In my experience that has been an issue with some of the older Tecumseh's, My Grandfather had a small engine repair business for about 50 years and that is the thing he never liked about the older 3-5HP Tecumseh engines, they were almost impossible to get tuned to where they would idle right and also rev up and maintain their RPM's, or you could get them to wind up real good but then they would not idle right or sometimes wouldn't idle at all without dying within seconds, we could get those old briggs engines to idle down so good you could count every stroke, but I never seen a Tecumseh engine that would do that, It is bad design of the carb's in mine and many others opinions whom I've talked to about it over the years, sometimes one of the good carb's off of the old Clinton engines would get the Tecumseh to run much better, The Old Briggs engines never had the power of the Tecumseh's but the Briggs was a much more reliable and much easier to tune engine, those pulse carb's on the briggs and even their bowl type was about the best there was.
Some of the Tecumseh's had 2 adjustments but most just had just a high speed needle adjustment, so you could lean them out and get them to idle but then they wouldnt rev, or you could get them rich enough to rev but then they would load up at idle and die after a few seconds or just idle rough, this was the problem I have encounted many time over the years with the Tecumseh carbs, even the OHV Powersport Tecumseh's had carb issues after a while.
You might be able to get one of those NT carbs tuned to run good on a 3HP 4stroke....it would be very easy to make an intake for it, even though it has no external adjustment at all, it might be possible to get it tuned by soldering the jet and redrilling it along with needle clip adjustment just like is done to tune the china girl 2 smokers, the carbs are cheap and it might be a good alternative, I have a 3HP Briggs I plan to try this on in the future just to see if it can be done.
Good luck with what ever you do, hope you get it running great before long!
Peace
Oh.! one more thing you should do is check the throttle shaft in the carb. they are bad about getting wore to the point that the engine has a vacuum leak around the shaft, when this happens they dont like to idle well at all.
I have a Tecumseh I bought on ebay. I never had one before but this one runs top end fine. It just won't bottom end. The guy who owned it before me got the linkage to the carb messed up. So there is no idle adjustment but I think it is more than that. I think there is a low end adjustment screw that is missing from the carburetor but I can't be sure so much is missing from where it was pulled out of service. Then I broke and removed recoil starter as well.
So my question is, was there supposed to be a second carb adjustment on that engine. 3 hp I think its off a snow blower.
