Alcohol Stove sputtering

I suspect the fuel. Try some yellow HEET. IFF you can get some, a pint of 95% Everclear will burn cleanly, too. (HEET is methanol which will start/prime a little easier. Everclear is ethanol, a little harder to get going, but will burn hotter once it is.)

Generally, water and alcohol will mix and not sputter. While water will not cause this when mixed with alcohol, other stuff can (sugars, gasoline-several parts, and, oils.) I also suspect that the stove’s ports are dirty/rimmed with brown. This can cut down on heat output. While it is possible to clean them out, be very careful. The aluminum is soft enough to be damaged. Many stove cleaners eat away at aluminum, too. If you made the stove, it might be easier to simply make another.

Also, what is the ambient temperature? Does it sputter less when you prime? Does it eventually start running okay once it has thoroughly warmed up? Or maybe this stove design gives up a lot of heat to the environment and is not the best when ambient temps are low.

When it’s cold it takes longer for stove and fuel to warm up so that the alcohol boils and produces a steady alcohol vapor… and vaporization makes the world go ’round, after all. :^)

I had one trip where I grabbed a partially filled alcohol bottle, topped it up with alcohol, then took it out on the trail. My stove sputtered, smoked something awful, and took forever to boil. When I held the bottle up to the light, I could see “swirly” lines, where clearly 2 different liquids were not mixing.  I have no idea what else might have been in the bottle, as I have specific designated bottles that are colored and don’t look like any other bottle I have.  I always suspected water got in it, but maybe not.  After that, if I’m grabbing a half-used bottle, I examine it carefully before and after I fill it.  I’ve never seen that problem since, and my stoves have worked well.  So, yes, suspect the fuel.

The two halves are not completely sealed. Fuel enters the gap, boils/sputters. That’s my wild guess ;)