[andy135]

Hi I just bought a used craftman lawnmower with a 6 3/4 hp B & S engine (not OHV). It start up fine but later when I stop the motor there is quite a bit of blue smoke came out from exhaust before it totally stalled. Restart have some blue smoke too but during the running looks fine. I don't have previous history of this engine. My experience is limited to rebuild some Lawnboy two stroke engine and replace valve seal for my own car. I did put 10w30 oil instead of #30 oil into this engine and already changed too. Do I need to take this engine apart and work on the piston rings or valve area? Thanks ahead.

 

[Mark777]

Hello, Andy and Welcome to MLMF !

I have to ask the dumb questions first, OK?

Is it possible you've overfilled the crankcase oil?
How does your air cleaner element look? If it's really dirty, that's enough to siphon oil from the crankcase.
Loose or poorly adjusted valves can also create leakage and create oil in the combustion chamber too.

If all those items look to be OK, then it just might be time for new rings. Fortunately, most small engine rings are manufactured from a relatively soft alloy. Which makes them wear quicker and cause little damage to your cylinder walls.

But wait, there's more !

There are guys way smarter than me and, hopefully they will have more to offer .

 

[andy135]

Thanks for your reply. I didn't overfill the oil, and I put on a new paper air filter. The engine still have compression and it start up as normal. What I want to be sure is whether this problem is within the engine or else. If so besides the oil and compression rings what else I shall replace. I think you could be right about the valve issue. The mower is only 5 yrs. old use by a senior for a small lawn It is a FWD selfpropell with electric starter. This can be a good project this winter.

 

[Mark / Ohio]

...later when I stop the motor there is quite a bit of blue smoke came out from exhaust before it totally stalled. ....

Sounds like could be ring blow by.

.. The mower is only 5 yrs. old use by a senior for a small lawn ...

Thing you don't now is how often or if they changed the oil and if they might have run it low on oil at some point. Might also have been one of those folk with the belief that adding oil was as good as an oil change.

 

[BOSMECH]

Throw some new rings on it and give it a try that should work.

 

[andy135]

Thanks for helping, fellows.

 

[andy135]

I was using my other lawnmowers until last week I make up my mind to take this motor apart to find out what's wrong with it. Once I took the engine cover off I was surprise to find that between the cover and that horizontal cylinder was pack up with dead grass. So pack up that it cause the engine to overheat. That explain why it start up fine then when warm up it start to burn oil. It even loosen the cylinder head bolt a little bit. How the grass pack up there I don't know but I do realize that the blade of this mower is not a good mulching blade. Look like it was design for the rear bag grass pick up so it do have lot of lift which I seen grass clippings flying around the mower when mulching. But to pack so much grass so hard in that spot is I never even seen before.
So I didn't take engine apart, clean everything up around the cylinder, tighten the cylinder head bolt and the engine start up no problem. It runs good for quite a while then I smell burning oil again, not too bad I can keep going and seen very little blue smoke came from exhaust. When I stop the engine then restart right away there is quite a bit of blue smoke show up. So the symptoms is hot engine burning oil and heavy blue smoke when restart.
Will oil treatment(thick oil) for old car engine help reduce the smoke?
If I have to change the rings do I need to doing anything to the valve seal? I did change the valve seals of my car engine for smoke problem.
May be the easiest way is to change the engine? A used one is quite easy to find.

 

[Austen]

You can try running slightly thicker oil, but you don't want to go up too high in viscosity as small engines like this that don't have an oil pump are going to have a harder time splashing that thicker oil around and getting everything lubricated sufficiently. This can happen especially when it is cold as you run the risk of allowing the internal parts to run dry briefly before the oil heats up and starts cirulating as it should.

I agree, it sounds like blow-by and that oil is getting past the rings. If it were me, I would fix the one that you've got rather than replace it with another used one because you don't always know the history of how a used engine was ran and who knows, that one may need rings down the road too. While you're at it like you also stated, doing the valve seals definitely wouldn't be a bad idea either when you have it apart.

You might find the Briggs service manual helpful for reference. If you put in the model number shown in this link, you can download the service manual for it in a PDF format. Small Engine Manuals | Lawn Mower Manuals | Briggs & Stratton