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I have a Briggs and Stratton 6.25 HP motor on a yard machines lawn mower. The motor will not go up to the operating RPM, It stays at a low rpm and bounces around a lot. I've already replaced the spark plug, clean and disassembled the carburetor, new fuel line, new air filter and screen, And I bought a bunch of other random parts such as new wheels, skirt, and blade.

Video of problem here:
I still cant figure out what's wrong, any ideas?

Your engine is surging
Surging is cause by a fuel : air ratio that is too lean.
So either the fuel supply is too low or there is air leaking between the carburettor & the intake valve.

A common problem with these Briggs engines is the manifold cracking or coming loose.

bertsmobile1 said:

Your engine is surging
Surging is cause by a fuel : air ratio that is too lean.
So either the fuel supply is too low or there is air leaking between the carburettor & the intake valve.

A common problem with these Briggs engines is the manifold cracking or coming loose.

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Are you referring to the manifold on the carburetor itself? I took it off the engine and cleaned everything out, but I didnt notice any obvious cracks. Do you think it's time for a new carb?

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.

There is a long plastic tube that takes the fuel from the carb to the opposite side of the head .
It passes under the magneto coil.
It is made from a thermo setting plastic so it will not soften from the engine heat but this material gets quite brittle with age.
There is a very small gasket under the 2 x 1/16" dia bolt that hold it in place.
The bolts are prone to coming loose and chewing up the gasket. Do not over tighten them

ILENGINE said:

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.

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X2
what ilengine said.

ILENGINE said:

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.

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The engine its surrounded by plastic covers that cover the carb and spring area, (removed when I took the video) and they are not damaged, so I think it's safe to say that the tab is not bent.

bertsmobile1 said:

There is a long plastic tube that takes the fuel from the carb to the opposite side of the head .
It passes under the magneto coil.
It is made from a thermo setting plastic so it will not soften from the engine heat but this material gets quite brittle with age.
There is a very small gasket under the 2 x 1/16" dia bolt that hold it in place.
The bolts are prone to coming loose and chewing up the gasket. Do not over tighten them

Click to expand...

I checked the bolts and they were still tight. Also, the tube doesnt draw air from the carb, but rather the air filter box, so I dont see how this could cause a bouncy idle.

ILENGINE said:

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.

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I tried to pull on the spring to readjust it, and I was able to increase the idle speed, but it was still bouncing around. Something else is off with this engine that wont allow it to idle smooth. Any other ideas?

Your video proves that it is an engine speed issue not a carb issue when the engine smoothed out when you increased and held the throttle on the carb. You are running around 1600 rpm on an engine that is designed to operate at 2800-3600 depending on blade length. Until the engine gets into that range it will hunt and surge. And you cannot properly diagnose anything else until that is corrected.

ILENGINE said:

And you cannot properly diagnose anything else until that is corrected.

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That's what I'm trying to figure out, how do I correct that low/surging idle?

Compare your spring tab location to this one I have attached. Wish the quality was better but you can see what I am looking at.

ILENGINE said:

Compare your spring tab location to this one I have attached.

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Looks about the same as mine. Mine had no damage, and no signs of bending. I bent it on my own trying to play with the adjustment, but I out it back to where it was originally.

I would take the carb off and give it a good cleaning.
make sure the holes in the bowl nut are clean etc.

You would want to bent in the direction that puts more tension on the governor spring to increase rpms.

Scrubcadet10 said:

I would take the carb off and give it a good cleaning. make sure the holes in the bowl nut are clean etc.

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Carb cleaning was one of the first things I did when I put on all these new parts.

AVB said:

You would want to bent in the direction that puts more tension on the governor spring to increase rpms.

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I did that by manually holding tension on the spring, and it did increase the RMPs, but the idle was still bouncing all over the place. I'm lost what to do.

So it ran fine at higher RPMs, but at lower (idle) RPMs it surged?
As Ilengine said these engines are made to run 2800-3600 rpm.
no need for a pushmower to idle.

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?

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?

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.

AVB said:

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?

Click to expand...

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.

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.

ILENGINE said:

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.

Click to expand...

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.

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.

MEDEL514 said:

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?

Click to expand...

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.

bertsmobile1 said:

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.

Click to expand...

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.

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.

Did you ever check the plastic intake manifold for cracks?

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.

Scrubcadet10 said:

Did you ever check the plastic intake manifold for cracks?

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Yes I checked the intake manifold and the tube that runs to the other side, no noticeable cracks and gaskets are all in good shape include the gaskets I just changed.

bertsmobile1 said:

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.

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No, the engine still surges if I place the spring tension higher. I'm not having a low idle issue per se, I'm having a idle stability issue. The low idle seems to be a secondary side effect.

Now I am confused.
Didn't we establish that you have a fixed speed engine with no throttle cable that has the rabbit & turtle on it .
If so it has no idle
It is supposed to run at high idle all the time.

bertsmobile1 said:

If so it has no idle
It is supposed to run at high idle all the time.

Click to expand...

By idle speed I was referring to the operating speed. I'm sorry for the confusion. It's still bouncing around.

Since the engine model and type number has never been posted that I can tell take the following with a grain of salt.

IF this engine has a float type carburetor using fuel bowl retaining bolt type main jet it may need enlarge one or more jet sizes. We are talking of less than a few hundredths of millimeter but before attempting this make all wax like residue is removed as have seen this to be the cause of the lean burn governor hunting.

An example of this resizing is going a #29 drill bit to a #28 can a lot of the time make a correction. The size used depends on the current size and up sizing is done step at a time and test as to not over richen the main jet size. Anyway this can over done and a new main jet would be required.

Since it is surging the govenor is working. That carb has no idle circuit. If the engine runs slow enough to not be on the high speed circuit it bounce on the govenor. You need to adjust the govenor spring anchor point so that the engine runs about 3400 RPM. Unless you change to a different carb it will not idle smoothly. I have seen where people have shoved the mower under stuff and hit the carb linkage and bend it and get the problem you have. That mower is made to just run full speed all the time.

AVB said:

Since the engine model and type number has never been posted that I can tell take the following with a grain of salt.

IF this engine has a float type carburetor using fuel bowl retaining bolt type main jet it may need enlarge one or more jet sizes. We are talking of less than a few hundredths of millimeter but before attempting this make all wax like residue is removed as have seen this to be the cause of the lean burn governor hunting.

An example of this resizing is going a #29 drill bit to a #28 can a lot of the time make a correction. The size used depends on the current size and up sizing is done step at a time and test as to not over richen the main jet size. Anyway this can over done and a new main jet would be required.

Click to expand...

it's a 12 series Quantum.

Sorry Scrubcadet, I have yet to learn the various series code names. I prefer to go by the actual model and type codes for looking up an engine's IPL.

This is especially true for me as I working across multiple brands, multiple OEM engines, and multiple equipment types. Basically if it is a machine I can work on it. Currently I got two Stihl trimmers, 1 Stihl backpack blower, 2 Husqvarna chainsaws, a walk behind mower, Yamaha ATV, and a Farmall Super M in the shop currently. Just sent out a Honda ATV, Polaris ATV, JD LT133 rider, and few other chainsaws. Then I got Hustler Super Z due in late next week for hydro work.

The biggest pain in the backside currently is the Yamaha since is has 50 psi compression where it should have a minimum of 150 psi. I got to pull the engine so I can disassemble the engine.

AVB said:

Sorry Scrubcadet, I have yet to learn the various series code names. I prefer to go by the actual model and type codes for looking up an engine's IPL.

Click to expand...

no harm in that at all

Im just wondering if there is an issue with the float and or needle. Might not be letting enough fuel into the bowl.
I had that problem once with the same mower, replaced the needle,seat and float and fixed it.

I ended up buying a cheap used mower for $25 I the neighborhood. This should get me through a couple seasons, but I still want to fix my mower. My dad bought this for me when I bought my first house, and I want to keep it going.