09 YZ450 Boiling Gas

I have a 2009 YZ450 and I am having issues with the bike boiling the gas. After about 15 to 20 miles of riding the bike with begin to studdard while on the throttle. Stop, turn the bike off, and pull the gas cap and its like the tank was pressured. With the gas cap on you can see a steady stream of vapor exiting the gas cap breather hose. The bike is backfiring on the decel but is not losing coolant. The bike is basically stock except for the FMF exhaust. Also, the bike ran excellent for 90 hours, it has only been acting up for about the last 5 hours.

So far here what I have done and nothing has solved my issue:

Cleaned Carb

Main Jet - 168 to 170

Pilot jet - 48

red needle on the 5th clip

Fuel screw - out 2 turns

(all of these were the JD jet kit recommendations)

New Petcock

New gas cap

New top End (piston, rings, valves)

heavier coolant cap 1.6

Any help would be appreciated.

Gas isn't what it used to be. The problem from hot gas is a vapor lock if the gasoline is heated to the point where it vaporizes before it reaches the intake, it can often lead to studdard while on the throttle. I read this article where someone suggested using fuel line with braided stainless-steel sheathing to protect the rubber hose from heat.

Are you sure it is a positive pressure in the tank, and not a vacuum? With the one way valve in the vent line, there should not be vapor leaving the line, only entering, if the vent is working properly. Also, I can't imagine getting the fuel hot enough to boil in 15 to 20 miles. I've done wide open 100 mile desert races and hours of riding tight mountain trails in the heat at a snails pace and never had a problem with either the stock tank or my Clarke desert tank boiling fuel.

The only time my bikes boiled gas is here in Arizona during the summer.. i think locomike might be on to something, do you have a regular vent hose or do you have one of those little shorty breather pieces on the gas cap? I've seen those little vents cause a lot of issues in the AZ heat.

I have the same problem on an '08. I was looking at that that heat shield that goes on the bottom of the tank. Anyone have any info on them??

Have you checked the color of your sparkplug to look for signs of a rich/lean condition?

My guess is the gas cap vent hose got put back on backwards or is defective and is not venting properly.

Look at the arrow on the tank breather hose check valve. The arrow should point towards the tank.


Check your valves. A friend I was riding with last summer had the same problem with his KX450F. It was hard to start and then would run ok at first but at a high idle, then after awhile, it would run worse and his fuel was pretty much boiling. He adjusted the valves and no more problems.

i had this problem as well, take the one way out for a bit next time it happens. I think you will find that it runs perfect. I had to clean out the one way check and take the cap apart and clean that as well, put it all back together and was able to suck air throught he cap like it would be coming out of the tank, worked ever since. not sure if it clogged or what

The cap and one-way check are supposed to allow air IN to the tank so as to replace the fuel as it drains to the carb, and to resist, but not completely stop, air or fuel from running out. Bear that in mind.

Just ran into the same problem and same bike!I too had a short breather and i replaced the cap and walla problem solved!

Fixed it, Thanks for all the comments it was the mixture screw was running to lean.

... walla problem solved!

"voilà" :cheers:

It seems like you guys are providing a band-aid, not a fix. The problem is boiling fuel. Try the CVS Fuel Kool, it gaurds the tank from the heat of the engine. One of the dirtbike magazines tried it and said it worked great.

10 to 1, the OP has never actually seen it boil, and I'm sure you haven't. Running a 3 gallon IMS tank in the heat of the SoCal desert, I haven't ever had a problem with heating the fuel in the tank, either, so I'm just going to suggest that the guys who are addressing this as venting issue are on the right track here.

I'm sure it is a fix not a band-aid, I have been running the bike for over a year and not had this problem. Also ok maybe not a boil cause i didn't have my thermometer for a accurate temperature reading, for some reason i left that at home. I guess i should have said bubbling anyways it was running lean the venting was fine and cause it sat for a little bit one of the jets junked up.

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