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Monster Scooter Parts | SKU:
P33-4158
Main Jet for CVK 50cc 4-Stroke QMB139 Engine Carburetors with M4x0.70 Threads
$2.25
$2.99
Unit price
/
Unavailable
Main Jet for CVK 50cc 4-Stroke QMB139 Engine Carburetors with M4x0.70 Threads - 72 (0.72 mm) is backordered and will ship as soon as it is back in stock.
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- Does not include expedited shipping services.
Main Jet for CVK 50cc 4-Stroke QMB139 Engine Carburetors with M4x0....
$2.25
$2.99
Unit price
/
Unavailable
Description
Description
Main jet for QMB139 CVK 50cc carburetors.
Carburetor jets come in different orifice sizes. Carburetor jet sizes are measured in hundredths of a millimeter. For example, a 87 jet has an opening that is 0.87 mm in diameter. An 90 jet has a size of 0.90 mm, etc.
To obtain maximum performance from your scooter, the carburetor must supply the correct fuel/air mixture all the way though the full range of throttle openings. What controls how well the scooter runs when the throttle is most of the way open is the carburetor main jet. This is basically a hole, and the bigger the hole, the more fuel can flow through it. How big the main jet need to be depends on the size of the carburetor and how much air is flowing. Since the air is less dense at high elevations and at high temperatures, you may need a smaller hole (less fuel) at 95 degrees in the high desert than you do at sea level and freezing temperatures.
Manufacturers tend to use the smallest possible main jet for two reasons. First, you get slightly better mileage. Second, exhaust emissions are lower if the engine runs slightly lean (less fuel in the fuel/air mixture) than slightly rich (more fuel in the fuel/air mixture). Manufacturers don't know what temperature you'll be riding in or what elevation you'll be riding at and so to stay on the lean side, they often use a jet smaller than the optimum size for performance, just to make sure that wherever you are, the engine will be running on the lean side at full throttle rather than the rich side. The ideal condition is to have the correct air/fuel ratio at low speeds, not to run lean or rich, and under that condition emissions will still be well controlled and the engine will give good performance. However at high speeds and under hard acceleration it's best to run a slightly richer mixture in order to keep temperatures down and prevent pre-detonation ("knocking" or "pinging"). Maximum power is usually developed when the mixture is slightly rich.
The process of changing the main jet size is pretty simple. First, remove the carburetor. To do this you'll probably need to take some parts off the scooter, such as the under seat storage container and possibly some body panels. Every scooter will be different. Disconnect the fuel line and vacuum lines, auto enricher (choke), throttle cable, air intake hose, and the inlet manifold. At this point you should be able to wiggle the carburetor out of the scooter.
At the bottom of most carburetors you'll find the float chamber, usually secured to the carburetor body by four screws. Remove these screws and the float chamber should separate from the body. If you haven't drained it, it will be full of gas, so be careful.
Be sure to use the right sized screwdriver to remove the main jet. Brass is quite soft and you don't want to damage the screw slot. Unscrew the main jet (you might need a wrench on the atomizer tube) and replace it with the new one. Screw it in tight, but you don't need to use a lot of force. Just make sure it's not loose. That's it!
Now put it all back together. Mount the carburetor back in the scooter. Make sure the hoses are secure on the air inlet in intake manifold sides of the carburetor. You don't want any air leaks. Then reconnect the throttle cable, fuel line and vacuum hoses. Finally, plug the auto enricher back in. At this point you should be done, but check around to make sure you really have reconnected all the vacuum hoses properly of the scooter won't run well. You may have to crank the scooter over a few times since the carburetor has to fill with gas before it will start. If all is well, it will start up and idle well. Changing the main jet should have absolutely no effect on the idle, so if the engine stalls or won't start, it's not the main jet that's the problem. You've probably forgotten to connect something, or you've connected the wrong hose to the wrong place or you've accidentally disturbed the idle speed adjustment screw.
Finally, take your scooter for a ride. If the scooter runs better, you made a good choice for the new main jet. If it doesn't, then you picked the wrong size and you may need to try again.
⚠ WARNING California Residents: Proposition 65 Warning
Carburetor jets come in different orifice sizes. Carburetor jet sizes are measured in hundredths of a millimeter. For example, a 87 jet has an opening that is 0.87 mm in diameter. An 90 jet has a size of 0.90 mm, etc.
To obtain maximum performance from your scooter, the carburetor must supply the correct fuel/air mixture all the way though the full range of throttle openings. What controls how well the scooter runs when the throttle is most of the way open is the carburetor main jet. This is basically a hole, and the bigger the hole, the more fuel can flow through it. How big the main jet need to be depends on the size of the carburetor and how much air is flowing. Since the air is less dense at high elevations and at high temperatures, you may need a smaller hole (less fuel) at 95 degrees in the high desert than you do at sea level and freezing temperatures.
Manufacturers tend to use the smallest possible main jet for two reasons. First, you get slightly better mileage. Second, exhaust emissions are lower if the engine runs slightly lean (less fuel in the fuel/air mixture) than slightly rich (more fuel in the fuel/air mixture). Manufacturers don't know what temperature you'll be riding in or what elevation you'll be riding at and so to stay on the lean side, they often use a jet smaller than the optimum size for performance, just to make sure that wherever you are, the engine will be running on the lean side at full throttle rather than the rich side. The ideal condition is to have the correct air/fuel ratio at low speeds, not to run lean or rich, and under that condition emissions will still be well controlled and the engine will give good performance. However at high speeds and under hard acceleration it's best to run a slightly richer mixture in order to keep temperatures down and prevent pre-detonation ("knocking" or "pinging"). Maximum power is usually developed when the mixture is slightly rich.
The process of changing the main jet size is pretty simple. First, remove the carburetor. To do this you'll probably need to take some parts off the scooter, such as the under seat storage container and possibly some body panels. Every scooter will be different. Disconnect the fuel line and vacuum lines, auto enricher (choke), throttle cable, air intake hose, and the inlet manifold. At this point you should be able to wiggle the carburetor out of the scooter.
At the bottom of most carburetors you'll find the float chamber, usually secured to the carburetor body by four screws. Remove these screws and the float chamber should separate from the body. If you haven't drained it, it will be full of gas, so be careful.
Be sure to use the right sized screwdriver to remove the main jet. Brass is quite soft and you don't want to damage the screw slot. Unscrew the main jet (you might need a wrench on the atomizer tube) and replace it with the new one. Screw it in tight, but you don't need to use a lot of force. Just make sure it's not loose. That's it!
Now put it all back together. Mount the carburetor back in the scooter. Make sure the hoses are secure on the air inlet in intake manifold sides of the carburetor. You don't want any air leaks. Then reconnect the throttle cable, fuel line and vacuum hoses. Finally, plug the auto enricher back in. At this point you should be done, but check around to make sure you really have reconnected all the vacuum hoses properly of the scooter won't run well. You may have to crank the scooter over a few times since the carburetor has to fill with gas before it will start. If all is well, it will start up and idle well. Changing the main jet should have absolutely no effect on the idle, so if the engine stalls or won't start, it's not the main jet that's the problem. You've probably forgotten to connect something, or you've connected the wrong hose to the wrong place or you've accidentally disturbed the idle speed adjustment screw.
Finally, take your scooter for a ride. If the scooter runs better, you made a good choice for the new main jet. If it doesn't, then you picked the wrong size and you may need to try again.
⚠ WARNING California Residents: Proposition 65 Warning
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Specifications
Specifications
Thread: M4x0.70
Thread Diameter: 4.2mm