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Engine Model: 31P777

531 hours on a 18.5 Intek running something terrible. Checked flywheel key and it is fine.Popping out of the carb is erratic in inconsistent just no rhythm to this. Next I plan to check the valve adjustment, but I have seen this same engine with a "flat" camshaft. Mower also has signs of hard cranking with a known good battery consistent with compression relief failure. Did have a missing screw in the throttle plate that I replaced today which did not solve the high erratic idle (carb cleaned and dipped in chem-dip and ultrasonic cleaner as well). Idle goes down to normal when I force idle down with governor arm. Governor arm is bouncing rapidly when engine is running. With spark plug removed engine turns over fine.

Wanting to get some opinions on this array of symptoms, before I tear into this thing any further. Appreciate any suggestions. Thanks.

Sounds initially like a bad inlet seat
Thank you for posting the whole story it makes writing a sensible reply a lot easier
Pull the engine out & apart
I would hazard a guess that the missing bolt went into the engine, taking out the valve seat or inlet valve on the way through
May have bounced around inside the cylinder doing more damage .
Governor actions may be because of random bad combustion from the head damage or the ACR could have broken & taken the governor with it.
So don't fiddle around the edges, rip it out & rip it apart
Strain the oil and check for bits of magnetic metal ( ACR & Governor )
Once apart we can make an informed evaluation.
If for instance the holes the cam run in are damaged then the entire engine is scrap metal unless you have a friendly machinist who works for booze .

Worn cam take the valve cover off and check the lift of the cam. See how much the valves open.

Shook the muffler around like it owed me money and found this stuff. Not one penny came out of this thing! One is clearly the flat-head screw from the Walbro carburetor. The other metal chunks...no idea.
Cam lift for exhaust was .074 and Intake .246. These specs close for 31P777? I checked Briggs online for service manual (31p777-0348-e1) and couldn't find service manual only parts and operator's. Thanks Again for the help and suggestions.

Cams & valves are identical
So if the lashes are set correctly the lift will be the same
Time to pull it out and apart

Couple small knots in piston from the screw-not too bad though. Start tearing it down in the next few days. Anywhere to get specs/service manual for reassembly?

briggs 276781 repair manual should have what you need, it's for the Intek singles

Engine torn down. Exhaust lobe is almost gone; 1.070 specs call for 1.240. Intake lobe is exactly 1.240. I'm curious how the intake lobe is perfect and the exhaust nearly gone-valve spring pressure or heat radiation maybe, heck I don't know. No major scars in the cylinder walls. Didn't feel a ring groove.

No metal chunks laying around inside the engine. Compression relief is fine.

Any suggestion for reassembly? Plan to order a camshaft, new lower crank seal, "oil pan" gasket and head gasket, but anything else? I replaced that elusive-but found in muffler-throttle plate screw, but the throttle shaft is still so loose at the bottom I'm going to have to buy a carburetor.

The Push mower said:

Engine torn down. Exhaust lobe is almost gone; 1.070 specs call for 1.240. Intake lobe is exactly 1.240. I'm curious how the intake lobe is perfect and the exhaust nearly gone-valve spring pressure or heat radiation maybe, heck I don't know. No major scars in the cylinder walls. Didn't feel a ring groove.

No metal chunks laying around inside the engine. Compression relief is fine.

Any suggestion for reassembly? Plan to order a camshaft, new lower crank seal, "oil pan" gasket and head gasket, but anything else? I replaced that elusive-but found in muffler-throttle plate screw, but the throttle shaft is still so loose at the bottom I'm going to have to buy a carburetor.

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Damn. Could you see when turning the engine over by hand that the intake opened a noticeable amount more than the exhaust?

Oh yeah, it was obvious. But I used a dial gauge when the engine was still assembled to avoid guess work. Then confirmed with calipers when the cam was removed.

This thing due for a DIY crank polish and a fresh rod bearing? EDIT: This thing even have a rod bearing? Not seeing one in the diagrams.

The Push mower said:

Oh yeah, it was obvious. But I used a dial gauge when the engine was still assembled to avoid guess work. Then confirmed with calipers when the cam was removed.

This thing due for a DIY crank polish and a fresh rod bearing?

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99.99% of modern small engines do not use rod bearings. just the crankshaft and the connecting rod.

Scrubcadet10 said:

99.99% of modern small engines do not use rod bearings. just the crankshaft and the connecting rod.

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I guess there is not much more to do than replace the cam and gaskets then...? Just trying to make a decent mower out of this one for my personal yard.

Yep, cam, sump gasket, and PTO oil seal.