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82' cx500c custom
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Discussion Starter · #1 · (Edited)
I'm curious if anyone has experienced a slapping sound coming from the carburetor's vacuum piston? I had the bike running great and it sat for about 2 weeks and I went to start it today and the right side carb's piston seems to be slapping inside. I'm going to try changing out the spring to see if it corrects the issue.
The bike will barely start and bog down and die quickly. Then won't start.
I'm still curious if anyone else has experienced this.
 

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'84 CX650E that is evolving into a GL500
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If you have the original type vacuum petcock they don't like sitting for more than a few days.

As you probably know, the vacuum petcock has 2 valves, a manually controlled valve that works the same as a non-vacuum petcock, allowing you to select between ON, OFF and REServe and a vacuum valve that prevents fuel from entering the manual valve when the engine is not running.

The carbs on these bikes have a tendency for the fuel in the float bowls to evaporate and fast enough that if they are not used for a week or so the level can get low enough that there isn't enough for the engine to start. The problem is that the vacuum petcock doesn't let any fuel into the carbs unless the engine is turning fast enough to produce vacuum so you have to crank the starter long enough for the engine to supply enough vacuum to the petcock to open the valve for a long enough time for the float bowls to re-fill and it is pretty easy to run the battery down doing that.

You have a few options when that happens:
  • If it has been sitting for a few days and you expect that it will be hard to start, before you touch the Start Button, vigorously whack the throttle open several times to operate the accelerator pump and squirt raw fuel into the carb throats.
  • If it has been sitting for a week or more and whacking the throttle doesn't help there are 3 ways to fill the bowls:
1) Disconnect the fuel line from the petcock, connect a funnel and pour 90cc of fuel in directly.
2) Disconnect the vacuum line from the petcock (this is the small barb farthest from the petcock's main body), connect a short piece of clean tubing in its place and apply vacuum (with a vacuum pump or just suck with your mouth) for about 10-15 seconds.
3) Disconnect the drain/vent line from the petcock (this is the small barb between the vacuum line and the main body), connect a short piece of clean tubing in its place and blow gently into it for 15-20 seconds.
 

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82' cx500c custom
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Discussion Starter · #5 ·
Thanks for all the information. It turned out to be a mixture of things. The vacuum petcock decided to stop working period no fuel coming out the outlet and the face of it started leaking... also the right cylinder's plug was fouled out preventing ignition.
I have a bottle I hooked up to give it fuel and cleaned the plug. Fired right up. I ordered a non vacuum petcock to use while I rebuild the old one following phreaks guide.
I'm assuming the vacuum was off causing the butterfly or the vacuum piston to slap. It felt like it was the vacuum piston when I touched the top of the carb but they are so close to eachother I could be wrong.
Thanks again for all the help.
 
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