Engine Problems.
Engines seem to be my first weakness. Seem to have more engines than planes, and don’t have enough room for the planes.
Hate to contradict any of the posts, but don’t understand most.
I had so many dead stick landings when I started this hobby, I was almost afraid to land with the engine running.
The problem? #1. I didn’t know how to properly tune one. #2. Junk glow plugs from a garage sale.
Life got good, when a couple “Old Timers”, showed me how to properly tune the engine, and I ran out of the old glow plugs.
With the exception of the four strokes, these things are about as simple as it gets!
Most are just copies of each other. Other than the type of bearings, and maybe metal, they are the same.

First of all - break in that new engine! Out of the box, it is going to be so tight, that it will quit about the time you get into the air.

Next problem. Find someone who can help you tune it. * The directions came with the engine but no one reads them.*

Air leaks. Bad gaskets, “O Rings,” holes in the fuel line, and foaming fuel from improperly mount fuel tanks, will cover about 90% of the problems. Don’t junk a good engine over a 10 cent O ring, or bad fuel line. Check for loose screws - Pad the fuel tank. They can shift in the plane. Just because it worked one time, doesn’t mean it will work, if the foam shifted.

Most air leaks can be fixed for no money, unless you have broken the case, or carb.

Of course a little dirt in the fuel will give you problems.

Engine starts, and then gives you problems? Try another glow plug, Just because it turns red, is no indication that it works properly.

Sorry guys! A Little tender loving care, and these things will outlive us, unless you run them into a brick wall, or use them to dig post holes. Doesn’t matter what brand, or even the ones from some ones junk pile.