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31p977
0575 e1

Hi,
The motor runs normally for awhile. Then drops to a lower speed. After a minute it kicks up into high again. It also has a surge when the blades aren't engaged.
Sometimes the blades will engage ok, and sometimes the mower has to be moving or the motor will lug down and die.
It originally had a nikki/ briggs carburetor and it ran badly.
I stuck a walbro on and it does better.
Any suggestions? Thanks

buy a horse and some goats?

ba_50 said:

31p977
0575 e1

Hi,
The motor runs normally for awhile. Then drops to a lower speed. After a minute it kicks up into high again. It also has a surge when the blades aren't engaged.
Sometimes the blades will engage ok, and sometimes the mower has to be moving or the motor will lug down and die.
It originally had a nikki/ briggs carburetor and it ran badly.
I stuck a walbro on and it does better.
Any suggestions? Thanks

Click to expand...

Start with the easy
Adjust the valve lash and check the valves are opening & closing an equal amount
They are both the same so should move the same amount & sit at the same height
Check the timing key & check the top bolt was tight
Behind the carb, ensure the choke completely opens and that there is nothing to snag the governor arm or throttle linkage .
Remove the fuel tank wash it out & blow it dry
Replace the fuel lines all the way

It would also be worth cleaning the carb out. Have a good look at the float and the jet in the carb, it could be a blockage in there or something restricting fuel flow.

Couple goodies from your engine manual.

ba_50 said:

31p977
0575 e1

Hi,
The motor runs normally for awhile. Then drops to a lower speed. After a minute it kicks up into high again. It also has a surge when the blades aren't engaged.
Sometimes the blades will engage ok, and sometimes the mower has to be moving or the motor will lug down and die.
It originally had a nikki/ briggs carburetor and it ran badly.
I stuck a walbro on and it does better.
Any suggestions? Thanks

Click to expand...

Dump fuel tank and blow it out like a new one. Same with fuel lines. If you have a pump, verify it pumps into a glass jar AT the carb. Try the ol' trick of cracking open the gas cap for a bit. See if it's not venting the tank.

Install The OEM Briggs carb just as the engine came new.

It looks like we forgot an important piece of the puzzle, a dirty air filter.
I took it out and the surge seemed to disappear.
The things you mentioned, carb clean, lash, sticking linkages, new gas lines, oil and filter change, and clean gas tank were already addressed.
Not sure if that caused the motor to die when the blades were engaged.

Will see.
Thought about goats but still need watching.

Wildflowers and red clover are a possibility though.

The air filter helped but not completely.
Now the govenor rocks back and forth just above low idle and in mid range with the blades disengaged. It runs normally with the blades engaged.
The throttle shaft might stick once in awhile. It has some play in it.

These engines tends to do this on high idle when unloaded as they are jetted too lean to meet EPA specs. Sometimes all you got to do is to increase the idle speed stop a little.

Thank you.

StarTech said:

These engines tends to do this on high idle when unloaded as they are jetted too lean to meet EPA specs. Sometimes all you got to do is to increase the idle speed stop a little.

Click to expand...

If it surges just off idle with no load then the low speed circuit in the carb is running too lean. Wither dirty or jetted wrong. Very common on aftermarket carbs or carb not specifically designed for the engine.