I borrowed an AquaCraft Miss Vegas U-18 (the orange one with the Aqu .18 engine)
The only fuel with 50% nitro that I have is Cox fuel for .049 engines.
Will this fuel be ok?

I think the nitro content is ridiculous for anything but racing, but it would work provided the engine’s head is shimmed properly and a cold glow plug is used.

You must not know much about 1/2a stuff. Those engines thrive on nitro and need lots of oil. Cox fuel is 20% oil if I’m not mistaken - possibly 25% oil with most/all of it being castor.

Also, engine displacement has nothing to do with how much nitro it wants. While many small ones love Nitromethane and run well, some do not - especially at 50% nitro. An .049 Cox is a far cry to an Asian .18. Not being semantic, just Pointing out the “exceptions” to the rules. .

I have some Norvel .049s that came from the factory setup for just 0-5% nitro. They would blow their heads off if I tried 50% nitro in them without shimming the head and using a colder glow plug.

With all due respect (and no disrespect Intended); The statement that your engines won’t run on ANY given fuel simply means the engine needs to be setup properly. Out of the box, they’re setup for a range of fuels and plugs. Go outside that range, and something in the engine must change. That’s just a fact. My OS .21 runs fine on 5-10% and it runs hot as hell on 25%. Cool the plug off a range or two and shim the head .001-.002” and the excess heat goes away and it runs as well as it does on 5-10%. Comparing an HB .21 and OS FP .25 to a Nova or CMB is like
comparing a Yugo to a corvette. Big difference between those engines. The HB and FP are ancient engines designed around lower nitro fuels (5-15% typically) and lower rpm. Neither are screamers compared to Nova, Picco, OPS, Etc. And honestly - I’d expect 22-25% oil (mostly castor) to be ideal for the HB and FP, whereas the Nova and CMB or any of the fine Italian stallions will have the best balance with 10-12%oil for regular running and probably 16% oil for breaking in. Novarossi specs 6% castor 4% synthetic for their marine engines.

But to say that any given engine just flat out won’t run or run with a load on it means something is improperly setup. Sometimes adding a shim or removing a shim is all it takes. I run car engines mostly, but some boat and aircraft as well. I have one engine that is my old tried and true beater car engine. I’ve run as little as 0% nitro and as much as 40% nitro, 4% oil to 25% oil. As long as the right changes are made, it runs on anything. Combustion chamber shape does play a role, but only on the most high strung engines and even then it’s not a massive difference in performance.

That’s just the facts. YMMV.

The Cox fuel is actually 30% Nitro- not 50. my bad.

That Cox fuel should run just fine. If you blow glow plugs, run it a little richer. If it still blows plugs or randomly flames out when warm, try a cooler glow plug.

But those engines you’ve run 60% nitro in are what kind of engines - modern design engines or 30 year old designs? Will those engines still run on 15% nitro?

All I’m getting at is that some engines perform better on heaps of nitro, but it’s not necessary. Nitromethane is not required for a glow plug engine to run. Methanol is the fuel, nitromethane is the oxidizer (primarily). Some engines do not run correctly on high nitro fuel, but they’ll still run. My 30 year old OS .21 SE-M runs good on 10-15% nitro, but it runs hot as a pistol and pops plugs on 30% nitro. Adding a head shim and a colder plug fixed it right up; however the performance advantage of 30% over 15% isn’t much in my ancient OS. A modern Novarossi will show a more poignant result on higher nitro fuel because it’s designed to make big power. I’ll guarantee it’ll still run just fine on 10%, just with a little less power than the 25% nitro it’s designed to use from the box.