I have a JD LA105 riding mower with a 19 HP engine (Briggs and Stratton? I think). I completed a full maintenance routine at the beginning of the season including oil and filter change, air cleaner change, fuel filter change, spark plug change, lube and belts, the works. It's been running fine. Yesterday, I add some fuel (recently purchased and burns in my other push mower just fine so I don't think it's stale) and get to mowing. It ran fine for about three minutes, then the governor started to idle down and back up and down again. When it ran slow, smoke came out the exhaust. I pulled the throttle with a finger and the engine revs up to full speed and sounds great again. When I let go, it cycled up and down in speed. I drove it to the garage, shut the engine off and now it doesn't start at all. It's still getting fuel to the carb, spark on the plug and seems to be getting air, but not starting. Do I need to pull and clean the carb or is there something else?

 

Yep, clean the carb. Something got in it.

 

I'll do that this afternoon although, when I had the spark plug out in my hand looking for a spark as I turned the key, fuel was coming out the open hole to the chamber where the plug should've been. I took that to mean the carb isn't plugged, but I'll check it anyway.

 

The needle is not seating,,,replace needle and clean seat.

 

what causes the needle to not seat? I only ask because it was running fine for three minutes or so before it started hunting (revs up and down)

 

Bet you pull that carburetor off and take it apart your going to find it full of funky looking stuff.
Caused from ethanol fuel.
Time to remove the whole thing, clean it with carburetor cleaner and install a rebuild kit.
You need to be adding an additive to the fuel or using non ethanol fuel.
Ethanol Fuel Additive - Compare Prices, Reviews and Buy at Nextag!

 

No ethanol here. I cleaned a carb at the beginning of this season on my much older push mower (10 years old). Although a jet was clogged, the carburetor looked to be in good shape, clean no gunk at all. That push mower doesn't have a fuel filter though.

 

It doesn't sound like a needle and seat problem to me, it just sounds like a bit of trash or something in one of the jets. Does applying partial choke help smooth it out?

 

Was totally surprised at the gunk in the carb. I just did the same thing to a 10 year old push mower and it wasn't in nearly as bad a shape as the carb on this newer riding mower. Removal, disassembly, cleaning and reassembly was easy though and it works like a champ now. Thanks to all who replied on this thread.

 

Great! Glad we could help!