On Thu, 27 Jan 2011 07:46:06 -0800 (PST), Hank
wrote:

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
Run some "SeaFoam" in the gas - and some "marvel mystery oil" in the
oil - the valves will free up if they are the problem, and the jets in
the carb will too - often exhaust backfiring is due to running too
lean.
Lighter oil won't help in this case - you can almost guarantee that.

Removing the muffler and squirting a solvent like carb and choke
cleaner on the valve stem may help too. = When the crap softens,
scrape it off with a wooden stick.