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18.5 b&s twin on craftsman popping exhaust

3255 Views 4 Replies 3 Participants Last post by  Brad
i have a 11 year old lt1000 craftsman with a 18.5 twin- maintence has been kept up- this season did fresh plugs, cleaned filters, oil change ,battery etc. last summer had carb cleaned, new float needle and #1 cyl gasket replaced. running great till last cutting- mid way threw using it developed a popping noise like a backfire from exhaust. I let it cool and next day doing the same thing immediatly. I pulled & checked plug gap, pulled air& fuel filters and even went as far as draining tank and refilling with fresh fuel & seafoam. the popping from exhaust is still their and you can feel it at the mufler when it pops- both headers are hot . i let it run a few min to circulate the new gas and drove it around for 5 min hoping it would clear. My property is clean with the ocasional hit of a pinecone. it fires right up, no diseling after shut off, no smoking noticed, oil level is good and looks clean .

Any ideas before i talk to the mower shop- if it turns into a costly job it may endup getting replaced with a newer rider.
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Hello, Llang60979, and welcome to the forum.

Well, unburnt fuel must be going into the muffler and getting ignited in there from some source. I might suggest trying new plugs again. possibly a bad on that's not burning the fuel. One more thought, check your oil level. If the carb float is sticking and allowing too much fuel into the cylinders, you might notice a rise in the oil level from some excess gas seeping into there when the mower isn't running.

Some of the other folks on here might have some more in-depth thoughts. I would have guessed valve clearance had this developed over a period of time, but since it was immediate, it appears that something "broke" all of a sudden.
My unprofessional opinion would say the exhaust valve is sticking open and when the piston on that side fires, it blows back thru the exhaust. Keep in mind-unprofessional.
actually dropped it off to the shop this morning- its only 2 blocks away so i drove it over. The light popping got worse and giving a heavy backfire and some surging at all speeds and throttle . It was loud enought that they heard me coming down their road. Time will tell if its minor its getting fixed if its costly the mower will be parted out and i will be looking for a new rider on craigslist
Please let us know how this turns out. That way, we'll know if we were leading you in the right direction.

Thanks and good luck.
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