Engine Honda GCV135 engine: throttle valve open in idle position when not running?

fourstroke

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Hi all, this is my first post here. Very happy to discover such a treasure trove of expertise! :smile:

My trusty 20-year-old Honda HRB475C mower with GCV135 engine has been idling very poorly for some time. It was time for a new spark plug and air filter and I decided to have a go at cleaning the carburetor as well. It turned out to be much easier than I expected thanks to the excellent guides; the hardest part was getting it off in the first place. Curse those hose clamps!

As I was reinstalling the carb, I noticed something odd: when the throttle handle is set to idle, the throttle's butterfly valve remains open! I'd expect that to be closed, only progressively opening when I push the throttle handle forward. This retracts the throttle cable, rotating the governor assembly clockwise (viewed from the left), which pulls on the bottom of the rear governor arm via the spring, tilting the top of that arm backwards, which pulls the carb's throttle arm clockwise (viewed from top), opening up the butterfly valve.

However, that rear governor arm already assumes the "bottom forward, top backward" position by itself at rest. The spring going to the governor can just slide up in the spring's slot on the governor assembly. Nothing is broken, jammed or misaligned, it just settles naturally by gravity and/or the small throttle damping spring. I only undid the linkages at the carb's throttle and choke plate, the rest of the mechanism including the springs was not disturbed.

Is this behavior supposed to happen? I didn't pay particular attention to it before. Perhaps the engine vacuum flips the throttle closed but that seems like an odd design choice.
 

ILENGINE

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It is not unusual for the throttle to be partially to fully open when the engine is not running. When started the governor should then move the throttle to idle unless you increase the throttle which in turn pulls more on the governor spring to increase the engine speed.
 

bertsmobile1

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It is not unusual for the throttle to be partially to fully open when the engine is not running. When started the governor should then move the throttle to idle unless you increase the throttle which in turn pulls more on the governor spring to increase the engine speed.
People think that the throttle control moves the throttle valve.
This is the case with your car but not with a governed engine
What it actually does is limit how far the governor can close down the throttle
 
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fourstroke

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Ah ha, suddenly it all made sense! I assumed that the term "governor" just referred to the entire linkage-with-springs assembly connected to the throttle cable. I thought that it was governing the throttle (as in a car) and that the rear arm was simply pivoting like a see-saw. But now I realize there is more to it than meets the eye; that arm actually governs the engine speed via a centrifugal weights assembly hidden inside the crankcase, controlling the carb valve at one end and being limited by the throttle lever setting at the other end. So it's definitely not a simple pivot after all, hence the name "governor arm". Learned something new today; that's a clever system!
 
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