What closes the throttle on a Honda GX120?

Question

I've a problem with a GX120 in that I start it and it just revs up to maximum rpm and stays that way. The throttle butterfly is linked to the governor arm via the usual arrangement (weak spring plus rigid control rod) and the whole assembly moves freely. The governor arm is also linked to the throttle lever/plate assembly via a spring that is only just mildly tensioned when the throttle butterfly is closed. In its resting state there's a good amount of play in that spring and it's not the case that the spring is too short and continuously pulling the throttle open. The throttle lever isn't jammed open; the whole assembly is new and the return spring works to close the throttle lever position back to normal/idle position

While the engine is running, if I grab the governor arm and move it myself then the engine behaves normally. I can push it all the way to the kill switch end of the engine and rev the engine down to a slightly too fast idle, I can move it to the other side and rev the engine up but what I can't work out is supposed to shut the throttle down/move the governor arm towards idle.

If I let it go then the assembly moves to about 85% WOT and stays there, revving the engine high.

If this engine behaved normally, what would make the governor arm move towards tick-over position? I'm trying to work out if there is something faulty with the governor inside the crankcase - it seems to move freely enough on the outside; if I remove the arm I can twiddle the governor shaft through about 90 degrees with a hard stop at each end with fingers - no idea if any of this is normal

Answer 1

Summary

The answer is, essentially, that the governor closes the throttle. The faster the engine goes, the more the governor tries to close the throttle. A governed engine is relatively throttle-open when the engine is off. As soon as the engine is started and spinning under its own power the governor internals are being spun too. A governed engine operates on the concept of a fight between the throttle cable trying to open the throttle and the governor trying to close the throttle. When the engine is placed under load, it slows down, giving the governor less ability to win the fight, opening the throttle more. This counteracts the load by applying more throttle, restoring the balance

Detail

Governors are typically a pair of weights that pull on a shaft, that presses a lever (we'll call this the governor lever). The faster the engine spins the more centripetal (centrifugal) force the weight experience, the harder they press on the governor lever.

The governor lever has a rigid rod connected to the throttle butterfly valve on the carburettor. This rod also has a weak spring connected with it, to reduce the risk of the butterfly valve chattering/vibrating.

The action of the governor lever, moving under the influence of the governor weights, is to close the throttle butterfly. This means, when well adjusted, the mere running of the engine is pushing on the rod force the throttle closed. To counter this, and to give you control of the engine speed via a lever that behaves a bit like a throttle, there is another lever (we'll call the throttle lever) connected to the throttle cable/speed control via a spring (we'll call the throttle spring).

When you set the throttle lever to some partly-open configuration the throttle spring pulls on the governor lever, pulling on it in such a way that it's trying to pull the throttle open. When combined with the fact that a running engine has a governor trying to push the throttle closed, we'll reach a point where the two mechanisms are balanced. The throttle/speed control tries to speed the engine up/open the throttle valve. The governor tries to push it closed. The faster it goes, the harder the governor pushes. By contrast the throttle spring pulls the same. This mean in this fight between the two systems, a stable state can be reached where the governor is pushing closed with X force and the throttle spring is pulling open with X force

In this state the throttle opening is constant, as is the engine speed. If some extra load is introduced to the engine, the engine slows down. Because the engine slows down, the governor exerts less force, and the throttle spring wins, pulling the throttle valve open more. This in turn speeds the engine up to it can reach the same steady state, it was in before. It's using more throttle to achieve the same speed because it has the added drag of overcoming the load too.

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For an engine that just starts and revs up to max, it could be that the governor mechanism is not working. Though the throttle moves freely and can be pushed closed manually, the governor doesn't seem to be doing its normal job of pushing the throttle closed