Presure in tank leaking fuel out of bowl overflow

4 replies to this topic
  • Gjc814

Posted August 02, 2011 - 06:41 PM


Finally got my 04 wr 450 running great and went for a ride yesterday. About 4 miles into the ride we stopped and I noticed fuel leaking out of the overflow hose from the bowl on the carb. I shut off the petcock, cracked the gas tank and it was like opening a bottle of soda. Gas poured out and and after a few attempts I was finally able to open the tank. The fuel was bubbling alot so I lefft the tank open while I took off the vent hose to the tank and blew through it. No obstructions. Blew through the gas cap and still no obstructions. Put the cap back on and turned the fuel back on. I continued to ride and after another 8 miles noticed the fuel was leaking out of not only the bowl tube but all of them. Turned the fuel off and the leaking stopped. The only thing I can think of would be the float needle and possibly the o- ring underneath the brass fitting in the needle port or possibly a float level problem/ stuck float. I have not taken the carb apart yet to check it out. Anyone have any ideas as to whats going wrong and how to fix it?

  • MANIAC998

Posted August 03, 2011 - 04:14 AM


I had this happen to me a few years ago. I'm assuming your running an extra large capacity fuel tank, and that your also still running the stock Yamaha check valve in the gas cap hose. When the heat of the engine starts to warm up the fuel in the gas tank, the fuel starts to build pressure and tries to vent it out thru the gas cap hose. But with the stock check valve in there, it can't. That valve is designed to let air in, so fuel can flow out. It's not designed to let any fuel or vapors to escape out of the gas tank. If you remove this "check valve" your problem will be fixed. Let us know how you make out. Maniac

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  • Gjc814

Posted August 03, 2011 - 08:56 AM


Thanks! I took off the one way valve last night but havent checked out how its working yet. I am still running the stock 2.2 galon tank though. what could be causing the build up of heat?

  • dgcars

Posted August 03, 2011 - 09:14 AM


Higher ambient temperature = More vaporisation = Pressure build up in a sealed tank

  • Gjc814

Posted August 03, 2011 - 03:50 PM


So just an update. After taking off the one way tank valve and taking off the carb to look at the float, I found a small piece of rubber in the float needle seat area. I blew through the gas port to check for leaks and it sealed with no leaks when it was supposed to. Before I took the carb off, I blew into the gas tank breather hose and had the fuel leak Idescribed before. After I cleaned the float needle and got everything put back together, I tried to blow back into the gas tank to see if I could replicate the fuel leak as I had before. So far no leaks. Fingers crossed I think I fixed the problem. I will update this when I go for a ride next.


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