of a dealership maybe that is but because these scre screws are indeed hidden away it may well be because they don't need adjusting anywhere near as much as the H screw does but if you do feel that you need to adjust these and you want to save time and money by adjusting it yourself or you are just interesting in how this screw works and Gathering a better understanding of it or both then I will not only show you how to do this I will also show how this screw Alters the air to fuel mix and the effects that has on the engine to provide you with a better understanding of this system so that when you adjust these screws it's clear to you what you are actually doing to the engine and why okay then so as we've established this is the low screw and if we really need to adjust this carburetor then we will start with this screw so I have this type of carburetor on my weed eater and so we'll go ahead and start the engine and make the adjustments we can note here that from the engine idling if we turn the screw out anticlockwise then the engine will eventually sound like this it becomes lumpy and struggles to idle and from here if we screw the idling screw inwards clockwise then you'll see that the engine revs will start to increase and increase more the more we turn it so what just happened there what did this do well with these carburetors there is a needle that protrudes into the main jet and it's the low screw that we've been adjusting at the top that is directly connected to this needle and so when the needle screw is fitted in its correct place within the carburetor it's sheathed within the protruding plastic part of the main jet and if we could see the side of this main jet from this angle we'd see a structure there that's significant to the way this Main Jet Works I've actually removed this part of the main jet so we can have a closer look and when we do what we see on the side of it the very thing I was talking about is this cutout structure here in the shape of an elongated triangle So within the carburetor when the needle screw is fitted in place in here in a controlled manner the needle can move up and down thus opening and closing this Gap so how does this allow the carburetor to fall function and the engine to run well during the carburetor normal functioning fuel is of course brought up the main jet but at the moment the needle is low and closing off the Gap but if the needle was to raise slightly opening up some of that Gap then it would let some of that fuel out into the Venturi of the carburetor and to the engine and because of the very nature of this Gap from bottom to top it gets wider as the needle raises we're naturally going to get an increasing volume of fuel out from the Gap and of course as we lower the needle we're going to start to restrict some of that fuel so there's less available for the engine so if the needle raises and lowers to govern the amount of fuel coming out of the main jet then what exactly is operating the needle as we've already seen the needle will protrude into the main jet at such a level to allow just enough fuel out to hopefully regulate a decent idling speed and just so we know the way we raise the engine revs is to pull the throttle which pulls the throttle cable which rotates this throttle lever and as the lever rotates the whole throttle Barrel mechanism raises and the needle raises with it and that exposes more of the Gap in the main jet to allow more fuel out to allow for higher engine revs so let's now get back to Gathering an understanding of the low screw adjustments themselves because we're talking about the needle screw being the low screw that adjusts the air to fuel ratio at low speed we'll start the explanation with the engine idling okay so in order to explain that what we must make clear is that this needle is directly connected to the screw on the tip and so of course when it's all fitted in place this screw is turned to fit the needle into its correct position so if we screw the screw in clockwise the needle will lower and if we screw it out anticlockwise the needle will raise so when the engine is idling screwing the screw out anticlockwise will lift the needle and allow a little more fuel out this will be adding more fuel to the air going into the engine and so rich up the mixture and of course if we were to screw this screw in clockwise it would lower the needle restricting some of that fuel going into the engine and so the fuel to mixture would be leaner so if we've got a situation where the engine is idling like this that's obviously not running at its best because this sound is a symptom that the engine is running too lean so screwing the screw anticlockwise outwards raises the needle out of the main jet slightly this allows more fuel out of the main jet thus riching up the fuel to- a mix at low revs and so of course we adjust this screw slowly while we're listening to the engine where the engine sounds like it's running much much better on the other hand if the engine is idling like this then this is a symptom of too much fuel reaching the engine at low revs and so in this case we take the opposite stance we screw the screw in slowly lower ing the needle into the jet restricting some of that excess fuel coming out of the main jet thus leaning out the fuel slightly so that the engine is receiving the right amount of fuel and therefore runs much better so when we feel that we've adjusted this low screw to a point where the engine seems to be running well we can then put a further tweak onto the adjustments with the idling screw either to speed up the low RPM slightly if it needs that or to slow it down slightly if it needs that either way we need to bring the idling RPM to a reasonable level on this weedy to strimer I need to bring the idling RPM to a point where the head of the strimer is no longer turning but not too low that it's going to stall so I get that nice sweet spot after adjusting the low screw so that's the hidden L screw explained and that's the low RPM adjusted if you want to go ahead and watch the full video where I explain how to adjust the L screw the idling and the high screw then please click through this link here in the meantime I want to thank you so much for watching and I'll be back soon\n"