'06 YZ 450F gas cap/tank vapor lock?

Last weekend both mine and my buddies YZ450's started cutting out/sputtering off the jumps at about the same time of day at Lakewood. This was the first warm Colorado day (79 degrees), and both had major pressure relief (hissing)when the gas caps were loosened. Is there a known problem with the gas caps? My thinking is they are running lean with reduced gas flow, but not sure why it primarily happens over jumps. Both are jetted and have run great to this point (in cooler weather). Never had to re-jet for temperature variation in Colorado before. Any suggestions?

hey brian, i am having that exact same problem. have you figured it out yet? it was running fine in cooler weather, and now that it has warmed up to like 75 its bogging through whoops and while coming down from jumps. it starts to do it after like 3 laps on the track. i put a 170 main jet and that didnt help. ???

Dont know if this is the case but I had worked on a YZ 426 that would do that and the owner had put those shorty style gas cap vents on it. I guess is does not vent enough or could have been blocked??? I put the old stock one on hte bike and hasnt had the same problem. Thought it might be worth the shot. Lemme know if you figure it out

Check the plastic vent piece in the middle of your gas cap breather hose. The plastic piece in the middle of the hose is a one way check valve (air in but no gas out). The plastic piece has an arrow on it and tells you what direction the piece must be. Yes you are starving your carb of fuel because of a vacuum condition in your fuel tank. When fuel leaves the tank for the carb, air has to go in place of the fuel that exited. Fix it before you ride again. That is exactly how Jeremy McGrath broke his hip on his KTM when the motor coughed on a jump because the fuel tank vent line was not working properly.

How does your bike idle? Does it get hot fast and expel coolant, more so than normal?

no it doesn't. if i let it sit there for too long idleing it will over heat of course but otherwise it doesnt get too hot. i pulled the one way valve off the tank and am going riding tomorrow (sunday). i will try some jetting if that doesn't work. have you gotten any further on yours?

That is exactly how Jeremy McGrath broke his hip on his KTM when the motor coughed on a jump because the fuel tank vent line was not working properly.

Yes, MC 's bike did suffer from a vapour lock, but it wasn't a one-way valve in the vent tube that caused it. It was a one-way rubber valve thingy in the gas cap that malfunctioned(bad design idea). Most of the KTM guys(that found out about it) just cut the middle out of the rubber and it solved the problem.

I had this problem in 95 degree weather and it bogged right before a triple. I am still recovering from the wreck! I removed the one way valve and now it seems to be ok.

When you say you hear hissing, watch or feel the tank, and try to see if it's going in or out. If the tank shrinks and air escapes out of the cap as you loosen it, then the one way check is building pressure, and this is not your problem.

If, OTOH, it feels like it swells a little and air rushes IN, then you have trouble with your vent. BTW, this is NOT a vapor lock.

That being said, problems resulting from a blocked vent should be the most prevalent after the longest, hardest pulls, as in the case of an uphill straight, much more than when landing after a jump. If I were you, I would suspect the float level first.

I'm pretty sure that I read in MXA mag ages ago that the Pro's don't run a one-way valve at all on their gas tank breather hoses, coz it's not worth the risk. I think I might do the same.

I took the ball-check valve out of the vent tube and the problem went away. Same for my buddy. No further issues since.

Good luck!


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