CHINESE ATV CARBURETOR (50cc to 110cc 4-STROKE)
SERVICE & REPAIR MANUAL
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This Manual is designed to help the novice & expert understand, adjust and rebuild your carburetor on your Chinese ATV. It is divided into 3 sections; 1. Understanding the fundamentals. 2. Adjusting your Carburetor. 3. Rebuilding (proper cleaning)
Carburetor "The Fundamentals" Its operation is a mystery to most, and because of that carburetor troubleshooting and tuning has earned a reputation akin to black magic, when in reality a fundamental understanding coupled with a little experience in interpreting engine feedback is enough to become fluent in the ways of old fashioned air-fuel mixing Basic Function Fundamentally, and entirely, the carburetor's job is twofold: first - to throttle (or control) the airflow to the engine, and second - to add fuel in the appropriate quantity to that airflow. That's it. Period. But notice the subtleties of that definition. First we need airflow, then we need fuel. Put another way, your right hand is not connected to the "gas", but more accurately to the "air". The ability to produce power is directly linked to engine's ability to injest air. Porting, low restriction intake, and tuned exhaust all are designed to improve airflow into the engine. Of course, air has no energy content, gasoline does. But because the two are mixed together in a precise ratio, more air translates to more fuel, and more fuel (which does have energy content) translates to more power. However, somehow, power needs to be controlled from zero to maximum. That's where the first function of the carb comes in - the throttle limits the airflow, and therefore the power. Nothing particularly magical about the throttle. Suffice it to say that the throttle accomplishes two things - it limits airflow, and creates vacuum between it and the engine. So let's talk about fuel now. There is a chemically correct ratio of air to fuel, which is called the stoichiometric ratio. For gasoline, this ratio is about 14.5 lbs of air to 1 lb of fuel. Your fuel injected car runs that ratio nearly exactly all the time thanks to things like computer control, and oxygen sensors. Its catalyst needs that type of control to keep the exhaust clean. But for off-road, performance-oriented engines, a little richer is better, like around 12-13:1. But how exactly do we accomplish that with an antiquated hunk of die-cast aluminum? With some pretty basic fluid mechanics... The "Primary" System Let's pretend we're inventing the carburetor, and start out with a pretty simple scenario: your engine is idling, therefore not much air and fuel is required. That means the throttle is almost complete closed, and there is a high amount of vacuum between the engine and carburetor. If we want to match the air stream with some fuel we could tap into the float bowl and direct that fuel into the vacuum side of the throttle. The vacuum created by the engine with the throttle closed will draw the fuel up from the bowl. We would drill this passage pretty big, but then limit the fuel by installing a smaller orifice inline. We'd keep changing this orifice until just the right amount of fuel was passing into the carburetor. To fine tune that fuel flow, we could add a needle valve that's adjustable by hand. In a nutshell, this is your primary fuel circuit. In our simplistic schematic
below the inline orifice is more commonly identified as the pilot jet. The hand adjustment is the low-speed mixture screw, or idle mixture screw, or air screw. From the diagram below you'll notice that there's a path from the air-cleaner side of the carburetor to the near the pilot jet. This illustration shows an alternate way to control fuel flow through the primary circuit, which is by displacing the fuel by air fed in through the air screw. The more we open the air screw the leaner the mixture will be.
The "Secondary" System But sitting around idling will get old pretty quick. We need to make some power to get the machine moving. So we start to crack open the throttle, which has a dual effect. First it increases the airflow to the engine, and second it drops the vacuum. As far as the primary circuit is concerned, this is problematic for a couple reasons. The drop in vacuum means there's less "force" pulling the fuel into the intake stream. To compound this, there's a lot more air going into the engine without an accordingly higher amount amount of fuel. We need a new method to draw fuel from the float bowl... Here's where a bit a fluid dynamics comes into play, thanks to a guy named Bernoulli who a century ago realized that if you neck a tube down in the center (like an hour-glass laying down) then a vacuum will be produced at the center that's proportional to the airflow, aka a venturi. That'll be perfect when the throttle is wide open. If we add an orifice (a main jet!) between the float bowl and the venturi port we can vary the amount of fuel that's delivered. Theoretically, this shouldn't change much, but adjustments might be
necessary to compensate for air density variations (due to humidity, temperature, barometer), and for things that might affect the vacuum signal between the venturi and bowl (like air-cleaner restriction).
The "Everything Else"System Alright, it looks like we got both ends of the operational spectrum covered. But not too many of us work the throttle like a light switch. We need a system to handle the varying fuel delivery requirements at part throttle. Problem is, at part throttle the venturi is really sucking (literally), and although the main jet is sized perfect for wide open throttle, it's just too big and supplying too much fuel at part throttle. We're running way too rich. A system for trimming the fuel flow according to throttle position is what's really required. Since the throttle slide is moving up and down why not attach a long tapered needle to the slide and move it in and out of another orifice, so that the cross-sectional area available for fuel flow changes as the needle moves up and down? Perfect. At idle (throttle closed and needle down) it pretty much blocks any fuel flow. And at full throttle the needle is completely out of the way, thus allowing the main to perform full fuel metering duty.
Carburetor Adjustment & Troubleshooting Carburetor adjustment is another one of those great mysteries of engine tuning that some perceive as being a black art. And true enough, it is a bit of an art if you lack the sophisticated equipment to do it scientifically such as CO analyzers and dynamometers. And as an art form, it takes practice to get good at it. Carburetor adjustment can be done by the garage tinkerer quite successfully, and has been for years. Unfortunately, it does take some trial and error and experience developed over time to do it well. The topic of carb tuning has been covered by just about every motorsport website, and has had countless books written on it. The intention with this installment in our carburetor series is to tackle the subject in the context of the asian mini-quad. Specifically, what are the pitfalls and troubleshooting techniques that are unique to these machines. The most fundamental idea regarding carb adjustment is that there is an ideal air to fuel mixture for every engine operating condition. This is not only defined by the engine’s own unique requirements, but also by the air density (temperature, pressure /altitude, humidity). What this means is that not only must a tuner determine what the engine wants at a certain environmental condition, but that adjustments may need to be made to accommodate a colder day, a more humid climate, or a higher altitude.
The idle mixture screw on the Asian mini-quad is located near the air-cleaner side of the carburetor (slotted screw on upper right hand of the photo).
Before getting into the environmental adjustments, the first step is to get the adjustments and jetting correct for that engine configuration. Anything that would affect the engine’s breathing ability will affect carb adjustment. Obviously, installing an aftermarket carb will require tuning, but so will changing to a different air filter. Cylinder porting and a pipe will not directly require a tuning adjustment since they only affect the engine’s ability to create airflow. The carb really doesn’t know what’s downstream. However, it might care what’s downstream – for example, if a pretty hot cylinder is installed with lots of compression and/or lowend porting, then the carb might want to be a little richer, or fatter, to decrease peak combustion temperatures. Ideally, tuning should be done at the “near normal” conditions you expect to run so as to minimize the need for fine tuning when conditions change. Whether stock or aftermarket, the first step in carb adjustment is to dial in the idle circuit. Remember that a relatively high vacuum exists downstream of the throttle slide when it’s closed, or at idle. A passage in this area is connected to the float bowl that allows fuel to be sucked into the engine while the throttle is closed. As the throttle is opened, this vacuum drops and the amount of fuel drawn into the engine through this passage decreases accordingly. We can adjust this amount of fuel by turning the idle mixture screw. On these carburetors the idle mixture screw actually controls the amount of ‘’ air that’s bleed into this passage. The more the screw is turned out, the more the bleed passage is opened, and therefore more air instead of fuel that’s being sucked into the engine via the primary or idle circuit.
The idle circuit air supply passage can become blocked with dirt and cause a rich running condition. This is the hole at about 7 o'clock on the carb inlet.
The question is just how do we know when the adjustment is correct. The simple answer is when you’ve obtained the highest idle rpm. The highest rpm is achieved when the air-fuel ratio (AFR) is optimized. Remember that while this adjustment is being made the throttle is essentially fixed, that is, held at idle. We are not changing the amount of air being drawn into the engine. If the engine rpms increase, that would imply that the engine is doing a more effective job converting the fuel and the given amount of air to useful mechanical power. Too little fuel, and the revs are too low. Too much fuel, and the revs are also too low. Somewhere in between, and combustion efficiency and the revs are maximized. With the engine off, turn the idle mixture screw in (CW) until the screw seats and count the number of turns. Note this number as a “fall-back” if you decide that your adjustments are not working and you want to get back to the baseline, or ground zero point. Next, turn the screw back to the baseline adjustment and start the engine and warm it up thoroughly, maybe even ride the quad a bit before starting the adjustment. With the engine running at idle turn the screw in (CW). This has the effect of closing the ‘air bleed’ passage to the idle circuit, thereby allowing more fuel to be drawn up from the float bowl, richening the fuel mixture. Make 1/4 turn increments until you notice a drop in engine speed. Then turn the screw out (CCW) in 1/4 turn increments. You should notice the engine revs going back up, then dropping again. Take note of how many turns it took to reach this point. Divide this number in half to determine where the theoretical optimized point is. Turn the screw in to reach this point. You may need to experiment with this procedure a few times to satisfy yourself that you’ve found the sweet spot. If by turning the screw you were never able to find the point where the revs peaked, i.e, the revs keep climbing/dropping until you bottomed the screw, or the screw came out, then you may need a different size pilot jet. As noted above, the idle circuit is fed by a passage that connects a port in the carb to the float bowl. The amount of fuel drawn by this circuit is not only controlled by the idle mixture screw, but also by the size of the orifice in the pilot jet at the float bowl. If you’ve turned the screw in completely (air bleed totally closed) and the revs seemed to keep increasing up to that point, it would indicate that the engine is wanting an even richer mixture. Install a bigger pilot jet. If you’ve backed the screw completely out and the revs still haven’t peaked, it would indicate a need for even more bleed air, or less fuel. Install a smaller pilot jet. If adjustment of the idle mixture screw doesn’t seem to have any affect, then one of two things is going on. The first most likely possibility is the air bleed passage may be blocked. This can occur if the air-filter has passed too much dirt which has gotten into the passage at the mouth of the carb. If this happens the engine will be running very rich, but can still run. The second possibility is that the pilot jet itself is blocked from dirt getting into the float bowl. If this happens, very little or no fuel will enter the carb.
It’s quite likely that engine will not even run since the primary ingredient in the recipe for combustion is missing – fuel. Engine’s generally have a higher tolerance for too rich than for too lean. If it seems you need quite a bit of throttle to keep the engine idling (no longer using the idle circuit for fuel supply) then this could be your problem.
Carburetor Cleaning: Ninety-five per-cent of the time, carb rebuilding is nothing more than taking the thing apart, cleaning it all up, replacing a few worn parts, and putting it back together. A thorough, immaculate cleaning job should not take any longer than an hour to per-form, and more like 15 minutes if you’re in a hurry. First, remove the carb from the atv. In most cases, it’s easiest to remove the top of the carb and pull the slide out before you pull the carb out of the manifold and air-box boots. This is fine—just leave the slide hanging there from the cable and take the carb over to the bench. Drain the old fuel out of the carb body and remove the four screws that hold the float bowl on. Remove the floats, and the float needle and seat. Remove the main jet and pilot jet; push the needle jet up with a blunt object and pull it out the top of the
carb. Moving up to the outside of the carb, re-move the idle adjust screw and the idle air screw. Remove the choke lever, and care-fully remove the nut that holds in the choke assembly. Pull the choke assembly, spring, and related parts out of the body, and sit them on a clean rag with the rest of the jets. A good example of a slightly worn slide. If the grooves get any deeper than this, it will be necessary to replace it. The new one will last longer if dirt is
kept out. The body is now stripped. Take the body and submerge it in a bucket of solvent )or carb clearner) and scrub it down with a small (clean) toothbrush. Clean off every speck of dirt and crud, inside and out. If you want to get the body totally shiny-clean, dunk it in a bucket of commercial carburetor cleaner. It’ll come out looking brand new. Carb cleaner is expensive, however, and you don’t really need it. Just make sure that the body is perfectly clean. Once you’re satisfied that the body is clean, do the same thing to the loose parts, taking care not to lose any of them. Re-move the top of the carb, the spring and the throttle slide from the end of the throt-tle cable; clean them, too. As you remove the parts from the solvent, lay them out on a clean rag. Your basic choke mechanism. At the very bottom of the plunger is a rubber gasket—if it wears out, it will give you grief. No matter what your friends say, the main jet and the pilot jet cannot wear out. Just blow them out with air or a spray of contact cleaner, and set them aside. Never clean them by running a wire through the holes—this can wear them out and make them useless. Check the throttle slide, needle jet and jet needle for wear or scuffing, and replace them if they look at all worn. Separate the float needle and its seat, and inspect the tip. If there’s any indication of wear, get a new setup. It’s cheap and critical to prevent leaks.