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Hello All.
My neighbor picked up a mower for $20, asked me to get it going. It’s a Troy Bilt with a Briggs and Stratton 550EX, 140cc motor. Motor model 09P7020144F1.
Engine made in 2019.
Initial issue was it would start but quit quickly. Found fuel was saturated with water. Drained, refilled, started up pretty well but had a miss and backfire.

Checked oil, was overfilled, probably a full quart in there. Drained oil, could tell some fine metallic in it. New oil.

Cleaned spark plug, had carbon.
Restarted, backfiring. Thought to add couple ounces of Marvel Mystery Oil, pulled oil fill slightly while running, when whoosh…pressure released, rpm’s came up, misfire/backfire stopped.

Not sure if I want to go any further. Is this related to the breather reed valve? Engine is pretty new. Had only been used for maybe 3 years. Is this a common issue? Any other causes for the crankcase pressure? It’s a pretty crappy, cheap mower that has many bad reviews online.

I suspect you do not have a crankcase pressure issue. We have a operator that broke the seal on the crankcase when the fill was loosened allowing air to be drawn into the crankcase when the large plunger ie the piston when up and then started expelling that introduced air on the down stroke. Small single and twin cylinder small engines are not car engines which you can remove the fill on while running to check operation for blowby. lawnmower engines will try to expel all the oil from the crankcase in just a few strokes if the fill is removed while running. And with single cylinders that piston moving up and down causes large swings in crankcase volume.

I suspect the real problem is a fuel/carb/spark plug problem.

ILENGINE said:

I suspect you do not have a crankcase pressure issue. We have a operator that broke the seal on the crankcase when the fill was loosened allowing air to be drawn into the crankcase when the large plunger ie the piston when up and then started expelling that introduced air on the down stroke. Small single and twin cylinder small engines are not car engines which you can remove the fill on while running to check operation for blowby. lawnmower engines will try to expel all the oil from the crankcase in just a few strokes if the fill is removed while running. And with single cylinders that piston moving up and down causes large swings in crankcase volume.

I suspect the real problem is a fuel/carb/spark plug problem.

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Thanks for the reply. I still suspect a pressurized crankcase, as as soon as I turned the fill cap, air was expelled and the engine rpms increased, and the miss/backfire disappeared. Retightened the cap/dipstick and the rpms dropped and misfire came back immediately.

Sounds like you go a very usual issue here. Reed valves don't tend to go bad. I would do a leak down test to see if there is a cylinder or head gasket problem. Could be bad rings or blown head gasket but this is one going take some investigation on your part. Assume nothing until it is ruled out.

fish56 said:

Hello All.
My neighbor picked up a mower for $20, asked me to get it going. It’s a Troy Bilt with a Briggs and Stratton 550EX, 140cc motor. Motor model 09P7020144F1.
Engine made in 2019.
Initial issue was it would start but quit quickly. Found fuel was saturated with water. Drained, refilled, started up pretty well but had a miss and backfire.

Checked oil, was overfilled, probably a full quart in there. Drained oil, could tell some fine metallic in it. New oil.

Cleaned spark plug, had carbon.
Restarted, backfiring. Thought to add couple ounces of Marvel Mystery Oil, pulled oil fill slightly while running, when whoosh…pressure released, rpm’s came up, misfire/backfire stopped.

Not sure if I want to go any further. Is this related to the breather reed valve? Engine is pretty new. Had only been used for maybe 3 years. Is this a common issue? Any other causes for the crankcase pressure? It’s a pretty crappy, cheap mower that has many bad reviews online.

Click to expand...

Excess oil initially, with metallic residue in oil is not good. As suggested, if available, do compression and leak down test, despite the engine only being 3-4 years old. Also, maybe pull valve cover and check valve clearance.

Tiger Small Engine said:

Excess oil initially, with metallic residue in oil is not good. As suggested, if available, do compression and leak down test, despite the engine only being 3-4 years old. Also, maybe pull valve cover and check valve clearance.

Click to expand...

Probably never had an initial oil dump. All the break-in metal still floating around. Is this one of those you never have to change the oil on?