Sorry about the test post. I was having problems responding.

Anyway, in response to the OP:

Running it won't hurt it.

In older engines gunk builds up on the valve stem. When this happens,
the valve may either sticks open or close slower than normal,
therefore (in your case) allowing the engine to fire when the
"exhaust" valve is slightly open. If the valve were to stay open, it
wouldn't run. It could be a broken/weak spring, but unlikely. After it
warms up the stem gets a little oil and runs properly or at least
better. This is normal for gunk build up.

Using a lighter weight oil MAY help the problem. But to fix it, the
valves have to be removed and cleaned. "lapping" them in at this time
would be suggested also.

Hank ~~~~ having trouble posting