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Hey everybody, So I have a Craftsman 917.28712 riding mower, I think from 2007, that was sitting a while, but didn't see a ton of use. It starts fine, runs fine, idles fine, power is fine, didn't overfill the oil, air filter doesn't have oil in it.....but it tears through oil. It puffs white smoke out which comes and goes. It's not thick smoke, but sometimes it can be pretty heavy, and then it will stop for a while then smoke again. Sort of fades in and out. I did a head gasket, same thing, so I did it again with another gasket thinking I did it wrong, same thing. At this point I'm thinking bad piston rings? I read it could be a clogged breather tube too. Does that sound possible? Where do you guys and gals think I should go from here? Thanks!

Post B&S Model xxxxxx, Type xxxx, Trim xx, Code xxxxxxx from engine please.
Perform a compression test on each cylinder. Auto parts stores rent them out & you will get a full return upon returning compression test kit. Please let us know how it goes, thanks, Mark

B&S 31P977 0635 E1 070328ZD I did a compression test and it was 120psi The plug looked clean, oil around the threads. I did a leak down test and it was 60%. Noise coming our of the oil fill tube. I could hear noise coming out from the valve cover too (probably expected). So I guess it's piston rings? What do y'all think?

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Anyone have any thoughts? Should I buy a set of piston rings? Do you think I'd need to hone the cylinder or anything?

Make sure Air Filter & Breather are clean & not obstructed. Have you ever removed Blower Housing topside of engine cover & cleaned engine cooling fins so engine can breathe, if not try this first. You may be surprised at the amount of crud you remove. Let us know how it goes, thanks, Mark
Your engine parts manual is posted above. Always use B&S OEM Parts.
Adjust valves with engine cold per specs. in Operators Manual.

Hi Mark, I had the blower housing off, and besides some caked-on dust, there's no other obstructions. I took off the breather tube and it's clear. Air filter's fine. valve lash is set per spec. What's my next move? Thanks so much for the help, btw.

Did you lap the head and block gasket surfaces prior to installing the then new head gasket?

If not you left out a step in properly replacing a head gasket.

slomo said:

Did you lap the head and block gasket surfaces prior to installing the then new head gasket?

If not you left out a step in properly replacing a head gasket.

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I gently cleaned the surfaces of all the old gasket before I put the new one on. When I put the 2nd head gasket on, because I thought I did something wrong the first time since it was smoking exactly the same, I didn't see any damage, marks or discoloration on the old gasket. Honestly didn't on the first one I removed either. The leak-down test I did produced air noise from the oil fill tube. Cylinder at TDC. Could I have a stuck ring? I was going to try the seafoam treatment and see if that helps: pour in small amounts of seafoam a little at a time while it's running, and then enough to stall the engine. I wish I could soak the piston in seafoam for a while, but it's horizontal. I also bought some marvel mystery oil and thought I'd try that too.

jlatenight said:

The leak-down test I did produced air noise from the oil fill tube.

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Cold or hot test? Air leaking past the rings and valves.

jlatenight said:

Could I have a stuck ring?

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Possible.

jlatenight said:

I was going to try the seafoam treatment and see if that helps

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Don't expect anything on this.

jlatenight said:

I wish I could soak the piston in seafoam for a while, but it's horizontal.

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Dump the oil and gas. Then soak the rings at your leisure. This is a great winter project. Pull the head and soak away.

Marvel Mystery Oil, good idea, give it a try. Remove spark plug & dump a table spoon of MMO down throat of carb. & spin engine over with key several times & let soak overnight.

Best way is to use some kind of heat while doing this. Cold soaking with PB Blaster or MMO is kind of a waste of time. If you can get the piston out and do a proper cleaning....... Rings are not that much if still available. That way you will get a 100% visible on her.

Thanks for the replies. If it turns out I do need new rings, do I have to hone the cylinder, or could I try just replacing the rings and see what happens?

Absolutely yes use a cylinder hone on the end of a drill.
If you have the time watch Derrick Bieri on youtube "Vise Grip Garage", or Motor Trend "Roadworthy Rescues".

White smoke?
and 120 compression does not indicate bad rings.

What is the chance that the engine oil is contaminated with gasoline?

I would do a leak down test and remove the OHV valve cover and listen for air into the push rod galley if it's blue smoke and eating oil. Air going into the push rod galley indicates a blown or leaking head gasket which is common on the single cylinder inteks 31 and 33 series engines.
White smoke usually indicates burning water.
Air cooled engines do not burn water unless ??????????

mechanic mark said:

Marvel Mystery Oil, good idea, give it a try. Remove spark plug & dump a table spoon of MMO down throat of carb. & spin engine over with key several times & let soak overnight.

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So I did exactly this and same thing. I replaced the carb (cheap) and same thing. I'm thinking I should crack her open now. Not sure what else to try. When I do, I'll check the rings. if they're stuck, should I just clean and free them, or put new ones on. If new ones, do I need to hone the cylinder? I hope not. I don't have access. If I need to take this to the shop it's going to cost me big $$.

It runs fine, it sounds fine, plenty of power, no oil in the air filter. Just smokes like Chong.

If you have not cracked her open yet.

take a oil squirt can and inject some motor oil into the spark plug hole spin the engine over by hand couple turns then do the leak down test again and if it's the rings your 60% leakage will get to less leakage and the compression should go up also if it's the rings.
The motor oil only temporary seals the rings blowby. Another test is to use a vacuum/pressure gauge (compound gauge 0-10 pressure, 0-30 vac, automotive type for about $15) and measure the crankcase vac/pressure. (you can review this test on-line) On a good engine the crankcase will have a slight vacuum, bad rings will produce a pressure. Just another thought you might pull off the breather hose from the carb throat and start the engine and see if you see any smoke or oil coming out of the breather tube. A leaking breather check valve should also not be overlooked. You can review it's operation and how to test on-line.
The 60% leak down test you made is a hint of internal leakage and you are now trying to determine if it's the rings and worn cylinder. You need to have the cylinder mic'ed to see if it's within specs if you are going to re-build/re-ring for yourself. New rings will not do any good for very long if the cylinder is out of spec's.
Sometimes what causes the rings to go away fast is someone when doing carb work accidently left off the breather hose at the throat of the carb and this bypasses the air filter and the engine will suck lots of dirt into the carb throat. This breather hose is in behind the carb throat intake on yours and most Briggs and Kohler small engines and is easily overlooked and not readily apparent that it's off until the engine has already sucked lots of debris into the carb throat bypassing the air filter.

Those engines are also know for head gsket leaking between the oil galley and the cylinder, but you said you replaced the head gasket.

Add to my previous post:

Also something I've encountered when doing a leak down test on a small engine that is a oil burner.
The rings and cylinder is already loaded up with oil already and a leak down test will sometimes indicate that the leakage past the rings is not as severe as it really is due to excess oil already in the cylinder and on the rings.
When I see a small engine that is showing the leak down as on the edge cold, I run the engine to get the oil warmed up and the leak down will sometimes quite a bit lower.
Keep in mind on that engine you need to do the leak down test with the piston 1/4 past TDC. (or remove both push rods to make sure that both valves are closed when at TDC. (at TDC the intake valve can be partly open on that engine)

Well I see one problem and that is the 120 psi on single cylinder Briggs with an ACR. It indicates a bad camshaft ACR. Also these 310000 engines are bad about blowing hte head gasket between the cylinder and the push rod galley. When this happen oil is blown thru the breather tube and the carburetor inhales it and the engine burns the oil.

Removing the rocker valve during the leakdown test will show more air escaping from the push rod galley then the dip stick tube.

Also these engine can pass a standard leakdown test and still have a worn out cylinder. It takes a modified leakdown test to fully check the piston stroke in the cylinder. This requires disabling the valve train which is easily done on an OHV engine.

StarTech:

Right about the camshaft ACR failures on the 31 and 33 series and also the head gasket leaking into the push rod galley.
I see he has replaced the head gasket maybe twice and no immediate results when he first started the engine.
He has not complained about the engine hanging up on the compression stroke when cranking indicating that his ACR on the cam is bad YET.
He calls that smoke white. I would bet that it's actually the BLUE oil smoke since it's gulping oil.
I've also seen them Briggs engines pass a leak down test if the test is not done correctly, especially a oil burner.
I do not put a lot of money and time into the single cylinder Briggs Inteks 31 and 33 series due to the high failure rates of the engine internals and the usually wimpy carbs. It is also a waste of time and money throwing China parts into them especially the camshaft and the $100 Briggs OEM camshaft has high failure rates and can fail anytime. When it fails small pieces will usually destroy the rest of the engine if the engine is running at a high rpm when the ACR flies apart internally.
I usually just find a good used old model Briggs twin L-head, Single cylinder L-head or even a twin Intek as a replacement for the 31 and 33 series if the tractor and deck is ok and worth the effort the restore, if not it goes to my salvage for parts.
Really a shame how sorry the Briggs OHV 31 and 33 series engines are made.
Briggs should be ashamed of themselves.

Even if the standard leakdown test is done correctly it will not show a tapered,oval, or both cylinder problems this why a modified version must be done.

Also the heads do warp.

StarTech said:

Removing the rocker valve during the leakdown test will show more air escaping from the push rod galley then the dip stick tube.

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I already returned the leak down tester, or I'd do a test with the rocker removed so valves stay closed, and with some oil in the cylinder. Might be worth me getting another one. There was definitely air noise from both the OHV cover and the dip stick.

shadetree#1 said:

engine hanging up on the compression stroke when cranking

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It doesn't have any problem starting up.

StarTech said:

oil is blown thru the breather tube and the carburetor inhales it

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There's no sign of oil in the breather. Maybe the breather valve is blocked? I could take the head off and check the head gasket again. I didn't see any sign of it being blown on either gasket I replaced and I looked really close on the spot between the cylinder and valve galley.

shadetree#1 said:

I would bet that it's actually the BLUE oil smoke since it's gulping oil.

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I would think it would be blue too, but it looks pretty white to me. My neighbor who's a gear head commented it's white and not blue. Not sure how that could be though.

shadetree#1 said:

I usually just find a good used old model Briggs

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The tractor itself is in great shape. Garage kept. Is there a cross reference for which motors would be compatible? Is it just the shaft length and basic layout?

I saw you can put a vacuum gauge on the oil fill and see if there's any indication of positive pressure (I think)?

Hone cylinder with tool & replace rings with OEM B&S Part 791936, superceded to 594437 part #.

Just wanted to report back that I replaced the rings and no more smoke! I put a new carb on it and it need a little adjustment. It's surging so I think that means not enough fuel is getting through, right? It's a brand new carb and wasn't doing it at first, but started to after running for 45min or so. Something in the main jet clogging it up?

You posted Nov. 8th you installed cheap new carb.
https://www.amazon.com/Carburetor-Briggs-Stratton-792768-799727/dp/B07DSZ5Q63 Brand is not B&S OEM Carburetor here, cheap knockoff.
Above is Genuine B&S OEM Carburetor for your engine, lowest OEM price I could find.