First off, how about posting a photo of the gun?
As somebody said, if the vent hole is 1/16", you are fine. As for powder coming out the vent hole, no problem.
The spark, as Gary mentioned, is the real worry. Your gun should send a shower of sparks down when the flint hits the metal. What is happening is that the sharp (note that for a paragraph or so from here) edge of the flint is actually shaving metal from the face of the frizzen and that heats it to a point where it glows red hot. That is actually your sparks.
If you do not have sparks, chances are that you have a dull flint. Feel the edge of it. Does it feel like a sharp knife or a butter knife? Flint quality varies, as does the wear characteristics of flints. I won't get into knapping here, as I barely can do it (though I am told it is easy) but recommend a new flint if it is dull (save the old one as it can probably be knapped.
Next, unlike Gary, I like lead wrap of the flint. To to this, take any old round lead ball and start flattening it with a hammer. Don't worry about the round pizza-pie shape it takes, you can trim that with a pair of scissors. When sufficiently flat that you can easily bend it. put it around the flint and put it in the lock. After five or so hits, retighten the lock, and you should be golden for a while.
Now, as you are putting the flint in, but not tightening the lock yet, lower the lock so the flint touches the frizzen. Make certain that the edge of the flint is as square to the frizzen as you can get. Also, your flint should be striking the frizzen about 1/3 of the length down from the top. If not, you need a shorter flint if it is striking high, and if it is striking lower, there is a fix for that, that is remarkably cheap. Get a tree twig and put it behind the flint. This will push the flint out to meet the frizzen where it should.
Use only black powder, too. Synthetics have a higher flash temperature than flints can reliably give. Black powder, only.
Let's see how you go from there.
The Doc is out now.
