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Craftsman DYT4000
917.273643
Briggs & Stratton 18.5hp Intek (single cylinder)
Engine Family: YBSXS.5012VP 274826

This mower came included with a house I purchased ~2 years ago and I've had to tinker with it all along the way. The latest issue is that it just seems to struggle to make power. That seemed to happen after mowing once with some out-of-balance blades, though I don't know if that would truly cause lasting engine damage, I feel I should note it anyway.

This weekend I've thrown a few parts at it:
Genuine Briggs carb
Intake breather hose (noticed the edge going into the case was wrinkled and out of shape, no longer sealing)
Briggs vacuum fuel pump

I also took a look at the valves and tightened up the clearances a touch. Oil level was right on the full mark. I did check compression in the fall, the tester read about 160psi.

Now once started, the revs only rise with the smallest opening of the throttle plate, and beyond that it just hits a wall and becomes a sputtering mess, unable to gain any rpm. With the engine cover removed I can see either exhaust or fuel vapor pulsing out of the intake manifold.

Let me know if you have any ideas of what could be wrong or if there's anything I haven't provided.

Thanks!

remove the valve cover and the spark plug, turn the engine over by hand while observing the opening and closing of the valves. Make sure they are both opening the same distance. Acts like a worn camshaft lobe preventing one of the valves from opening properly.

Looks like the intake valve seat has come loose , intake vale is stuck & not closing properly , head gasket is leaking

Remove the breather tube from the back of the air filter and repeat what yo did in the video
If it stops blowing excessively out of the carb then the head gasket is gone and you can double check this buy pulling the dip stick after 5 minutes of running.
With the engine off there should be nothing coming out of the dip stick tube , white smoke confirms leaking head gasket.

I do not think that is the problem but it is the easiest one to rule out

To check the inlet valve remove the spark plug and carb manifold
rotate the piston to TDC compression stroke. both valves closed then down another 1"
Using a pencit type torch shine a strong beam down the inlet hole
If you see any light whatsoever in the cylinder then the inlet valve is open when it should be closed .
If this happens double check you set the valve lash correctly on the compression stroke & not the exhaust stroke ( we have all done this at least once )

Requiring a bit more skill , you can measure the height of the valves .
Both valve guides and the valves themselves are the same length and the cams have the same lift
So the valves should sit at the same height when closed .

Now jus one point.
I am assuming that you took the blower housing off for the purpose of testing the engine and making the video and have not been running the mower like that .

Thanks guys, I'll see what I can check tomorrow.

A friend suggested maybe some issue with the exhaust, clogged/blockage etc, so I did pull the exhaust off this evening and it sounds like it's full of loose change. I could blow compressed air through it okay, though I guess it's possible shaking it could have moved the debris around and opened a blockage.

Biggs said:

Thanks guys, I'll see what I can check tomorrow.

A friend suggested maybe some issue with the exhaust, clogged/blockage etc, so I did pull the exhaust off this evening and it sounds like it's full of loose change. I could blow compressed air through it okay, though I guess it's possible shaking it could have moved the debris around and opened a blockage.

Click to expand...

"Sounds" like a baffle has come loose inside the muffler.