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Old 20-04-2014, 13:12   #1
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Yamaha Carb problems

I've got a 2002 15 hp Yamaha 4 stroke that has a carbureator problem that's giving me fits. I'm awaiting word on when I might get a carb rebuilding kit shipped to me here in the Bahamas, this fine holiday weekend (the Bahamas takes their Easter holiday very seriously). Ive got only one theory on what might be going on so I thought I would toss it out there and see if anyone had any better ones.

The problem is that the carb keeps flooding, that is fills up like the float valve is not stopping the fuel from entering the carb when it gets to the float level. I have had the carb apart 4 times and nothing I have done seems to fix it. In this carb the fuel connection is on the bottom half of the carb and the float valve assembly is in the top half. The fuel travels vertically up to a screen that is located in the bottom half of the carb at the interface between the bottom and top. There is a rubber gasket that seals the top and bottom halves and has a hole that completely seals the screen on both the inside and outside of the bowl. The fuel then enters a passage in the top half of the carb which passes across the top and then down into the float valve assembly. The float is plastic and has no liquid in it and the tab to the needle is not adjustable. I have tested the float valve and it appears to seal properly, though I grant you there's been no real pressure applied in my tests. When the carb is placed upside down the float valve seals. I tested this by filling the inlet line with gas while the carb was upside down which closes the valve. After 10 minutes the fuel level in the inlet tube did not go down at all. If the valve was leaking Fuel should have leaked through. When placed on the engine even having not been disassembled after passing the fuel leakdown test the carb fills with fuel. and it comes out the vent hole on the top. The mixture is of course too rich for the engine to run. On one occasion I only gently pumped the bulb until it got hard and the engine started, but ran rough and died withing 30 seconds. My current theory is that the float valve is in fact working properly but the seal around the fuel screen is failing under pressure from the fuel pump and allowing fuel to enter the carb directly without going through the float valve. The needle on the float valve has no ridges or signs of damage and looking at the valve seat with a magnifying glass I cannot see any damage or wear in the valve seat.

I'm open for any additional theories as to why the carb might be flodding with fuel. My concern is that there might be another issue with the carb that the new parts in the rebuld kit won't fix and that I should buy a whole new carb instead.
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Old 20-04-2014, 14:05   #2
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Re: Yamaha Carb problems

Bill I know your frustration having been through the same prob same engine there was another thread on here awhile back that discussed the same prob with mine it was the float valve pin ( that the float swings on ) kept moving sideways it either got stuck fully open or in the closed position it wasn't till one day the fuel bowl wouldn't go back on properly I then realised the float was skewed so it may this could be the prob and may be not
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Old 20-04-2014, 14:51   #3
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Re: Yamaha Carb problems

Check for debris in assembly screw holes, scored gasket mounting surfaces, check screws for length and placement (as there may be an odd one). Was this a factory carb and has anyone else, been in it before you. I've seen these issues on other carbs.
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Old 20-04-2014, 16:08   #4
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Re: Yamaha Carb problems

Quote:
Originally Posted by lonesoldier0408 View Post
Check for debris in assembly screw holes, scored gasket mounting surfaces, check screws for length and placement (as there may be an odd one). Was this a factory carb and has anyone else, been in it before you. I've seen these issues on other carbs.
I'll check those things. I looked the gasket surface over for scoring but didn't see anything. It's been open several times in the past 9 years. Ethanol based gas has been a problem and the carb has needed cleaning many times. I suspect the gasket has been opened one too many times and needs to be replaced.
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Old 20-04-2014, 17:44   #5
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Re: Yamaha Carb problems

Check for hairline crack or pinhole in float itself. Gas inside, and the float don't float.
Had it happen in the past with older brass floats.
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