Thanks Brett, I think you are 100% right. The flame out in June happened with fuel that contained 5 or 10% nitro, to cope with the altitude of the contest site. It was the first time I added nitro to the mix. I remember thinking back then that adding nitro could not possibly cause a flame out.
But this nitro has a shelf life of 25+ years and I have been using it since Jne. So I have the strong feeling this nitro has gone wrong and it is causing separation. I am not so much in chemistry, so my knowledge about mixtures is not so impressive. Thanks again!
That, also, would be extremely unusual to cause separation and if it is starting to decompose, it will have precipitates at the bottom.
But, if you are set up to run FAI fuel, and you go to 10% nitro, maybe it works at altitude in June, but not at sea level in November - lower altitude and cooler air. Unexpected flameouts, kicking back, "fierce" starting, are classic examples of overcompression. I submit that this sounds a lot more likely than having fuel separate because of nitro - since I have been doing this for 50+ years and never heard of anything like your stratification.
This is one of the problems with running weak fuel and FAI fuel in particular - you have to jack up the compression so much that it is very "hair-trigger" to adjust and highly prone to tiny changes. Part of it is that the fuel is so weak you need a lot of compression to get any power out of it, and also, you do not get any advantage of the catalytic decomposition of nitromethane with the platinum or rhodium acting as a catalyst.
Nitromethane is a moderately powerful rocket "monopropellant", because it will automatically "decompose" - separate into constituent components when exposed to high temperature or certain catalysts - like platinum. This is much like hydrogen peroxide and various forms of hydrazine - when it just touches platinum or something like it, it breaks the chemical bonds, generating extreme heat and then forming a hot gas. Without actually consuming anything else, like the platinum.
This is why the the early glow experimenters came to nitromethane and platinum glow plugs in the first place - the first popular glow fuels used 37.5% nitro, which is why they had so much more power than gasoline, and why they blew some old gasoline engines to bits when you tried it (Ohlsson, for example). Many spacecraft use the same thing for a rocket engine/thruster - spray some decomposable liquid onto a hot bed of platinum, etc, it breaks down, turns to steam, inside an enclosed chamber, then you shoot it out through a narrow orfice at high speed.
Nitromethane is not a preferred propellant, it's too stable, hydrogen peroxide (in the required 95+% solution, not the 2% you get at the drugstore) , AKA T-Stoff, is far, far, too unstable and prone to blowing up in the tank because any slight contamination might cause it to decompose and is extremely dangerous to store. Hydrazine is the preferred combination of power an stability. Straight hydrazine gives more power but is more unstable and gives poor handling qualties (freezing at 3-4 degrees F), MMH (monomethyl hydrazine) more stable and has better handling qualities but less power, etc. That's why hydrazine was also a common model fuel component back in the day until it was banned, also used in methanol or ethanol-powered Indycars until it was banned (and sometimes afterward...) - much more prone to decomposition so "ignited" it more reliably.
Methanol will also do something like this under certain circumstances - but is far more stable still than nitromethane. That's why you have to take very extreme measures like very high compression to get it to ignite at all. So, set up to run FAI at sea level, you might get away with nitro at high altitudes and temperatures, but cool or thicker air, the shock wave alone from the initial decomposition might also decompose the rest of it, no catalyst required, no flame front required. I will let you calculate the time it takes to get a shock wave across a 3/4" cylinder at the speed of sound in 1500 degree gas. Answer is "way faster than a flame front", it more-less decomposes it all almost instantly - which is what "detonation" actually means in engine terms.
So, I guess that you are running too much nitro or too much compression for your FAI-fuel engine, the fuel itself is fine. If you want to fix it, add a .005 head shim, or go back to FAI fuel.
Brett