HSR idle mixture screw

[sml1226]

Hey guys. I have an HSR42 that I’ve never been able to get right. I stuck an O2 sensor on to check AFR here recently and it doesn’t seem to want to move any while turning the air screw. I’m not really sure if it’s always been like this or not but I can’t really tune the rest of the carb if the idle is never right.

The needle, needle jet, main, etc shouldn't be affecting me here, should they? I’ve had a 20 and a 22.5 in there so far and both seem to settle right around 13:1. I can get it to go richer, but even there, not a ton. I think it hit 11.5:1 before it stumbled.

So I guess the first question is, is there anything that would lead to the screw being largely useless? I can’t see anything blocking the air passages. The air jet is a blank on these. The choke plunger, as far as I can tell, is fully seated. An air leak somewhere would have the opposite issue.

I can try a 17.5 jet too, as I do have one, but I’d still expect the AFR to move even if the jet were too large.

Any other suggestions?