tedsander

Yes, that is the issue. As you guessed, probably an o-ring that is faulty. It seals most of the time, but not all. It's been a long time since I had a Magnum, so I can't remember the carb design. If it has o-rings on the barrel of the carb, and the local hardware store doesn't have any of the same size, you could try to tighten it up by putting a few wraps of thread next to it, in the groove it sits in. Just enough to make it compress very, very slightly so it begins to bulge out of the groove. Too much, and the barrel will either not rotate easily, or it will be hard to even get the barrel back into the carb. If there is one on the carb bottom, that seals where the carb goes into the engine, the leak could also be there. Same fix, although as long as you get good compression of the o-ring between the carb and the engine, size is not critical, so you should be able to find one locally.