I looked at the ipl and the engine appears to be one of the earlier OHV's - perhaps a model 28xxxx. Most likely the carburetor needle valve is leaking or the float adjustment is allowing the fuel level too high - perhaps to the top of the venturi tube.

The best action is to fix or replace the carburetor. Be aware, the cheap carbs on the internet have a high failure rate. If you do not want to repair, add the shut off. During storage, mark the level of fuel in the fuel tank and compare the next time you use. Also check the oil level for fuel dilution.

After I did the maintenance last year I DID have gas in the oil tank! But I changed the oil and it didn't happen again, so I thought it was a fluke... guess not :-/

Can you suggest where to buy a good one? I usually use eReplacementParts.com, but I can't find the carburetor listed so I guess it's just too old. I don't have a local mower shop, so I'm usually limited to eReplacementParts or Amazon

If you can not find the part as a Murry part, determine who made the engine and ind the part by the engine manufacturer . My old Murry push mower had a Brigs and Straton engine. May be that your rider motor is Brigs and Straton also. There should be a plate on the engine with the manufacturer.

Once you find the maker of the engine search of their website.

May be that your rider motor is Brigs and Straton also

Yup it is, it's just 1,000 years old so parts are harder to find :-P

Update today, hoping you guys can suggest whether this new problem is also the carburetor or if I need to replace yet something else :-/

Today was the first chance I've had to mow since my last post... tons of personal problems in the last month, each day progressively worse than the last. So I wasn't at all surprised to have a new one today.

I was about 2/3 of the way down with mowing the back yard, with the throttle at about halfway. I had the speed level on 1 because the grass is high and just a little wet. It suddenly shuddered a little and had a flapping sound underneath, so I thought that it must have picked up a long weed or something in the blades (pretty common in my yard). I hit the brakes, then the engine started to die. Before I could disengage the blades, though, the engine picked back up and the flapping stopped. I didn't see anything shoot out, so I assumed that whatever it picked up must have been chopped in to little pieces. That whole thing was probably 2 seconds.

So I let it run for a few seconds with the brakes on, then start back to mowing. Within a few seconds, the throttle goes SUPER high! There aren't actual numbers on the throttle, but for the sake of discussion let's say that all the way down is 1, all the way up is 10, and Choke is 11... I had it on 6, but it felt like it was at 11! I pushed the throttle down to 1, and nothing changed.

At this point I thought the engine might blow, so I put the speed up to 3 so I could hurry and get it to the shed. But at that level the throttle dialed back a little, so I continued mowing at this higher speed.

When I was done I disengaged the blades, and then the engine throttle skyrocketed to 15! So I turned it off and pushed it out of the yard. It was obviously getting a LOT more gas than it was supposed to.

Looking underneath, I can see that when I move the throttle the spring still catches and moves the lever. I'm not sure what happens after that, though, it's kind of behind a piece of metal.

What do you think... still a problem with the carburetor, or am I know having a whole new problem?

If you can't tell based on this, can you suggest what I should look at to narrow it down?

After replacing the blades and everything last year, then the gas tank this year, and now the carburetor... I'm starting to wonder if I'm throwing good money after bad :-(