Back Firing

Looking for some feedback on a back firing issue I am having. It started after my last rebuild of the motor (carb is the same and not rebuilt). Have had a hard time getting it to idle properly (or close to properly for a 12.5:1 compression with a big stick). Before the overhaul I always idled around 1700 with a rough idle so the idle quality is nothing new. What is new is that I am backfiring when I start it up. This evening I was trying to start it to work on adjusting the idle mixture and the damn thing kept blowing a flame and catching the carb on fire:eek:. The carb is a HP4150 , 950 cfm. Thought it was the timing the other day and bumped it up a bit to about 37 degrees (was 35 to run pump gas). Almost seems to have made the backfiring worse.
My idle mixture screws are turned way out to 2 1/2 turns trying to get it to idle. This is what I was going to try adjusting today. I set them at 1.5 turns to start and that was when it backfired. After a couple of tries at 1.5 turns i turned them back to 2.5 and it still backfired :( I'm sure my new power valves are toast again....

I am now wondering if the timing pointer is set correctly. When I had the engine apart I had the shop install a new timing cover and one of those billet pointers that bolt on. I assume they set it up properly at the shop but maybe.... or it might have lossened up.

Any other thoughts to check?
Author: Stumper