When I rev briefly the engine close to WOT, either in park (free rev) or in gear, the revs don't come down, I have to shut the engine down. Now if I'm just giving it partial throttle, it's fine. Is it perhaps the vacuum secondaries that are hanging up? I put my hand over the front barrels and got no change. I don't think I tried the back and I tried to close the throttle more by hand but that didn't seem to help. This is a 4175, so it's got small front barrels and big back ones. It kind of even almost sounds like the secondaries kicking in when I rev it. I'm not sure of the spring colour as it's whatever it had in it, it's kind of a junkyard carb. When I shut the engine down, it doesn't even run-on, so that might be the secondaries closing when there's no more vacuum. Also, it's got a 6.5 inch powervalve but it's had a 4.5 inch one too.