This one kind of surprised me. I called a hobby shop today looking for fuel. It was a hobby shop I have no experience with and I talked to Joe. I asked Joe if he sold fuel for RC airplane engines and he affirmed that he did. I asked for brand names and prices. He said he sold Wildcat fuel at $15.00/gallon which sounded pretty good. So I asked him if he had a good stock and he said yes. I asked him what percent fuel he currently had, and he said 10%, 15% and some 20%. I said that was great, and I would like a couple gallons of 10%, and a gallon of 15%, and a gallon of 5% if he had any. He replied that he didn't carry any 5% but just buy another gallon of 10% and leave it out in the sun and it would soon be 5%

HUH??? Was he kidding or is there some truth in that? I know there have been hundreds of heated arguments on RCU regarding the shelf life of fuel, and I know it should be stored tightly capped in a cool dark place. But I never heard of changing the percentage of the nitro by leaving it in the sun. I'm naive, but was Joe pulling my leg?

How long do you leave it in the sun to become 0%?

He is wrong. Fuel breaks down much differently than just lowering nitro content. For example, fuel left out in the sun is often contaminated with water. Old 'dirty' fuel won't allow the engine to idle, transition, or run well at all. Reliability goes way down hill as fuel ages.

I did a search on nitromethane and found one site that talks about degradation by ultraviolet light. It only says that UV exposure causes nitromethane to change to di-nitromethane, and that it is "more interesting" to use. Unfortunately, it doesn't go into detail about WHY it is more interesting. Hmmmm....

No it does not, only an open cap would do that.

di-nitromethane is contact sensitive, but the methanol will help prevent it from breaking down. The major problem with leaving out a jug in the sun is that the pressure will break the seal on the cap. This will later show up as a collapsed jug, and sometimes some air will leak in. Not a problem as long as the cap is tightened. Also the some dye can turn dark, this is not a problem as the fuel will still run well.