Hello all!

I'm working on a carbeurated LT1 for a friend and am having a heck of a time figuring out what's wrong with it. It's running way too rich and no matter what I do I can't seem to correct it. It's running a Holley Brawler carb. Timing is set at 32°, fuel pressure is right around 5 psi, spark looks very good. We're in Pueblo, Colorado so the first thing I did was change out the jets to 68 thinking that the altitude was messing with it, but that made no difference. It's pretty much immediately carbon fouling the plugs. Vacuum is way lower than it should be at 1000 rpm idle, we don't get decent vacuum until about 2500 rpm. Adjusting the primary idle mixture screws does absolutely nothing, when screwed all the way in there is no change in how the engine runs, and zero change on the vacuum gauge. When adjusting the secondary screws, it will stall out when they are screwed all the way in, but still virtually no change on the vacuum gauge. Even when adjusting the timing there is no change on the vacuum gauge. I'm using one of the ports in the front bottom of the carb to check vacuum and also run the vacuum advance. I was thinking vacuum leak, but I can't seem to find any anywhere. The fact that the primary idle screw adjustment makes no difference either way makes me think that it's an issue with the carb. I pulled the power valve today to look at it, but it looks fine to me.

Does anyone have any suggestions as to where I might look next to try and solve this issue? If I left any info out that would be helpful, please let me know.

Thank you in advance for any tips/advice!!