Here is my experience.  I have a JD 420, a 1983 model.  The 420 and 318's both had Onan engines.  The 318 was 18 HP and the 420 was 20 HP, basically the same engine, maybe the jets in the carburetor were different.  My 420 would run full throttle, but if you tried to idle, it would die out.  I took the carburetor apart and found out the throttle shaft had wore into the body of the carburetor, so air was being pulled past the throttle shaft, and not through the idle jet, causing the engine not to run at idle speed.  So I checked with JD and the complete carburetor was not available...the last list price was over $400.  So I went on line and found a carburetor that would replace it.  It was $25+ and shipping.  When it came, you could see that it was made in China.  I put the carburetor on and it ran fine.  I tilled with it for about 3 hours, and then it started smoking and running poorly.  I ran it back in my shop.  I thought was the auxiliary pump bad, so I replaced that and adjusted the carburetor again, but to no avail.  I took the top of the carburetor off and removed the float.  As I shook it, I could tell there was gas in the float.  So the float was not doing its job.  The float was brass, which was soldered together and the gas was leaking past the solder and into the float.  I had saved my old carb and took it apart and took a look at the float.  That float was made of a hard plastic.  So I took the old float and put it in the carb that came from China, and put it back in the tractor and assembled it, and the tractor ran like a top.  It has been a year now, and there are no problems.  So be wary of carburetors that come out of China.