Briggs 257707 Hard to start / Keep running, Backfiring

Fireflite

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  • / Briggs 257707 Hard to start / Keep running, Backfiring
Hey guys, I have a Snapper LT14538DBV riding mower with a Briggs & Stratton 257707 engine. I'm having some bad issues trying to get the mower to start, and when it does start, keeping it running. Any help is greatly appreciated :smile:

First off, I have all the linkages to the carb set up right, but I can't move the throttle past midway, it will start to slow down and backfire for some reason. I have to start the mower with the throttle around this position, it takes a few tries but working with the throttle I can eventually get it sputtering into life and running strong, but eventually it will backfire and die out, and engaging the blades pretty much kills the engine and sends it backfiring. The mower won't start at all with the throttle all the way up/choke is on.

I can tell it must have been running very rich, the plug and carb is coated with carbon. I've tried adjusting the carb all day today and can't get it running reliably, and the carb doesn't explain at all what is up with the throttle.

This mower is really getting the best of me, is there a way I can reset the carb to factory specs? Any suggestions are really appreciated, It's been acting off for as long as I can remember and I just want to cut my grass!!! :laughing:
 

Fireflite

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  • / Briggs 257707 Hard to start / Keep running, Backfiring
Thank you for that!! ;)

It looks like I have the LMS style carb, with an idle speed screw and idle mix screw. Just to double check on page 86, I should remove my bowl and the plastic cover on my idle mix screw, turn the screw so it is flush with the seat, back it out 1 1/4 turns and put it back together. Start the engine, and adjust idle speed to reach 1750 RPM or more. I should be able to shift into higher speed no problem (fingers cross that throttle issue is fixed by this too but I'll be happy enough if I can just cut grass c:)
 
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Tinkerer200

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  • / Briggs 257707 Hard to start / Keep running, Backfiring
Sounds like crap in carb bowel.

Walt Conner
 

Telesis

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  • / Briggs 257707 Hard to start / Keep running, Backfiring
First off, I think you have an LMT, not an LMS carb. Refer to page 3-39 of the manual and you can see that the LMT is used on the 257700 Model Series. I agree that you should make sure the carb is clean. Pull the bowl off and then remove the 'emulsion tube'. Refer to page 3-42 of the manual and figure 104 shows what it looks like. It simply screws out of the center of carb bottom with a slotted screwdriver. It has several small orifices drilled in it so you need to make sure they are all clean. Dirt accumulates in the upper part it, and merely spraying carb cleaner up underneath the carb will not clean that out. Take it easy screwing it back in. Keep in mind that LMT carbs have a fixed main orifice jet (which is the emulsion tube in this case). The only thing you can adjust is the idle jet.

If cleaning the carb doesn't help solve the problem, it could have a partially sheared flywheel key that has the timing out of whack a bit. This too can cause hard starting and backfiring.

Good luck!
 
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