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I have an old lawn tractor that I just got. I think its a 1986 model. I got it all cleaned up, including the carb and air filter. Maybe I didn;t clean the carb well enough. The tractor will only run with the choke on and it backfires A LOT. When I turn the choke off the mower goes off. I think the backfiring problem is a timing problem. Anyone know whats wrong.

Mower info: Craftsman 917 255736
Engine info: craftsman B&S model: 252707, type: 0621-03, code: 36040111

crazyoldtractor said:

I have an old lawn tractor that I just got. I think its a 1986 model. I got it all cleaned up, including the carb and air filter. Maybe I didn;t clean the carb well enough. The tractor will only run with the choke on and it backfires A LOT. When I turn the choke off the mower goes off. I think the backfiring problem is a timing problem. Anyone know whats wrong.

Mower info: Craftsman 917 255736
Engine info: craftsman B&S model: 252707, type: 0621-03, code: 36040111

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I just had this problem with my old mower and it was indeed the carb. If your flow of fuel to the carb is good I would go there again.

You might try adding some Seafoam to the gas and perhaps it will clean the carb.

Sounds like it is running too lean, as when the choke is advanced, it lets less air in, and thus needs less fuel.

I think you should give your carburettor another clean and check the float is working properly.

Also maybe try changing the spark plug.

this is common....its fuel starvation.....you will need to completely go thru the carb....something is blocking the fuel..could be a bleed hole stopping pressure equilization too...take it apart and take pictures for us....ive just broke my own record and rebuilt 17 carbs this last week...so im in the carb mood:biggrin:......i know this carb well...its a 491 series...this one is great in the fact you can completely remove the emulsion tube assy.....the main jet has a needle at the bottom of the bowl and you can back it out and see if that helps right there.....thers also an idle screw...you want to get it to run good at high speed with the main jet and idle low with the idle screw on the side...is there any corrosion?..... http://outdoorpowerinfo.com/repairs/briggs_large_one-piece_flo-jet_carb.asp

I fixed the choke problem with the screw on the bottom of the bowl. But the thing still backfires a lot when its running. I think its the timing, like the shear key or something, but I don't know how to see if that is the problem.

youll need to pull the top shroud off to expose the flywheel....... Parts and Diagrams for Briggs and Stratton 252707-0621-03