Boiling Gas

Well, today was the first ride of year and I got to experience something new today, boiling gas.

I was riding behind my wife, mostly in 1st gear for the afternoon, riding approximately 15 miles with frequent stops. Weather was beautiful at about 75 deg F in Michigan, with low humidity and elevation of less than 1000 feet. We have ridden this trail MANY times under much warmer conditions, with the same jetting with the only issue being the coolant boiling over occasionally to the reservior. This is not a jetting issue.

I ride an '01 WR426 with WR timing, free mods, JD jet kit, and Vortip. I know my bike well, and how it reacts to temperature, humidity, trail conditions...etc.

At one stop, I heard the gas hissing out the tank overflow, and saw it running down the front fender. I pulled the hose off the gas cap and it was spitting and sputtering. The ball-check valves in both the gas cap and the one on the overflow tube wiggled around so they were not bound up. The coolant never boiled over once today.

I searched, and found that there are a few possibilities for why the gas was doing this.

1. bad gas cap

2. overheating, heating the gas above

Well, the gas cap IS 6 years old, but the ball-checks were moving freely. The bike did not boil over, however it did appear to be running hot, which would be expected since I was running mostly 1st gear at about 10 mph with minimal air flow.

My thoughts were that it may possibly be a plugged or stuck radiator cap. The hot, expanded coolant could not flow to the overflow, pressurizing itself. This would cause it to heat more and reduce cooling efficiency even further. I could not obviously check the radiator cap trailside as I don't like being scalded. I will check the cap tomorrow during the daylight, as well as checking my water pump, and changing the coolant. There was no steam, or the telltale stink of burning coolant in the exhaust indicating that I had blown a gasket so that is a plus.

What do you folks think? Stuck radiator cap, stuck gas cap, with the combination of slow speeds and reduced airflow and all the above? Is there anyway to check the gascap and radiator caps for proper function?

Thanks for the help.

never heard of gas boiling over before. My guess would be just like you said: combination of all factors, heat, slow speed etc.

My 03 WR 450 has done something similair since it was brand new. Engine heat get the fuel tank hot and causes the fuel to "hiss" as it escapes the tank. I have also noticed the fuel leaking down the front fender--I just don't fill gas to th white ring inside the tank then it dosen't spill out.

My 03 450 always hissed when hot, I would just remove the cap to release the pressure and the noise would go away. If you want to see gas boil Try my IMS tank! Its a 3.5 gallon clear unit and you can see and hear the gas boil!

Thanks for the replys folks, at least I know that I am not alone.

I still wonder why I have NEVER noticed it before though, even when riding when its 30 deg hotter outside. Guess I'll find out in a few weeks at the Rights of Spring if it continues with a new radiator cap, flushed coolant, new oil and freshly washed bike.

Pooley,

We have same problem in the spring with 4 strokes. They work fine in the winter and summer, its a spring thing. We think its the winter additives in the fuel, they don't boil when its 90 degrees, so what makes it happen when its 50. Drain your tank and replace with fresh premium and it should be back to normal.

Actually the gas was less than a week old, but the station may still have old winter gas?

Does "winter gas" get shipped to the south?

Definetly not "boiling"gas, rather the same thing you see when your extra fuel sitting in the back of your truck starts to make the container swell. Crack the cap and presto, back to normal. Different additives during different times of the year (winter and summer blends) react differently but the fact is they all do it. The slow riding pace surely adds to the issue but there are ill effects from it. Try streaking past the miss's for a while and see if the prolonged fast pace changes things. Cheers

It is expanding gasoline vapor, not boiling liquid, I believe.

I used VP U4 and started boiling on a hot day the vent hose was venting alot I haven't had it do it with pump gas

I covered the entire underside of my tank with fiberglass heat blanket and aluminum tape. No more heat issue.

ijust installed my 3.2 gal desert tank, and cant help but notice how close the bottom right side of it is to my megabomb header. (about a quarter inch) the pipe is ceramic coated, which holds back most of the heat, but the fuel line on that side is touching the header, i had extra heat sheilding material left over from when i covered my rad. hoses for the same header too close reason. so i wrapped the fuel line as well. hope fully i wont have this boiling problem. also, doesnt hot fuel = poor performance? or is that just a myth.

ijust installed my 3.2 gal desert tank, and cant help but notice how close the bottom right side of it is to my megabomb header. (about a quarter inch) the pipe is ceramic coated, which holds back most of the heat, but the fuel line on that side is touching the header, i had extra heat sheilding material left over from when i covered my rad. hoses for the same header too close reason. so i wrapped the fuel line as well. hope fully i wont have this boiling problem. also, doesnt hot fuel = poor performance? or is that just a myth.

cooler=denser=better performance. that said, wrapping the daylights out of that fuel line and insulating the bottom of the tank should help the issue quite a bit. vapor and expansion will always happen, but as long as you keep up the pace and don't fill the tank to the tippy-top you should be fine. also, I know you only have a 1/4" clearance between the header and the tank, but if you can gain any space at all, a properly placed shroud will do wonders. remember that the shroud must have an air gap (it doesn't have to be much) between the heat source and the shroud to work properly. i'm sure you know that but you would not beleivewhat i've seen in the past.

cheers

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