Mystery Smoke Scare

[gophersnake]

I have a '68 GMC 2500 that I drive mostly for short distances: a mile here, five miles there on city streets, occasionally 10 or 15 miles on the freeway.

It usually starts easily (if you know how much choke to give it, how quickly to push the choke back in, etc.) Thursday evening it was a little harder to start than usual. I wondered if I'd flooded it or something, but I couldn't tell for sure. I'd driven it a ways up into the hills, maybe 3 or 4 miles, then it had sat for a couple of hours. The temperature must've been in the fifties and falling but that engine takes many hours to get stone cold. Anyway, it did start -- and then ran just fine all the way home.

Next day -- Friday, around noon -- I went to start it again. This time it acted like it was barely firing, maybe on one cylinder here, one there. After 15 or 20 seconds of cranking I saw smoke pouring out from under the hood. It subsided as soon as I stopped cranking; no fire, obviously. A fried belt? Nope, all the belts looked fine and still do. I opened the oil filler cap. Smoke poured out of the crankcase. I opened the air cleaner. Smoke was wafting out of the crankcase breather too. Clearly the crankcase was full of smoke. I checked the oil level -- normal.

I went to crank it again. It started, ran a little rough as if missing on one cylinder, then after a few minutes smoothed out and ran fine. I didn't see any blue, black or other smoke from the tailpipe OR the hood. I drove the truck a couple of mikes, parked it for a few hours, drove it another mile or so, etc., for a total of four starts that day. It started fine, idled fine and ran fine. Whatever had been the problem seemed to have cleared up completely.

So my question: what could possibly have caused all that smoke, then cleared up by itself? I've been thinking of things like a stuck piston ring that allowed a lot of blowby, then reseated itself. Maybe a PCV valve that somehow, by a fluke, let some oil drip from the rocker arm area into the intake ports. Maybe a badly sealing valve guide that did the same thing.

Any ideas? The engine is a 351 V6 with Bendix Stromberg WWC carb, manual choke, a PCV valve screwed into each head, a breather hose that runs from one valve cover to the air cleaner -- and not much else worth mentioning. I've been driving this truck regularly since 1980 and had lots of other interesting things happen, but this was a first.