CJ750 toolbox
The CJ750 carburetor - a primer by Ross Kowalski
I had the carbs from the black bike sitting on the porch table after I cleaned them, so I figured I would photograph how they come apart and what the different parts are. This is a half disassembly guide, half tour, and half explanation. (I know...)
The throttle slide cover should have a little washer to allow the top to be screwed on without galling.
Removing the cable from the slide.
Pull back the spring and squeeze the cable, then push the cable towards the bottom of the slide to free it.
The cable and spring freed from the throttle slide.
The cable and spring held in position for reinstallation.
This the spot for some kind of break-in screw. I replaced it with a set screw as soon as I got the bike and just went easy when driving during break-in.
The set screw viewed from the bottom of the slide cover.
The pin for the needle valve can be removed by pressing it out with a screwdriver as shown.
The pin seemed very loose on this carb, so I pinched it closed before reinstalling it.
The throttle needle adjustment can be seen here. There are four holes on the needle and two on the throttle slide for a total of eight positions. Moving the needle towards the top of the carb makes the mixture start richer when the throttle slide is opened and reach full richness sooner. Moving it towards the bottom of the carb of course does the opposite.
Removing the float bowl cover.
The two screws should have lock washers.
The hole you see at the side of the float bowl is how the gas from the float bowl moves to main jet well.
Loosening the idle air circuit atmospheric inlet. Notice the little brass plug indicated by the arrow. The plug closes the hole that is left at manufacturing time when a passage is drilled from the main jet well to the idle jet well.
A shot of the idle air circuit atmospheric inlet removed from the carb body. The fitting allows additional air into the idle circut. Maybe it is very important that the back side of the idle circuit be at atmospheric pressure, though I really am not sure why that would be.
There is a tiny screen inside that should be clear.
The screw is only there to plug the hole, it should be tightened all the way down against the drain.
Loosen the idle air mixture screw.
The idle air mixture screw removed. This screw controls how much air is let into the idle mixture. It is sort of opposite to how most needle valves work. Tightening the screw in closes the supply of air bled in with the gas so the mixture is richer. Backing the screw out lets more air into the idle mixture making the mixture leaner.
Loosening the idle speed screw lock nut.
The idle speed screw out for inspection. Tightening it in raises the throttle slide increasing the idle speed.
Loosening the main jet fuel well.
The fuel well should have a fine screen wrapped around the outside and a washer to seal it to the carb body.
Loosening the main jet.
The main jet exposed for inspection and cleaning.
This is a shot of the needle valve bore from the bottom of the carb. It should be clear.
The main jet orifice. The main jet is a restriction located below the needle valve that limits the ammount of fuel that can be drawn through needle valve at full throttle or near full throttle operation. It really doesn't do a lot most of the time. If you plan on testing your main jet by the color of your plugs, remember to color the plugs under full throttle.
Removing the idle jet cover
The idle jet cover, note the washer. This is not an adjustable screw. You just screw it all the way in. The taper at the tip works in conjunction with a tiny idle jet to mix the air and gas in the idle mixture.
Looking into the idle jet hole, you can see the idle jet. The black 1.5mm hole in the center of the brass is NOT the idle jet, just the base of it. The acutal orifice of the idle jet is .010 inch or so.
It is almost impossible to see through this jet, and because it is by far the smallest passage on a Chang Jiang motorcycle. It is critical to proper operation of the bike and should be cleaned out with a thin wire even if you think it is clean. I am using a single straightened strand of bicycle brake cable to clean out the hole.
A good way to check if it is through is to look into the idle air mixture screw hole and see if the wire is visible there. If this hole is plugged, the idle mixture screws won't work and setting a reliable idle will be all but impossible. If your idle air mixture screws don't seem to do anything, a blocked idle jet is likely the cause.
Removing the needle valve venturi tube.
You can see it here for inspection.
This is the idle mixture hole visible at the bottom of the carb bore on the engine side of the throttle slide.
Here you can see two holes at the mouth of the carb. The larger one lets air in around the venturi tube to vaporized the gas more completely. The smaller hole lets air into the idle air circuit (which is why I am a little confused about the role of the atmospheric inlet listed above).
The red line shows the relative height of the venturi tube mount with the carb body. The venturi holder is just lightly pressed in so they can come loose. They top of the venture holder is almost level with the flats on the carb body on either side.