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I bought a zero turn mower with the 25 hp Briggs professional series engine (model 440000) that is 6 months old and has 8 hrs on the clock. After I changed the oil to 5w-30 synthetic, it started surging when it is not under load. Is this just a coincidence or could changing to synthetic oil require a carb adjustment? Since I bought the mower used, I don't have a warranty. Can someone help me eliminate this problem ?
Thanks!

Nothing to do with the oil.
Every thing to do with the fuel. it ain't getting enough.
SO it is off with the carb, careful clean of all the holes
reassembly, chack for air leaks etc etc etc.

I have an engine in that same series. I had the same problem, had to clean out the carburetor, and had to use sealant as my plastic intake manifold had warped. Also adjusted governor and throttle cable, magneto air gap and new plugs at the same time. I'm running full synthetic too, but my problems began because someone left the gas lid off and it rained. Mine has 350 hours on a yt 4500 riding mower

cryoburned said:

I have an engine in that same series. I had the same problem, had to clean out the carburetor, and had to use sealant as my plastic intake manifold had warped. Also adjusted governor and throttle cable, magneto air gap and new plugs at the same time. I'm running full synthetic too, but my problems began because someone left the gas lid off and it rained. Mine has 350 hours on a yt 4500 riding mower

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Is there a resource for making governor and throttle cable etc. adjustments?
I would like to try the simplest fixes first since it has such a low hours and it's been well cared for so I can't believe the carburetor is dirty.
My first thought is that motor now is broken in and Carb/linkage etc. may need to be re-adjusted.

Is it still under warranty?

You can take it back but a clogged carb is not covered under warrantee.
Left sitting with the carb in strong sunlight brand new never used carb can clog.
It has very little to do with you and everything to do with a slop that gets served up as fuel now days.

Heat drives off the thin bits of the fuel which then becomes a thick goop rather like pouring honey and in this state it will not redisolve back into clean fuel.
It sits on the bottom of the carb / fuel tank while the fresh gas sit on top running your engine.
Eventually it build up high enough to partially block the fule flow so the engine surges.
If left for long enough this crud becomes thicker eventually going like 1/2 dry varnish when it blocks off all the fuel holes.
Its presence acts as a catalyst promoting fuel to decompose even faster and any water will also separate and start corrosion.

I do about 40 Honda engined fire pumps a year and some of these have blocked carbs before they were sent to the fire trailers because they were tested when assembled and the fuel was not drained out after and the carb was not run dry.

Boba21 said:

Is there a resource for making governor and throttle cable etc. adjustments?
I would like to try the simplest fixes first since it has such a low hours and it's been well cared for so I can't believe the carburetor is dirty.
My first thought is that motor now is broken in and Carb/linkage etc. may need to be re-adjusted.

Click to expand...

I used youtube videos as a guide for the adjustments. Although I'm not sure your issues are caused by a misadjustment, you can try them first as they require little effort to complete and practically no disassembly.

Governor adjustment (My engine has full throttle all the to the right, so I adjusted the governor shaft to the right, with the arm all the way to the right and tightened to pinch bolt):
https://www.youtube.com/watch?v=9rO8AvDrdeM

Throttle cable adjustment: (My engine used a torx head bolt on the retainer. I set the throttle lever on the mower to full, and adjusted the cable to full open on the carb, but with the choke disengaged)
https://www.youtube.com/watch?v=1WWpeyDbHjA

Your issue sounds like an inconsistent fuel caused by sticking or debris/desposits restricting fuel/air flow in the carburetor. Even if the throttle cable was misadjusted, it should really only cause the carb to not be able fully or close enough for a lower idle, depending, but it should run smoothly no matter what rpm it is at.