Idling RPM should read 1000 or thereabouts. 1500 is too high and I imagine the engine would be jiggling around a bit at those RPM.
So to check what is going on, try screwing out the carburettor slide setting screws (idling screws) note where they are positioned before altering. If the RPM don't drop, you can bet the cable assembly is the culprit, or the twist grip is assembled incorrectly?
Unscrew the twist grip and see if that resolves the issue, if that does not work, check the routing of the cable to the junction box, you may have to remove the tank for access.
If you need to start from scratch, I assume you have the original Amal carbs? Remove the air filter, put some small drill bits under the carb slide cutaway, and adjust out the idle screws, then slowly adjust up so that the drill just twitches, from there screw in one turn. This is a base setting. Now set up the cables so you have equal free play, you can fine tune this with a couple of 1/4 inch balls, rest them against the cutaways, then carefully open the twist grip, the idea is to get the balls to drop past the cutaway, simultaneously, but don't let them go past the back of the slide, bad things will happen if you do!
The above will get the carbs mechanically syncronised, you may have to adjust the idle screws to get your desired RPM idle.
HTH Regards Richard