Briggs & Strat wont idle stable, rough/low idle

[Scrubcadet10]

That's the governor arm,
What is a Governor System?
The governor system is like a cruise control system in an automobile. It maintains the speed of your lawn mower or outdoor power products. When Briggs & Stratton governors are adjusted properly, they keep your speed steady regardless of engine load (the amount of work the engine must perform).
When powering a lawn mower, engine load can be affected by hills or height of grass. For a tiller engine, load may depend on depth of the tines where as a chipper’s load may be affected by the thickness of branches.
Without a governor, you would need to adjust the throttle manually each time your lawn mower ran across a dense patch of grass or the engine will stall. A governor does the job for you by detecting changes in the load and adjusting the throttle to compensate.
Your small engine contains either a mechanical governor, pneumatic governor or an electronic governor. The main difference between the three are how they detect speed.
How Mechanical Governors Work
A mechanical governor uses flyweights to create a force based off of crankshaft speed which is balanced by the force of the governor spring. The top engine speed is varied by increasing the spring force to run faster or decreasing the force to run slower. The governor spring wants to open the throttle and the governor tries to close the throttle. The interaction of the governor spring and mechanical governor holds the throttle at the desired engine rpms based upon a force balance and the load / operating conditions.

Its probably not a governor issue, try running it with the air filter off and see if that changes anything.

 

[Scrubcadet10]

What is the model and type number of this engine so I can look-up its IPL to see if it was ever intended to idle. But as noted does it surge at the higher rpms?

If it was intended to idle and the surging is only at idle it require a jet resizing as most of these later carburetors are non adjustable.

Also with it being a 6.25 hp I assume it is a OHV engine so have you checked the valve clearances?

In his video above its a Model 12xxxx flathead quantum.

I did have problem very similar to this, I tried cleaning it, checked the intake for cracks etc.I finally replaced the carb with a new OEM carb and it runs fine now.

 

[AVB]

Then without knowing the exact model and type number we are just guessing if this engine has a mechanical or air vane governor; although, it is likely it has a mechanical governor that may just need a static adjustment done.

And yes the black throttle shaft is control the overall speed by control the air/fuel flow into the engine. As I said if the governor is setup correctly and the engine still surges then it is likely that main jet is slightly lean and needs a minor resizing to correct the surging. But since it can run smooth with you holding the throttle shaft in one position it more likely that the static governor adjustment is off.

Now if you have the mechanical governor you loosen the clamp screw at the bell crank, move the throttle vane to full open while watching the direction the governor arm moves. Now while holding the throttle at full rotate the bell crank in the same direction until it stops and tighten the clamp screw.

 

[bertsmobile1]

I don't think is is as much of a carb issue is it is an engine speed issue. The tab with the hole in it closest to the front of the engine that the governor spring is connected to has most likely been bent from mowing around bushes etc and needs to be pulled back toward the front spark plug end of the engine until you reach the correct engine speed. This is better to be done with a tach to make sure you don't over rev past the safe blade turning speed which is lower than the normal 3600 rpm of the engine.

Since this engine is not set up with a throttle cable the carb most likely doesn't contain an idle circuit because it doesn't need one to run full throttle only. But if the engine is slowed down below a certain point without the idle circuit will cause a surging effect.

Rereaading this post .
Thanks for posting this.
Was not aware of fixed speed carbs not having an ide circuit.
We don't get them down here apart from the Chonda powered supermarket cheapies so I not come across this to date but am now aware of single circuit carbs.

 

[bertsmobile1]

And yes it sounds like the governor arm may have slipped on the governor shaft.
When the mower is running, the governor tries to SLOW the engine by CLOSING the throttle butterfly.
This is counterbalanced by the tension on the spring that you were pulling forward.
Thus it needs to be set as previously mentioned.

 

[ILENGINE]

I'm sorry to confuse, but you are correct, it doesnt have an idle. The normal operation RPM is not being reached because the RPMs are bouncing and keeping the motor running at a lower RPM than normal. When I tried to pull on the spring to get it into the normal RPM range, it still surged and would not smoothen out.

The only time I can get it to run smooth is when I hold steady the black plastic carb valve that the spring attaches to. That black plastic valve has an rod that connects to another arm in the back of the engine. I'm guessing that this is what's supposed to open/close the carb to a stable idle. What controls this arm, and how is it calibrated?

NO The engine speed is controlled by the governor spring. The governor spring speeds up the engine and the governor arm slows down the engine. unless there is enough tension on the spring to put the operating speed in the normal operating range for that engine it will surge.

You sped the engine up one and then decided to slow it back down. Why. Unless that engine is running at 2800-3600 rpm depending on blade length it will surge because there is no idle jet in the carb to draw fuel through at the low speed.

 

[ILENGINE]

Rereaading this post .
Thanks for posting this.
Was not aware of fixed speed carbs not having an ide circuit.
We don't get them down here apart from the Chonda powered supermarket cheapies so I not come across this to date but am now aware of single circuit carbs.

Started around 2005 with the first tier emissions ratings on small engines. The first engines or at least the briggs that use to have a throttle cable bent a tab down to hold the what was a movable throttle to make it a fixed throttle, and people would straighten out the tab and then try to install a throttle cable and they would run like crap at slower speeds that people tried to run them at.

 

[MEDEL514]

Happy new year everyone! I'm still stuck with a inop mower, and spring time is just around the corner. Any ideas? I'm about to seek out a local repair shop since I'm at a loss.

 

[Scrubcadet10]

Did you ever check the plastic intake manifold for cracks?

 

[bertsmobile1]

If this is a single speed mower and it runs fine if you stretch the spring, then bend the anchor point to stretch the spring.