ORIGINAL: fizzwater2
I would think if it's leaking at the head, it wouldn't run very well.
If it's leaking out the front bearing, which ends up getting fuel everywhere, then it may still run ok but use a lot of fuel.
Kinda tough to diagnose a problem like this without seeing it, could be all kinds of reasons.
Is the backplate secure? o-ring under the carb intact and in good shape? Like someone else mentioned, don't overtighten things, you can ruin a head doing that.
I agree if the head was leaking it wouldn't run well. Its a fact that almost all model engines spit a little fuel while running, more noticeable at higher rpm. I'm willing to bet thats the problem. Otherwise there would be bubbles showing up around the head while running.
Obviously there are no pictures or videos posted, so this is an assumption but I've noted it on all of the engines I own. The sloppiest engine for spitting back is my Fox .40 that has killed itself. The next worst spitter is my Enya 60-4C but this one is more of a dripper than a spitter.
To the OP: If you still think the head is leaking, get a video of the engine running, being throttled up and down. This will prove one way or another whats going on.