Pressurized Fuel Tank Issue

[Guest]

Look for some words of wisdom other than "Get a new Bike"

My Son has a Hongdou CG 125, all was well and runs fine until hot weather, when the fuel tank is nearly full it comes out of the Tank vent pipe at the front of the tank. I ma thinking that this could be caused by the vapors getting hot and pressurizing the tank forcing the fuel up the vent pipe and all other the place. it also ensures a constant drip from the carb leak of tube on the bottom of the Carb. I have only just changed the carb for a new one so know that is OK. as he had filled it up yesterday the fuel started coming out of the vent, it looks to me that the vent pipe is lower than it should be on the tank so is about half way down the tank. so when you fill her up on a very hot day the vapor will expand and pressurize the tank either forcing the fuel out of the vent pipe or the leqak off pipe on the CARB,

any thoughts would be appreciated.

I would have expected the filler cap to e vented as this i believe would sort the problem of pressure in the tank. I did take the cap off and all seemed ok, so then put my hand over the filling hole and sealed it and then it started coming out again.

[Guest akey]

Your diagnosis seem to be about right, a tank vent pipe should not extend below the level of the fuel, it should (as you rightly say) be above the fuel line to allow for overflow and gas venting in both directions.

Can you pull the vent tube up?

[Guest]

without the petrol cap it is OK, with or with a hand blocking the filler it leaks,

sounds like the vent in the filler cap is obstructed..

heard about this but never come across it...

[eastanglianbiker]

have seen this problem before have a look behind rubber seal on cap to see if you can see a vent hole