Odd fuel leak

Odd fuel leak

Postby sin486 » Sat May 04, 2019 4:59 am

Around 24 hours after my last ride I came back to the garage to see a sizeable (about twice the size of my hand) pool of gasoline under the left-hand side of the bike. The petcock was in the OFF position and the bike was on its centerstand.

I have tested the petcock by removing the fuel tank, setting the off position, waiting ten mins and then watching for any fuel. Seems like the petcock is working correctly. I also assumed that the vacuum operated fuel cutoff could be the culprit, but that would require a petcock that was leaky.

I also tested each of the left hand side float bowl drain screws to make they weren't loose. It took more than half a turn for each to start draining fuel. The fuel leak is coming from the same 3-to-1 pipe that drains the float bowls.

It seems like the left-hand bank of carb bowls is leaking fuel. My next step is to bypass the vacuum operated cutoff, feed fuel into the left bank of carbs and find which carb is leaking. Any other diagnostic suggestions?
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Re: Odd fuel leak

Postby daves79x » Sat May 04, 2019 5:13 am

The only thing that could drain a bowl of fuel (other than an open or leaking drain screw), is a cracked standpipe. This assumes that no additional fuel can enter the bowl, if that's the case, then a leaky float valve would likely be the problem. But proceed with great caution in any case as you do not want a hydrolock situation. I'd pull the plugs on 1-3 and make sure you don't have a cylinder full of fuel. DO NOT turn the engine over until you do this.

Dave
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Re: Odd fuel leak

Postby steve murdoch icoa #5322 » Sat May 04, 2019 6:19 am

If it is a cracked standpipe it can be repaired with a bit of solder.
This excellent tutorial on replacing the pipe has been posted before but bears repeating.
https://cx500forum.com/forum/general-di ... ement.html
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Re: Odd fuel leak

Postby sin486 » Sat May 04, 2019 6:32 pm

Thanks for the info, Dave and Steve.

I have done some tinkering. Firstly, whoever last assembled vacuum operated cutoff valve did it bass-ackwards (see attached image). The diaphragm wasn't installed correctly and the spring was on the wrong side of the diaphragm! This means that the cutoff valve was persistently open regardless of vacuum. I have fixed the valve and tested it by attaching it to the fuel tank in the ON position; no fuel was making it through the valve.

Secondly, I disconnected the left hand side carb bowl drain lines and then poured fuel into the left hand side carb fuel intake line (to try recreate the conditions where fuel was always able to flow to the carb). Interestingly I got different results, see: https://www.youtube.com/watch?v=p2uuEoEAkQ4

There was some fuel leakage at the front screws. Is this to be expected?
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Re: Odd fuel leak

Postby daves79x » Sun May 05, 2019 7:52 am

Someone defeated the vacuum valve on purpose - a lot of us did that to eliminate the valve without altering the look. OK that you fixed it though. Knowing now that you did indeed have positive fuel flow, you very likely have a leaky float valve or two or, if you can trace the exact origin of the leak, it very may well be the o-rings at the fuel joints. I'd be pulling them for a full rebuild myself. Again, just be very aware of a potential hydrolock situation.

Dave
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