My 340 has recently been backfiring
Out the exhaust at slow speed when cold,
And running rough all the time.
Wound up being the brake booster leaking
Vacuum.
A rhythmic miss,once every other revolution,is typical when the vacuum line is plumbed to just one runner, as opposed to the plenum under the carb. I mean it's normal to route it that way, and it works fine as long as the diaphragm is good.
