I'll add what I think is happening due to all that initial timing and why on a street car it's not considered by many a good idea.
Any carb's throttle response and idle quality will are factory determined providing the airflow thru them is within their tolerance. Too little airflow or too little can lead to mixture issues, which is what your popping is. Coolant in an exh port is steam. Not popping. Unburnt fuel pops in the exhaust, and it pops because the burn in the cylinder is so bad that there is still oxygen left over too. The burn is bad because your lighting it so early the mixture in the cylinder doesnt have a chance to really mix well prior to the flame from being lit. That means some wet fuel out still of suspension and the possibility of the final few degrees of piston movement squishing out portions of the flame front. The piston is at TDC for about 10°. If you're lighting it at 30 or earlier at idle rpm the burn is making maximum pressure on the crank but from a smaller less-mixed volume of fuel and air and for a shorter number of degrees because the crank is preceeded by the burn. With pump gas the burn is even faster.
By igniting it just as the piston is stopping the upward stroke the mix is tumbled that last bit and by the time the flame kernel has formed and begun to expand out the piston is at 15° or so after TDC and in a much better position to leverage the crank throw over a larger number of degrees. The burn continues to expand the gasses even as the piston moves down the bore. Something currently in your car, that is not happening. The number of degrees the crank has to go doesnt change. The time it has to move that distance does change with rpm. At idle, it takes more clock time (we're talking hundreths and thousanths of a second) to cover the 30-40° that seperate the ending of the compression stroke and the point of maximum leverage on the crank. Stroke and rod legnth affect this amount of time too, but that is not adjustable once the parts are spec'd. At 3K rpm, it's a lot less time which is why the factories have self adjusting timing. Race cars that spend thier entire working life over 2-3K don't need any special timing control. You time it for the range it operates in. A street car operates in such a wide range, and needs to be fairly decent in all of them, that the advance controls are used.
Now when you have it so early, the carb has to be set to run with it. So by running the timing so early, the carb is almost shut and it shouldnt be. By retarding the initial timing to where it should be (15-18°) you will have to open the throttle plates with the speed screw. As a result of that airflow the fine adjustability will come back to the mixture screws. Once the mixture is set, the drop into gear will not be an issue, nor will the popping in the pipes.
My final setup recommendation is idle rpm: 900 in nuetral. Initial:18° Total:32° all in no earlier than 2200 and no later than 2600.
