ORIGINAL: Steve S. Helland
In my experience using these engines:
If it's new or nearly new you just have to set it right meaning needle adjustment.
If you bought it used and it has sat around for awhile on someones bench it always seems to be the pump and as before mentioned in this thread I agree that the only thing you can do is send it in to have the pump repaired/replaced by an authorized OS engines repair facility.
I steer away from buying them used because they always seem to have pump issues.
No more OS pumped engines for me.
Good Luck!
The pump is a replace only part. EXCEPT if you know a friend of mine here on RCU that decided buying a new pump for there price was silly and was able to take one of mine apart and replace the pump diaphragm with a home made one. My old 1.20 pumper works great again. I {Me} poked a hole in it getting out some old gunk. If the pump is pumping it's good. Just remove the glow plug, place the main fuel line in some fuel and roll the engine over, it should pulse out with some force. It really sounds like the engine is just out of tune but the pump and regulator can cause a problem. This is more of a heads up warning of something it can be. If you still have problems then Bill Baxter is the guy to talk to in the glow engine forum.