Re: wheel horse 520-hc
My experience with the Onan carb is that it is best to take it off, tear it down, soak it clean, blow out all of the passages and put it back together. (I have a friend who doesn't soak his, he just takes it apart and blows all of the passages out with carb cleaner) If you do this, pull the plastic cap off of the low speed needle valve and count the turns until you can gently seat it so you will know the nominal setting when you reassemble it. If I remember right the manual calls for 1.5 turns out and then replace the plastic cap which only alows a 1/4 turn in or out. I haven't had much luck with the idle adustment, typically my problems have always been gummy passages or a vacuum leak. If you do want to try adjusting the idle valve, there is a hole in the bracket under the muffler. You can reach the needle valve through the hole with a long screw driver.
I haven't seen many HC's either. They were made in 88 & 89 for the commercial guys that didn't want all of the chrome, guages and fancy stuff. They were no longer available when the 1990's and later models came out.
Have fun with your 520's. As an OBTW, if you do decide to take your carb off, the best way is to pull the muffler and intake manifold. When I go that far, I always pull the covers on the valves and check the clearances. The last two engines I checked had burnt valves because the clearances were too tight.