I have a Troy-Bilt tiller with a 4hp hs40 Tecumseh engine and (I believe) 631918 carb. I can't get a good idle on it. No matter where I set the screw nothing changes. I can screw it in all the way or take it out completely and hold it in my hand! High speed works just fine. I can use the tiller, just can't idle it down without it dying or surging up and down. I was told by a "professional" that the carb needs to be replaced, but I'm wondering if a rebuild would fix it or if there is anything else I can do.
Any help you can give would really be appreciated.

Danny

The low speed metering circuit is most likely restricted with some foreign matter or build up. It can probably be cleaned out unless there is damage from moisture (i.e. corrosion).

You can download a service manual for your engine at this link. Check out the section on carburetors and you can get a better idea of how they work.

http://www.cpdonline.com/692509.pdf

There is a small orifice in the nut that holds the float bowl on, you need to make sure that is is open as this feeds the low speed circuit.
Next you need to make sure the pathway from where the nut screws into the carburetor up to the adjustment screw is open. I use spray carburetor cleaner to blow through the passageway in the carburetor. Remove the low speed adjustment screw and spray cleaner in where the adjustment screw goes, it should dribble out the bottom of the carburetor where the bowl nut screws into the carburetor. This tells you that the passageway is clear. Next look inside the carburetor by the throttle plate and you will see a couple of small orifices in the side of barrel just opposite the welch plug on the outside of carb. When you spray into the adjustment screw you should get some cleaner spraying out of these small orifices, if not then the welch plug will need to be removed and the passageways cleared. Once you have the circuit cleaned and open, you should be able to adjust it to idle properly.

Good Luck...

The low speed metering circuit is most likely restricted with some foreign matter or build up. It can probably be cleaned out unless there is damage from moisture (i.e. corrosion).

You can download a service manual for your engine at this link. Check out the section on carburetors and you can get a better idea of how they work.
http://www.cpdonline.com/692509.pdf

I thought I'd write and thank you for posting this response, I too need to disassemble my carb and clean it; and your post was very helpful.

Might I also recommend that anyone who decided to do this aslo pick up a rebuild kit before the tear down. Includes gasket, main jet & valve, idle valve and plugs. There appears to be two types of kits avaliable, a p/n 31840 ($10.16) and a p/n 631029 ($9.31).
http://www.outdoordistributors.com/Tecumseh/tfuel.html

Kit no. 31840 is used on engines: H, HH, HS, HSK, HSSK, HT35, HXL35, LAV, TNT100, TVS105, TVXL105, V, VH70. Snowthrowers usually use the HSSK engine (Horz, Short block, "Snow King").
Kit no. 631029 is used on engines: H, LAV35, OVRM60, VM.

Again, thanks for taking the time to post.