Well, this one may be pretty nontechnical.
This sounds like a clogged fuel tank cap vent hole, with a full tank of gas.
The tank being full can make the problem appear more rapidly than if the tank was half full when you started the engine.
Basically, if the tank cap vent hole is clogged, air can't enter te tank as fuel is depleted, so a partial vacuum develops in the tank and fuel can't leave the tank any longer.
You can test this easily enough by simply removing the cap and see if the surging and dying stops.
If running performance improves with that, clean out the vent holes on both inside and outside of cap by probing with a stiff wire (think paperclip unfolded).

Thanks for your quick reply, mownie! Good to know info.

Here is some of the recent history of our LT180:

A few weeks ago, while we were getting it ready to cut for the first time this season, we noticed that the seal was shot.on the gas cap. We also noticed that the fuel pump was starting to leak. So we decided to replace both of them. (But I will double-check the new gas cap tomorrow.)

It ran fine for that first mowing. But yesterday, for the second mowing, after it warmed up, I cut for about 60 feet, and it started revving like I mentioned in the first post.

Today I did some more reading on here and decided to spray in some carburetor cleaner to see if that would help, but no change. Then I read somewhere that I should have opened the throttle before spraying the carb cleaner. *slapping forehead*

So you see, I really am a rookie here.

Thanks again.
- Mike

Any chance part of the bad gas cap seal wound up in the gas tank? That could be partially blocking the fuel line.

Carb cleaner does little good unless mixed IN the gas so it actually goes through the potentially clogged passages. otherwise, it just cleans off the external grime etc.

Tomorrow was yesterday. Did you try loosening the gas cap yet?
The fact that that is something that has changed from before is a red flag.

To add to mownies advice-
IF this engine doesn't have a fuel pump, a partially restricted fuel filter could cause similar symptoms.
As the fuel level in the tank decreases, the "pressure head" through the filter also decreases. You may have a fuel filter in the transition stage of good to clogged.

Well, I am so accustomed to reading posts where everything has already been changed by the time a thread appears here that I guess I must have imagined you had already changed the fuel filter.
Probably be best to get a new filter on there, at least to rule out that as a cause.
Good save Bill

Thanks for the replies txtom50, bill_kapaun, and mownie.

I haven't had a chance to work on it again yet, but I plan on going back over there later today, and will take your suggestions with me.

We replaced the fuel filter last summer. But since the old gas cap was a mess, I probably need to look at replacing it again, and also make sure the fuel tank and lines are all clear.

Thanks again.

Today I replaced the fuel filter and checked to make sure the vent was working on the new gas cap. Then I sprayed a good amount of carb cleaner into the carburetor (with the throttle open this time).

When I started it, I saw that the fuel filter filled with gas pretty quickly. And I was able to let the choke lever back in without it dying on me. (Alright!) So, I let it warm up and idle for a few minutes. Next I turned it back off, and sprayed the carb again, thinking it might help, and probably couldn't hurt. Then I restarted it and took it for a test spin.

I eased out onto the road and opened the throttle. Got her cruising at top-speed, and she was humming along just fine. I went about 100 yards or so, turned it around, and started heading back. I got about 15 yards from the driveway, and it started revving up-and-down again. So I backed-off the throttle and it died. (Crap!) After restarting it, I had to ride the choke a little bit to get it back to the house.

So what do you guys think I should be looking at next?

The surging is a classic symptom of fuel starvation. Whether it be from a clogged filter or a clogged tank vent.
Since you have eliminated those 2 items, it is time to get the fuel tank off for a thorough clean out.
I also recommend replacing all the fuel hoses from tank to carb inlet.
When you clean the tank out, try to dump the gas into a strainer (old cloth or paper towel) so you can examine what comes out of the tank.
If you don't find anything incriminating in this process, the next step I would take would be to remove the flywheel sheet metal cover and look to see if mice have packed the cooling fins with their nesting and food cache materials.
An overheating engine can sometimes mimic a fuel starvation issue.

Whilst you are servicing the tank- look real close at the formed pick up line for cracks

I stopped by to remove the fuel tank today, but decided I'd better ask first. Do I have to remove the seat & back frame cover section (fenders)? What's the best way to go about it? I don't want to remove anything that isn't necessary to get to the tank.
*edit* - I did a Google search but couldn't find anything that looked like it would apply to replacing the LT180 tank.

While I was there, I removed the float bowl and found some light-pale-green pulpy goop in it. And now that I'm looking at the drawing, I don't remember seeing a gasket (#23) in there. I'll have to check that again.

Also, should I get JD fuel lines, or buy a length and cut it myself? I saw that Tractor Supply sells a 24" piece for $5.

This post was edited by Mike.D on Wed, May 8, 13 at 9:11

*update* - Drained the fuel into a bucket, straining it through an old t-shirt. I was surprised that I did not find anything other than a speck or two that would have been handled by the filter.

I also removed the fuel tank, which was easier than I expected on the LT180. Other models require lifting the fender deck, but not this one. You only have to remove the two bolts at the bottom of the tank, and the bracket (pin fastener) under the top-left section of the tank.

I checked the fuel pickup tube (thanks, tomplum) for cracks. The tube and the rubber bushing look to be in good shape.

As txtom50 and mownie suspected, there were fragments of the seal from the old gas cap inside the bottom of the tank. I will be cleaning the tank and removing the fuel lines, on my next visit, hopefully this afternoon.

Next, I'm going to remove the flywheel cover and make sure everything's clear in there, with no surprises.

Making some progress. Thanks, guys!

I'm thinking you might have had enough "soup" in the tank to clog the new filter???

Yeah, think "vegetable soup", not clear broth :^)

Got all of the old gas cap seal out of the tank.

Nothing going on under the flywheel cover. All clear.

The fuel lines are just starting to crack, so I will be getting enough to replace them all.

I double-checked and found no gasket in the float bowl (maybe that's what that pulpy stuff was), so I will be getting one of those, too.

Could the float bowl gasket cause this revving problem?

I think that highly unlikely (in regard to the gasket causing a surging issue).
Where I was leaning (and maybe Bill too) regarding piece of anything inside the tank is that these things can get sucked against the pick up tube and act like a "flapper valve".
The only problem in this case is that your surging seems to come on after only a short run time. A flapper situation is almost a "random act" in that it must drift past the pick up tube at just time right time for it to get sucked into place............that simply will not happen like clockwork.
Knowing that this is a new fuel pump, I have to ask: Did you replace the pulse vacuum hose that operates the pump?
If that hose has a crack or fissure preventing the pump from getting a full stroke with each alternating pulse from the crankcase, the darn float bowl may be running out of fuel when fuel demand is high (wide open throttle).

mownie: (fwiw) When I was cleaning the fuel tank, I found two separate pieces of debris from the old gas cap seal that were probably large enough to block the pickup tube. Hoping that was at least some of the problem.

Yesterday I got enough fuel line to replace all of the hoses. I also got a gasket for the float bowl. After installing those, and re-installing the fuel tank, I had to leave it there.

Next I will re-install the flywheel cover and make sure everything is tightened down. Then I'll put some gas in it, and see what happens.

Roger Roger, you have clearance, Clarence.

I had a loss of fuel to the engine (23 HP Briggs twin) during my first mow this season. It's a 2007 JD 425 ZTR with only 88 hours on the clock. I had just completed a lot of maintenence on it this Spring including changing all the filters and engine oil, greasing the deck pulleys and installing new blades. i mowed for about 20 minutes and it started losing power and finally died. I loosened the gas fill cap and it just flat died again (weird I thought). Looked at the fuel pickup tube and noticed it was contacting the bottom of the fuel tank. So, I pulled the tube up a little, the motor started fine, and I finished mowing the lawn with no problems. The pickup tube is definitely something to look at. Not sure why I suddenly had this problem after 88 hours of trouble free operation..

Hey, guys! The LT180's engine is humming again. Thanks for all the help.

Now I need to figure out why the mower deck has started vibrating and making racket.

Take the belt off that runs the blade pulleys and rotate/spin each blade by hand (you may have to defeat a blade brake if) while you feel by hand, an area of metal near where the blade spindle mounts in the deck. If you feel any "rumbling or grinding" type of sensations/noises, the bearings in that spindle are "going down".
You can also move a blade tip up and down and observe how much movement (free play) is present. If you can detect 3/16" or more free play, the bearings are wearing out.

Same problem. Local dealer wants to rebuild the carb. Says it will take $350 in labor, and another $100 in rebuild carb parts. Said a new carb for this model LT180, 17 kaw, would be $600 for the new carb. Checked with John Deere, the new Carb only cost $200. Be careful of ripoff artists. I will be pursuing the matter. Be Aware.