Question about 1985 Yamaha XJ 600

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Throttle is stuck. Will not open carburetors.

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I am guessing the bike has been sitting for awhile. If so, chances are the slides are stuck in the throat of the carbs. The "glue" is gas that has evaporated, ( while sitting ), leaving varnish behind. Remove the air filter and then put a screwdriver into the filter side of each carb then gently pry the slide upward. If the slides do not easily lift upward you will need to spray some carb cleaner into the slide area and work the slides until they are free.

The bike has CV carbs. This means atmospheric pressure (or the lack of it really ), is what lifts the slides. You said the throttle is stuck. That is why I started with checking to see that the slides are actually stuck or actually free to move up and down. If the throttle grip opens and closes as any other bike does and the slides are free to move, the problem is that either no vacuum is getting to the rubber diaphragm or the diaphragm is cracked or punctured and thus is loosing the vacuum. Before I loose you in CV carb theory, let me ask you to rate my answer.

The throttle cable is not hooked up to the slides. It is hooked up to the butterfly. Without the engine running , the slides CAN NOT lift upward. When the butterfly valve is closed, very little air is moving in the carb bore. (The engine is getting some air and fuel through the pilot circuit, which we'll describe later.) With very little air flowing, the air in the carb bore and the air in the closed chamber above the diaphragm are at close to the atmospheric pressure of the outside air.

Open the butterfly and several things happen.
1. Air now speeds through and venturi effect (low air pressure) at the point of the slide (variable venturi) is created.

2. The low air pressure at the venturi is transmitted up through the holes in the slide to the closed chamber above the diaphragm. This lowers the air pressure in that chamber.

3. The open air below the diaphragm now wants to rush into that chamber to equalize the pressure, but it can't because there is no passage.

4. So it does the next best thing and tries to PUSH its way in through the underside of the soft diaphragm.

5. The diaphragm can't let the air in, but it is flexible so it gives way and is pushed upward by the outside air pressure.

6. As the diaphragm goes up, it pulls the slide with it, and with the slide, pulls the tapered jet needle upward in the needle jet to provide more fuel.

7. As more air flows, even more fuel is pushed into the air stream, and the engine rpm increases.

Posted on Sep 07, 2010


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