What do you mean by "When it swings "?
By "backfires" do you mean a pop or explosion in the exhaust/intake?
How have you lined up the timing marks?
Cams should be placed with the locating pins up and the intake cam sprocket arrows parallel with the cylinder head surface. The exhaust cam should be the same excepting that the front arrow on the sprocket should be one tooth below the cylinder head surface (Eagle Mike's exhaust cam advance). The crankshaft needs to be lined up with the "T" mark at the lines in the engine side cover inspection hole.
The exhaust cam advance will make for higher compression pressure when starting so makes starting easier.
I assume that valve clearances are correct?
The KACR could be holding the right exhaust valve open too far and reducing compression. You might try spacing the weights open to see if that helps starting.
I'd be doing a compression test as that will help to indicate whether things are close.
A bad plug can act exactly like that due to a cracked center insulator so suggest trying a different spark plug.
Low compression due to flooding/washing of the oil from the piston rings and cylinder wall is a common cause of a no start, so might squirt a few cc of oil into the spark plug hole, crank the engine 5-10 turns, then install the spark plug.
I assume that you have new fuel and have drained the carb the fuel is a month or more old? Fuel goes bad and won't ignite which makes for symptoms like that.