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Pipe Dreams: the upjet

August 2, 2007

I’ve decided to divide this little how-to exercise into two parts, because you may find it necessary to rejet your PX 150’s carburetor even if you don’t plan to change the exhaust pipe.

This is not rocket science. This is not even setting the parental controls on your TV. You’ll need a Philips and regular screwdriver, and some kind of a very small metal rod.

Pop off the engine-side cowl, then remove the top of the air cleaner cover by removing the Philips screws from the far ends. The big screw sticking up through the hole is the idle adjuster. Don’t touch that one.

In my case, the gasket stayed stuck to the bottom half of the air cleaner, which is fine because it won’t get lost or damaged that way. You’ll see the air filter, and rubber grommet on the idle adjuster. The screws you need to remove to get the air filter off should be obvious, so take ’em off and put the grommet somewhere safe (the hole in the top of the air cleaner cover is a good place).

If the air filter is really dirty, this would be a great time to clean it. The air filter in your PX is reusable. You clean it by soaking it in gasoline, then you coat the filter material with an oil/gas mix, according to the Haynes manual; if you can think of a reason not to use K&N filter oil, let me know.

PX 150 carburetor

And here it is. Carburettor in View! O the Joy!! The bit we’re interested in is the big brass fitting just to the right of the idle adjuster. Take a regular screwdriver, unscrew it and remove it. For me, this is old hat, having dealt with rejetting Weber carburetors in race cars. Tip: put a little rag in the carburetor throat so as to prevent some nasty foreign object from getting in there, and into the engine.

If what you take out doesn’t look like the following picture, put it back and get this:

Main jet assembly

The small end is your main jet, the thing you’re going to replace. Here’s what a main jet looks like; notice the little tiny numbers. The bigger the number, the bigger the jet. My PX came with a 98; Joe Flynn of Soundspeed Scooters said I should start with a 112. (Which turned out to be WAY too big. I finally settled on a 101.)

Carburetor jet.

The jet is a press-fit, so take a metal rod that’s small enough to fit in the big horizontal hole, push it through, and use the metal rod to pull the jet out. Don’t use pliers! You don’t want to chew up the old jet, ’cause you might need it later.

Once you’ve taken the old one out, put the new one in. Screw the jet assembly back in; snug will be fine, do not reef on it! Assembly is the reverse of disassembly. You’ll be so proud of yourself once you have everything back together. This took me all of five minutes, and I even cleaned the top of the air cleaner. Oh, yeah. While you have everything apart, clean, clean, clean. You may be doing this again.

Yes, there are jetting databases all over the Web, but the fact remains, everyone’s situation is different. Seattle is pretty much at sea level, but Denver isn’t called the Mile-High City for nothing. At higher altitudes, there’s less oxygen, so you have to compensate by rejetting. You’re going to have to run the bike, then check your spark plug for the ideal toasty brown color that says your fuel mixture is not too rich, not too lean, but juuusst right. Favicon

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