Mower RPM - Governor or Carburator?

[bertsmobile1]

I will hazard a guess here that the actual governor mechanism has failed
What usually happens is the engine then over revs and the con rod snaps just below the little end from over reving.
So the broken con rod was not the cause, it was the symptom .
To understand the way things work consider it to be a tug-O-war between the governor spring and the governor.
When you open the throttle the spring stretches
When at rest the governor is fully open so the throttle opens fully
as the speed increases the governor pulls ( pushes if you like ) against the spring
The governed speed happens when the tension on the spring is balanced by the governor working against it
So the governor is always trying to make the engine go slower but the force it does this with is dependent upon the speed the engine is rotating at.
zero rotation = zero governing force
High rotation = strong governing force .

In practice a governed engine should run forever so when something internal breaks you always have to find out why it broke before you replace it .
Mower engines are made very cheaply so the con rods are not particularly strong
Most will let go at around 5000 rpm because the engine is designed to never exceed 3600 rpm ( 3200 for most Hondas )
A rookie mistake when using a mower engine in a go cart or mini bike was to remove the governor and use a direct throttle control and a broken rod is the usual result .
"it was running great, then it just went bang & stopped " is something I hear regularly from the racing mower mob

 

[sgkent]

I will hazard a guess here that the actual governor mechanism has failed
What usually happens is the engine then over revs and the con rod snaps just below the little end from over reving.
So the broken con rod was not the cause, it was the symptom .
To understand the way things work consider it to be a tug-O-war between the governor spring and the governor.
When you open the throttle the spring stretches
When at rest the governor is fully open so the throttle opens fully
as the speed increases the governor pulls ( pushes if you like ) against the spring
The governed speed happens when the tension on the spring is balanced by the governor working against it
So the governor is always trying to make the engine go slower but the force it does this with is dependent upon the speed the engine is rotating at.
zero rotation = zero governing force
High rotation = strong governing force .

In practice a governed engine should run forever so when something internal breaks you always have to find out why it broke before you replace it .
Mower engines are made very cheaply so the con rods are not particularly strong
Most will let go at around 5000 rpm because the engine is designed to never exceed 3600 rpm ( 3200 for most Hondas )
A rookie mistake when using a mower engine in a go cart or mini bike was to remove the governor and use a direct throttle control and a broken rod is the usual result .
"it was running great, then it just went bang & stopped " is something I hear regularly from the racing mower mob

makes sense. Fix the symptom and not the root cause is a common issue in repairing and restoring things.

 

[NJDan]

I will hazard a guess here that the actual governor mechanism has failed
What usually happens is the engine then over revs and the con rod snaps just below the little end from over reving.
So the broken con rod was not the cause, it was the symptom .

Well, in my case the snapped rod was caused by lack of oil.

To understand the way things work consider it to be a tug-O-war between the governor spring and the governor.
When you open the throttle the spring stretches
When at rest the governor is fully open so the throttle opens fully
as the speed increases the governor pulls ( pushes if you like ) against the spring
The governed speed happens when the tension on the spring is balanced by the governor working against it
So the governor is always trying to make the engine go slower but the force it does this with is dependent upon the speed the engine is rotating at.
zero rotation = zero governing force
High rotation = strong governing force .

Thanks. Part 11 pushes against part 12 to govern the speed. If the throttle is lowered part 12 will be in a slightly different place since it is connected to the governor arm but that probably doesn't matter. It's a little hard to visualize what effect that small change makes. The net is that wherever the throttle is, the governor will maintain that speed.

 

[bertsmobile1]

All done with levers
The actual governor slider would be lucky to move 1/4" which becomes near 3 " by the time it gets to the end of the governor arm outside the engine.

 

[NJDan]

I got the engine apart to see what's going on with the governor and here is what I found. One of the weights was sticking and so it appears that the governor was not able to provide the force needed to push the arm. I did two short videos. The thing is that I saw this before I closed it up. The weight was stuck and I unstuck it thinking it was OK. That's the price of inexperience. I don't know if it is me or Honda but in disassembling the mower I snapped one of the engine mounting bolts. The first time I took everything apart both exhaust muffler bolts snapped off and I had to get a friend to tap new threads.

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[NJDan]

Just to close out this thread -- I put a new governor in the engine and got it running again. The mower ran at the perfect RPM right off the bat! Lesson learned. If the governor weights are not moving freely all the time when you touch them then replace it! I do have another issue now but that's a different thread.