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Yamaha Tech Originally the Yamaha XT600 Tech Forum, due to demand it now includes all Yamaha's technical / mechanical / repair / preparation questions.
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  #1  
Old 12 May 2010
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Unhappy Urgent help needed! Carb leaking fuel.


Hi all, I had just set off from Calabar in Nigeria today to make my way to the border and stopped for a bite to eat only to return to my bike leaking fuel from the overflow hose (not a small drip either). I decided to head back to Calabar to get it sorted but haven't found a mechanic yet - apparently bikes were banned here!? Visa running out in a few days so even a quick fix to get me to the Cameroon capital next week will do.
Two days ago I had to bring the bike to a mechanic who cleaned the carb (there had been a lot of loud popping from the exhaust and the engine then died, spark plug was also black) which worked fine after that but maybe something was damaged? I've read other threads which mention needle etc but tbh I have only basic mechanical knowledge from working on diesel vans so know nothing about carbs!
Help much appreciated but be specific and assume I know nothing! Thanks...
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  #2  
Old 12 May 2010
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gently tap the side of the carb and if that don't cure the problem, blow up the overflow!
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Old 12 May 2010
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That was a quick reply while watching football, and guess what? I missed the Fulham equaliser!
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Old 12 May 2010
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Its most likely that the float height has been messed with by the mechanic or that the float valve isnt closing properly. Maybe he's bent the tang or not clipping the valve back in properly. Possibly a bit of muck or grit is holding it open. Maybe the float is jammed.

You will have to remove the carb, take off the float bowl and have a look.

You can try the quick fix of tapping the float bowl to try and unstick it.

Have you got the manual ?? If you've never been inside a carb before, maybe it's better to look for some help. It's not rocket science though. Just be gentle with it.

There is a plastic float in the float bowl. When the fuel level fills up from the tank, the float rises and shuts a valve which prevents more fuel from entering the bowl. If the valve fails to close, or the float adjuster tang isn't set correctly, the carb will keep filling up and eventually overflow out of the pipe.

My XT did this once, and once only in Brazil. I gave the carb a couple of taps with a screwdriver and it stopped.
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Old 12 May 2010
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Thanks, nope no manual. I'll try tapping the float bowl but I could see that floaty thingy moving freely as he reassembled the carb. If not I'll take it apart and see how things look inside (I had to Google tang - thought it was a mis-spelling!). Any more advice tonight appreciated and if you don't hear back from me tomorrow it means it worked and I'm on the road, that or I've broken down in the middle of nowhere!
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Old 12 May 2010
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Quote:
Originally Posted by luadraman View Post
Thanks, nope no manual. I'll try tapping the float bowl but I could see that floaty thingy moving freely as he reassembled the carb. If not I'll take it apart and see how things look inside (I had to Google tang - thought it was a mis-spelling!). Any more advice tonight appreciated and if you don't hear back from me tomorrow it means it worked and I'm on the road, that or I've broken down in the middle of nowhere!
It's got to be the valve or the float... Nothing else regulates fuel getting into the carb.
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Old 12 May 2010
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Yeah float valve. Drain the carb, and give the carb a good blow of wd40 or simular, damn where the fuelline connects, out the fuelline on again, fill the carb (set the tap on prime) and start the bike.

It has helped me twice.
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Old 12 May 2010
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Here's a pic of my NSR carb for a bit of reference:



The tang is at the bottom of the picture, and the piece of wire running over it is connected to the float valve which should shut off the fuel when the carb is full.

The screwdriver is pushing the retaining pin out, so you can lift the floats off and withdraw the valve. Inspect the tip - should be no damage at all.



Look into the hole it came out of and check for dirt or pitting of the surface where the float tip rests.

If it's all good, re-assemble and check the float valve height. To do this you need to know what it is (13mm on the NSR) and rotate the carb so the float is dangling down, then slowly rotate it so the float JUST closes the valve (without compressing the little nipply bit). Measure from the carb surface to the highest point of the float.



If it's wrong, gently bend the tang until it is right.

Hope this helps!
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Old 13 May 2010
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Quote:
Originally Posted by *Touring Ted* View Post
Its most likely that the float height has been messed with by the mechanic or that the float valve isnt closing properly. Maybe he's bent the tang or not clipping the valve back in properly. Possibly a bit of muck or grit is holding it open. Maybe the float is jammed.

You will have to remove the carb, take off the float bowl and have a look.

You can try the quick fix of tapping the float bowl to try and unstick it.
I'm not familiar with Yamaha carbs, but had a similar problem with my Mikuni carb on a LC4...

Yes have a look at the float valve first... some time you get two faults at ones... I had the top membrane not set in as it should be, so it was squashed a bit and the lifter didn't close tight as well coursing the fuel dripping out of the overflow hose all the time.

good luck
spooky
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Old 13 May 2010
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My KTM LC4 has been doing this for a year now. I've been turning off the fuel at the petcock on the bottom of the tank when stopped for awhile. This is how I remind myself to turn it back on. I just flip the clothes pin. The other side says 'GAS ON'



On bikes with gravity feed fuel systems, there's added pressure against that needle valve and seat in the carburetor when the fuel tank is full or when the tank becomes somewhat pressurized due to outside temperature changes.

Over time the valve and the seat wear and combined with the added pressure causes the float chamber to overflow with fuel. It has to go somewhere and out the carburetor overflow tube it goes. Sometimes they stick or the float doesn't pivot freely. That’s why tapping the float bowl can stop the leaking.

Carburetor rebuilt kits have replacement needle valve and seat in them.

While I think of it....if the float has a pin hole in it, it can fill up with fuel. The extra weight effects how the float pivots.

IMHO....I wouldn't loose any sleep over it unless there seems to be a fuel starvation issue because the float level is off. If the gas is flowing out that tube in a steady stream as fast as its flowing into the carburetor, then yes I would look into it.

Be careful.....sometimes trying to fix something that’s simple or that’s not a show stopper can lead to creating something that is.

Good Luck!

daryl

Last edited by dlh62c; 13 May 2010 at 14:08.
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  #11  
Old 13 May 2010
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Quote:
Originally Posted by dave ett View Post
Here's a pic of my NSR carb for a bit of reference:



The tang is at the bottom of the picture, and the piece of wire running over it is connected to the float valve which should shut off the fuel when the carb is full.

The screwdriver is pushing the retaining pin out, so you can lift the floats off and withdraw the valve. Inspect the tip - should be no damage at all.



Look into the hole it came out of and check for dirt or pitting of the surface where the float tip rests.

If it's all good, re-assemble and check the float valve height. To do this you need to know what it is (13mm on the NSR) and rotate the carb so the float is dangling down, then slowly rotate it so the float JUST closes the valve (without compressing the little nipply bit). Measure from the carb surface to the highest point of the float.



If it's wrong, gently bend the tang until it is right.

Hope this helps!
Wow! Excellent explanation and pics there Dave. Should be a Sticky for sure!
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  #12  
Old 14 May 2010
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Thanks everyone for all the help, tapping the carb with a screwdriver did the trick - I'm relieved it was easy to fix but kinda gutted I got worked up and lost a day over such a simple solution! Thanks a mil everyone - DAve
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Old 14 May 2010
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I have a pair of SU carbs on my desk (from my MG B), in bits. same problem with over-flowing fuel. the float bowl's have some fine grit in them accumulated over the years (they're 35 years old..), so I'm cleaning/rebuilding. although a sharp tap would've stopped the leaking.

check your fuel filter is ok, as you have to wonder how the grit got in there. sometimes it just happens, and it's not worth tracking down. but if your filter is dirty then it could be time for a new one.

cheers
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