Minimoto Garage - Jet Replacement

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Jet Replacement
Step by step pictures and instructions listed below. (Note: The pictures shown are of a GRC RSR with a Polini 6.2hp water-cooled engine, but the basic setup is the same on any Pocketbike with the Dell'Orto carburetor.)
Step 1.
Below is a diagram showing the inner components of the carburetor bowl. Note that each component is marked with a legend to the right of the diagram. Number 4 is the Main Jet, which is what we are going to replace.


Step 2.
The picture below is of 3 main jets, you will only need one for your carburetor, but remember jetting is hit or miss, so you’ll probably need several different number sizes. You may have to try more than one before you get it right, but it’s not difficult, as you’ll see below. You may also want to have a new float bowl o’ring (item number 7). It’s always a good idea to go back together with a new one, especially if the bike has over 10 hours, or if the bowl has been previously removed.


Step 3.
Start by removing the fairing, it’s just a couple of screws depending on the model you have. Once you get the fairing off it will be easier to see and work. If you haven’t had your plastic off in a while, now is a good time to wipe everything down and do a little clean up on those areas of the bike that your not accustomed to seeing.


Step 4.
Turn off the gas. No matter what kind of bike you have, the valve is pretty much the same and usually on the right side of the bike. The picture below shows the valve in the OFF position (right angle to the fuel line). The lever will be parallel with the fuel line when it’s in the ON position.


Step 5.
Now get a good look at the carburetor located on the rear of the engine, in front of the rear tire. The picture below shows a carburetor with a K&N Style filter installed. Yours may have a black box with white holes in it, but the K&N type (part # 203.0066) allows more airflow. If you install a bigger jet, this type of air filter may be better to ensure the proper fuel air ratio.


Step 6.
There are two different types of manifolds. A rubber one, and a metal one. If yours is the rubber type, the carburetor will just pull out. But if it is metal there is a bolt, usually a 5mm hex head type as shown. You don’t have to remove it completely, just loosen it enough to slide the carburetor off.


Step 7.
Now after you remove the carburetor you can see the bowl. Usually it’s black plastic as pictured. Inside you’ll find the jet (remember, this is the orifice that determines how much fuel is pulled from the bowl by the air as it passes through the carburetor to create the fuel air mixture). Without removing the throttle cable, (and there’s really no need to) this is about as far as the carburetor comes off. Use a flat head screw driver to remove the two screws that hold the bowl to the carburetor.


Step 8.
Now remove the bowl. Just two screws hold it on as pictured.


Step 9.
Now you can see the main jet, removing it is as easy as turning a screwdriver.


Step 10.
Using a flat head screwdriver, turn the main jet counter clockwise to remove it. Always make sure you use the largest screwdriver that fits, otherwise you risk damaging the slot.


Step 11.
After you get the jet out, you can see what size it is by the numbers etched on top of it. If you have a stock 4.2 or 6.2 Polini powered bike, it’s probably a 68 jet.

Step 12.
Install the new jet, Use a good quality screwdriver that fits the slot and make sure you don’t risk damaging it by over tightening.


Step 13.
Replace the bowl the opposite way it came off, remembering to use a new o’ring if yours appears to be damaged. If you do re-use the old one, keep in mind this is a gasoline seal and any leakage could cause problems. Either way, make sure you have it properly seated before you tighten the bowl in place.


Step 14.
Slide the carburetor back over the manifold sleeve and tighten it up, or slide it back into the rubber manifold depending on what you have. Remember not to tighten it too much, you just want to snug it up. The carburetor is aluminum and will easily crack.


Step 15.
Turn your gas back on (parallel with the hose). At this point I wouldn’t put my fairing back on yet. Remember that replacing the jet is trial and error. So crank it up and ride until the engine has warmed up. You have to change to another size until it will idle right and run to top speed with out cutting out. At the end of each test ride, check your plug to make sure it’s not running too lean or too rich.


Step 16.
Now your ready to put your fairings back on your bike.
Finished now you're ready to ride.
Thanks again to Mid-South Minimoto for this great how-to!
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