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Weird carb problem. Fuel leak, but all parts move and look perfect. Weird carb problem. Fuel leak, but all parts move and look perfect.
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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 13 Feb 2010
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Weird carb problem. Fuel leak, but all parts move and look perfect.
Hi guys

I’m working on an ’83 XT550.

The carb is leaking fuel. Some facts:

1. The floats are fine, they are not leaking. They float if put in gas.
2. The float height is according to spec.
3. The carb needle looks perfect.
4. The valve seat looks perfect.
5. The valve is 100% tight if I tilt the carb to let gravity help the float push on the needle.

However if I put the carb in the bike (or hold it vertical) one of the following happens:

1. The carb does not leak fuel
2. Some fuel drips from the overflow.
3. Fuel gushes from the overflow AND the breather tube, meaning the carb us FULL of fuel
4. If I tap the carb with a screwdriver the carb stops leaking fuel..


SO – the valve or float somehow get stuck and let fuel by, until I tap the carb, then the needle valve closes and the leakage stops.

I have had the carb apart maybe 20 times, but can’t spot how the needle or floats get stuck. Everything moves smoothly and there is no snagging.

Any ideas???
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  #2  
Old 13 Feb 2010
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Change the needle valve, you really cant see when its slightly worn, its a close fit.
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Old 13 Feb 2010
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Did the carb leak fuel when you topped the tank up in the first place or just
after you stripped the carb down and put it back ? It may give the guys on here something more to go on because I think ive had a similar problem years ago and yeah it was the needle valve but I think it somehow came dislodged from that metal thing the float hangs on. not sure ive ever had to buy a new needle valve in my life but maybe others have.
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Old 13 Feb 2010
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Its not unknown for some carbs that have a brass overflow tube in the bowl, to leak.

The brass tube is prone to splitting (very hard to see) and it weeps out.
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Old 13 Feb 2010
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needle in 180 degrees out
You might have put the needle 180 degrees out. happened to me when cleaned carb, fuel pissed out overflow, turned needle around and it was perfect.

just a chance.

Cheers from Oz

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Old 14 Feb 2010
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Thanks guys.

I managed to fix the leak by polishing the float valve seat with some fine stranded steel wool. I’m not 100% convinced the problem has gone away completely (the carb leakage was sporadic) but the problem is certainly less than before. The seat appeared sparkly clean but somehow the polishing helped.

Jens, good point. I will buy a new valve if the leakage comes back.

Panzer, the carb leaked when I put some fuel in the tank. I did not open up the carb until after I tried to start the bike. The bike did indeed start, but only ran for a short while. When I had the carb disassembled I cleaned it, but the problem did not go away. Until after the steel wool treatment that is…

Pigfort, OK good to know. This carb has this overflow tube. I did not even think about this possibility.

Chuncky55, good point. In this case I don’t think I can put the needle in backwards as it can turn in the float valve seat (T-type connector to the float). Someone please correct me if I’m wrong.

I GOT THE BIKE RUNNING. I managed to free up the clutch and rode the bike a little in the parking lot. Lots of work ahead but I’m looking forward to it….
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