Briggs engine in Toro Snowthrower Governor Issue

I can't buy that spring temper idea, that the spring loses springiness when it's hot and regains it when it's cold? Nah. A spring isn't tempered. If one were to temper it (heat and quench), it would become stiff and brittle, and stay that way until you reheat and let it cool without quenching. Besides, I don't imagine that the spring gets that hot under normal use.
 
Let's instead consider whether the governor shaft is somehow impeded or binding. Can you slow it down by giving it just a little nudge? Does it feel stuck?
 
You know that lots of newer snow blowers and mowers have no manual speed control? They're made to rev up to standard operating speed right away, and stay there. You can't slow them down.
 
As usual, permit me to ask for more information.
 
1) What was happening before you swapped carburetors? That is, why did you swap?
2) When the engine is just started, not yet warmed up, can you manually speed it up and slow it down?
3) As the engine warms, does it speed up by itself, or do you manually throttle it up, and then it refuses to go slower when you try to manually throttle it down?
4) When it's running fast, can you poke at the throttle shaft on the carburetor, thereby making it momentarily go faster or slower, and does it by itself return to the same speed as before you poked?
5) If you have a way to measure RPM, let us know what you mean by "full RPM".
 
I'm sorry I don't have a ready answer for you; I'm still trying to understand the symptoms.

If, as you say, it has no manual speed control, then how (see No. 2) is it that you say you are manually speeding it up and slowing it down?
 
In response to No. 3, you say that the RPMs don't go back to idle. The goal of a governor is not to increase or decrease the RPM; it is to provide a constant RPM. By way of example, assuming the desired RPM is 3600, as you load the engine, the RPM starts to drop to, let's say, 3200, so the governor opens the throttle to get back up to 3600. Similarly, if I stop the auger and thereby remove the load, the RPM will momentarily increase to, let's say, 3800, before the governor brings it back down to 3600. It does not try to idle the motor.
 
I suspect that you simply have the governor set too high, or maybe your ears are not used to what 3600 sounds like. Set it lower, if it makes you more comfortable, but try to find a way to measure the RPM. You can buy these cheap ($15) little RPM/hour meter devices. It's easy to install, and you don't have to permanently mount it. Just wrap the sensor wire around the spark plug wire, start the motor, read the RPM, set the governor to whatever you like, and remove the unit. You can find the recommended RPM in the snow blower manual. It's probably 3600, but I've seen lower recommended numbers (3200, for example) for companies that can't reach their emission target at 3600.
 
One more lesson, but I'm sure you've learned it already: Never throw away old parts, until you're sure the new parts are working as expected.