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petcock vs floats 101

open1mind

open1mind

Forum Apprentice
Past Site Supporter
OK, lets start with this- I am about a 4 on the mechanical ability score, and about a 1 on the patience/having time to do my own repairs score. My question is pretty simple. How does a broken/clogged carb float allow fuel to seep out into the vent lines? Isnt the purpose of petcock to let gas in as it is drawn by vacuum? Confusing, in that if there is no "draw" created by a spinning motor, how is fuel getting past the petcock to begin with?

I had a problem in that I noticed a small drip under my 79 GS 1000e, and today at start up cranking I heard what sounded like an intake backfire (not out the exhaust). Definitely smelled flooded, and would not start. Significantly worse drip out of the fuel vent lines, exaggerated as I moved the bike around the parking spot. It was a no start, and given the pop this AM, and the fuel coming out of it I would rather not ride it. Looks like the dealer will be picking this one up to troubleshoot it. Should I just have petcock replaced along with a thorough carb cleaning and go from there? I did have them insert an in line filter for me a while back.
I checked some of the library/search options and didnt get to anything that explained that petcock basic function in a way that I can follow... thanks
 
To have a leak like that, you need to have TWO faults, a bad petcock and a bad float valve.

The problem is that you don't usually know when ONE of them fails, only when they BOTH fail.
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If the o-ring that seals the petcock is damaged (quite likely, in a 34-year-old bike), it won't be able to completely stop fuel flow, regardless of vacuum level.

If you have a damaged float valve needle or a mis-adjusted float level, it will not properly stop fuel flow there.

The combination will allow gas to flow freely. It is HIGHLY recommended that you check/change your engine oil to remove any gas that will have thinned it out.

Another possibility for a leak is if your carbs have brass floats. They can develop leaks and simply not float high enough to shut off fuel. Still need to have a bad petcock, though.

Yet another possibility is a bad diaphragm in the petcock. A running engine will provide enough vacuum to pull the diaphragm open, even with a modest leak in it. If the 'shut-off' o-ring in the petcock does not seal, fuel can simply go through the hole in the diaphragm, straight into the throat of #3 carb and into the engine. Probably won't even come out the overflow tube in the carb, either.

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In short,

a leaking petcock (easily replaceable by self) is normally caused by a ruptured/stuck open vacuum diaphragm. What should be closed off (in the ON and RES positions) until there is vacuum applied remains open, and there is free passage from tank through the barrel inside the petcock, through the fuel line to the carb.

Yes, get a new petcock. No, don't have the dealer do it unless you really don't have 1h to spare. Get a new petcock o-ring and petcock bolt gaskets (2) at the same time. The new petcock SHOULD come with the o-ring, but a spare for $2.50 is decent insurance.

EDIT- Steve is the first to post again. Is the man plugged into the GSR a la The Matrix? :-k
 
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Outstanding insights, especially the reminder about changing the oil. I know it puts me into the girly-man category on this forum, but my life really conspires to stop me from doing my own wrenching. Cant hurt to be well informed though.
Bike is off to the dealer this week, they do get it right (eventually).

I did hear that pop and I could swear that I felt something vent out on the clutch side of the bike/tank when I first started to crank. I wonder if the petcock or something went at that very moment when trying to start it.... hmmm. Could not be anything catastrophic or it would be leaking from there. All leaks appear to be coming from the vent lines that exit under the bike.
 
open1mind said:
Looks like the dealer will be picking this one up to troubleshoot it. Should I just have petcock replaced along with a thorough carb cleaning and go from there?
Click to expand...
No. If your paying the dealer to troubleshoot it, I'd let them propose a solution.
(No sense telling them to troubleshoot it and then tell them what to do to fix it.) :rolleyes:
 
open1mind said:
I did hear that pop and I could swear that I felt something vent out on the clutch side of the bike/tank when I first started to crank. I wonder if the petcock or something went at that very moment when trying to start it.... hmmm.
Click to expand...

That "pop" out the intake may have been the same carb that provides the vacuum source for the petcock. If so, its possible that pop may not have only gone out the air box/pod, it may have gone up the vacuum line and nailed the petcock diaphram. :eek:

Are we neighbors? :confused:
 
Circling back that the dealer says to replace both the petcock and to clean/adjust carbs. It looks like the entire petcock assembly is still available and it is on order. I took another look at the drip spots where the bike was parked-- and it is definitely a mix of oil and gas. I originally thought that it was just gas coming out the vent lines- but it looks like there was so much gas leaking into the motor that it overflowed and exited somehow? Where could that have been coming out? I probably road it a few short trips with the contaminated oil, but likely under 30 miles total. My question for anyone that cares to answer- 1) did I likely to any catastrophic harm to an otherwise pristine motor? 2) where could that oil have exited from on overflow, and does that mean that any other part needs attention? thanks
 
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