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Discussion Starter #1
Greetings all.

1981 CX500C. There are two small hoses on the vacuum petcock. I understand one is for the vacuum hose routed to the right intake manifold and one is a breather tube that hangs down by the swing arm but... which hose goes to which fitting on the petcock?

My idea is the vacuum hose goes at the far end of the petcock to pull the diaphragm open and the vent hose goes in the middle of the body but if I apply a vacuum to either fitting on the petcock it will flow fuel. Remove the vacuum and the fuel flow stops like it should.

Looked online and in my Haynes manual without any luck.

Next question is, has anyone swapped out the vacuum petcock for a non vacuum operated one? I really don't see any use for one.

Thanks all.

Peter Haynes
Grand Bend, Ontario
 

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You've got the order right. The diaphragm is complicated and thin. Not surprised that it will leak when applying vacuum to the other hose. That open hose is to drain fuel away in case of a diaphragm leak. No need to replace the petcock to get rid of the vacuum feature, which I agree with. Just fab a blank plate to replace the whole vacuum system, mounted to the same four holes. I cut the diaphragm to get an oring to seal it up. Make sure to poke a wire into the fuel cap vent hole on the forward edge underneath. These too small vent holes fill with dust and cause most of the problems with the vacuum petcocks. I know, I found out the very hard way.
 

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Discussion Starter #3
Excellent. Thanks for the reply and I think I'll check my fuel cap vents also.

Cheers

Peter
 

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I have a 82 gl500i. I vac-petcock started giving me problem. I swooped it out with a non-vac petcock. Works fine, bike start much easier. Even after when set for two week. I would not have a vac-petcock on a bet. You have to plug off the vac line to the carb.
Ev
Mi.
82 gl500i
 

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if I'm not mistaken I think you can disable the vac by opening the petcock and turning around the diaphragm and spring. this should hold it open.
 

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Discussion Starter #6
That's what I've been thinking. Ditching the vacuum petcock.

I'm doing a bunch of maintenance and straightening out 32 years of haphazard repairs and trying to get the bike to run first. After I'm happy with all that, I'm going to check and see if the petcock is causing the "hard start after sitting" problem by applying a constant vacuum to the tap for a minute or so before trying to start it. If it makes a difference then I'll disable the diaphragm.

Thanks
 

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Discussion Starter #7
That'll be easy enough to try.

Thanks.
 
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