As others have said, leave the baffle in there for now. You will pick up more power if you take it out later, but at the expense of making the engine a little harder to set (the OS engines actually generally respond pretty well to the baffle coming out, they don't get really finiky like some engines do). But the main point is that at this stage, you don't need the extra power anyway, so leave the baffle in there. Later one, after you've been flying for a while, and know how to run engines, you can start expiermenting with doing basic engine mods to get more power. Walk first, then run .

It's not clear to me if the fuel coming out of the muffler you are talking abotu is when you are trying to start the engine, or after it's running already. If you have fuel coming out while you are running, that's not a huge deal, it just means your needle valve is too rich. Lean out the mixture a little, and the raw fuel will go away. However, you will always get oil coming out. If you don't have oil in the exhaust, you're running way way too lean, and you're cooking off your engine.

The 13% nitro thing worries me, as does you referring to the plans as "nitro planes", a term that is common with car folk, but the plane community usually referrs to these kinds of planes as "glow" as opposed to "spark". (after all, you can run 0% nitro if you want, though I wouldn't unless you have a reason to (leaving out the reasons for the moemnt)).

Anyway, my point about the car reference is this: don't run car fuel in your airplane. Airplane engines need more oil.

I agree with the advice above that for sport flying 10-15% nitro is about right for most flying. Most guys can't tell 10% from 15% with out using a tach, so save your money and go with 10%.