Start up antics

Breaking in my rebuilt 74 850. It's only got 25 miles on it. When I first start it up it backfires and pops. After about a minute it's fine, runs down the road smoothly and even tries to idle. But she sure sounds terrible for that first minute or so. I have chokes still in the carbs and it doesn't seem to matter if they are raised or lowered.

My first thought is to insure that there isn't an air leak between the carbs and the head.

UNless you are cranking the throttle for that first minute, trying to keep it running while it warms up, it's trying to run on the idle jets, which are easily clogged, and especially if it hasn't been running for a while. If that's the issue, you must get to those jets and clean them with wire or some such, but I think you kn ow about that.
Did you already check the timing? Boyer or points working okay?
Here's another thread similar, turned out to be a faulty brand new sparking plug. https://www.accessnorton.com/NortonCommando/amal-every-going-to-get-this-right.6580/#post-77048

Check the chokes are lowering all the way. If they don’t go all the way down they have very little effect.

Lowering them SHOULD make a noticeable difference to the running of th engine.

Other way to check is to tickle a bit after its started, if the popping and banging stops then you need to tickle more or fully apply choke before starting.

If your floats are in the correct range, you will HAVE to use the choke lever to fire it up. If it fires without it, the engine is getting too much fuel, which could cause backfiring when cold.

Your air screws may be out too far. I had similar symptoms on a fresh Mk3 years ago and was amazed at how much influence the air screws had on the idle mixture.

The normal situation is that if your bike is jetted correctly in the mid-range, it should spit back through the carbs if you try to start it without the chokes. So the usual approach is to adjust the jetting with the motor hot. Then rely on the chokes for cold starts. So what should happen, is you get your motor hot then adjust the idle settings and the needle and needle jets to get best performance. Then when you do the cold start, you should know whether the chokes are ineffective.

Okay, think I found the culprit. I checked the timing and it was way retarded. I could see what happened. When I initially timed it I somehow set it off the edge of the pad and not the timing mark in the center of the pad. May have been a bright day or I had my wrong glasses on or whatever. Now she starts and runs like a Commando should. Also, I pulled the plugs and while the jetting looks okay, I didn't see a change in color at the bend in ground strap showing the timing mark is okay. Just goes to show you start with the basics when trouble shooting. Thanks for all of the replies.