The shower of sparks concerns me

So after much tinkering I finally get my (grandpas) 1935 Chevy truck started today. She runs very well for not running for 30 years and not having the best mechanical care the thirty years prior. I rebuilt the carb and fuel system but didn't check the timing or anything yet. That's where the fireworks come in.

I fired it up in the machine shed and pulled it out lest it burst into flames or something. I have this odd thing about that when I've just rebuilt an engine or started something after a long rest. All of my years working on air cooled VW's made me cautious. It was just about dusk and I noticed sparks coming from the exhaust. It's just a straight pipe as the muffler left sometime in the 70's and didn't seem necessary for feeding cattle. Of course I was a tad concerned because the fuel tank is right above that straight pipe (under the seat). So I let the old girl idle there for about 15 minutes and she continues this sparking business. Not like a sparkler but about like mig welding. My first thought is a nest in there, but it isn't bits of flaming material - it's like slag. I immediately think carbon buildup. Did I mention my uncle never changed the oil in the 30 years he drove it everyday?

The thing is, it never stopped. I drove it about a quarter mile down the road and when I got back I got a "talking to" from the wife about how I WAS NOT driving that truck again until I sorted that out because there were "millions" of sparks up and down the road.

Could carbon buildup take that long to burn off? She was still sparking when I shut it down after 30 minutes. The thermostat had opened many times and it was at full temp for quite a while.

Hot rust flacks will do that and with one that has sat that long could well be a whole lot of rust in the pipe that will blow out and be hot and in dim light you will see them

You bet. Sparks is good, flames is bad. Just rust flakes or carbon as others have said.

You can trickle some water through the carburetor when the engine is hot and it will break all the carbon loose in the combustion chamber and head and most of the sparks will stop. John

You nailed it 'old'. Was part of the same movie with a '43 GMC Maple leaf 5 ton.(30 years ago). It was SPECTACULAR with the sparkelin' dept. When all was said and done, there were great freakin' holes in the exhaust system. Replaced same and that unit is still operating to this day
not every day, but parades and such.

millions......

That's the same kind of "exaggeration" a female member of my family displays! Her favorite is "Destroy" though!!

Depending on how bad the carbon is. It could take a few trips to get rid of it. I have put generators on load banks. Some took as long as five hours under load to clear out.

Sounds like the build up of time. rust in the manifolds and exhaust or just anywhere in between. If it dont stop i would say a could be a valve going but you said it was running fine . could be build up of rust on the valves