Seems to idle fine once running full choke then when you rev engine backfires through carb
Backfire through carb often means timing too advanced.
SOURCE: 1983 FLHS starts and idles fine,pops and backfires when riding
In 1983 the carb used on this model was a kehin. They can be made to work well, with a couple of tweaks. First, on the top about midway, you will see a boss sticking straight up. Unless it has already been done, use a jabsaw and cut the top 1/8 inch off the top. Using a tiny improvised screwdriver, back out the idle adjustment screw which is down inside about a half a turn. Then remove the float bowl. You will see 2 tubes hanging down. Remove the rubber plug from the larger one and remove the low speed jet. The jet and the mixture screw are soft brass, so make sure you don't shred them. The jet has a number stamped on the end. Go to a knowlegble shop and get the next size up. For example if you have a 52 and they say the next size is a 54, that's what you want. Take the old jet with you and compair the size of the air holes along the side. They should be the same size. If not, they are selling you a jet for a cv carb and that will not work. (common mistake) Don't forget to shove that rubber plug back when you reassemble it. If I have your problem pegged correctly, the bike will be less cold blooded and the driveability will be much better.
SOURCE: Backfire out of carb!
Remove and clean the carbs. Clean the air filter and install new stock plugs. Re-mount the carbs and then balance and sync them. Now would also be a good time for a compression check. Valves and rings do give problems sometimes but these problems show up first as a lack of power. Since you didn't report a power problem I doubt if valves are the problem.
SOURCE: Bike is backfiring through carb.
do u have the correct spark plug the one recommended by the manufacturer
SOURCE: Backfire through the intake at cruising speed,
Intake:
Lean, or spark control problem, valve problem, bad fuel, plugged carburetor.
Exhaust:
excessively lean or rich with exhaust leak, valve problem, spark control problem.
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