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.