OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Walk Behind Lawn Mower Frames Question On Scott Bonnar Cylinder Mowers Scott Bonnar Original Engine - Rebuild or Replace?

The smoke emitted was blue.

So, the engine is burning oil and liquid oil is running around the top of the carburetor and fuel tank. You can see a black hose in that area, connecting to the carburetor just below the air cleaner. Is that where the oil is coming from? That is the breather hose. If the breather is malfunctioning, crankcase pressure can push oil up that hose. If that happens, chances are some of the oil will leak out and run everywhere, while most of it will go through the hose into the carburetor air intake, then be burned by the engine, causing blue smoke from the exhaust.

If oil is passing through the breather hose, you will need to take a look at the crankcase ventilation valve. This is part of the valve chest cover, which is screwed to the side of the engine below the fuel tank. If you remove that, you can see whether it is stuck in some way: it is a simple disc valve on the inside of the cover. Please post a picture of it after you remove it, and we'll discuss it from there. Do not perform any acts of violence on the valve, just show us the picture.

If oil is not passing through the breather hose, we'll need to do some head-scratching and take it from there.

Thanks for your help grumpy, yes it appears the oil came through the hose you stated and also got a little in the air cleaner.

I'll put the sledge hammer away and get the camera out again. I'll post the pictures up as soon as I have done what you suggested.

Cheers
Peter

Hope this it what you are talking about..

Yes Peter, that is the positive crankcase ventilation valve, and it looks completely clean. The disc valve you can see through the hole, is supposed to be able to move back and forth as the pressure in the crankcase pulses. This is what the manual says:

If the fiber disc valve is stuck or binding, the breather
cannot function properly and must be replaced. A .045"
(1.1 mm) wire gauge should not enter the space between
the fiber disc valve and body. (A spark plug wire gauge
may be used.) Check as shown in Fig. 10. NOTE: The
fiber disc valve is held in place by an internal bracket
which will be distorted if pressure is applied to the fiber
disc valve. Therefore, do not apply force when checking
with wire gauge.
Fig. 10 Checking Breather & Fig. 11 Breather Hose

It appears to be stuck on one side as I cannot insert the metal strip.

You should not be able to insert the metal strip anywhere around the fibre disk. If you can, the breather is worn out.

1. Did you clean the breather before taking that picture, or was it already clean?
2. Was the disk stuck to the metal, or could it move back and forth (transverse to the surface of the disk, not laterally)?

If the disk was crudded up and stuck, and you cleaned it and freed it up, we have an explanation for your oil-from-the-breather-hose problem. If it was pristine and working, we cannot blame the breather valve and have to consider uglier possibilities for the problem (like a broken piston, for example, since it happened suddenly).

I can insert the metal strip on most sides and can feel the disk popping back in position.

1. Only oil around the seal of the valve was wiped off. I did not touch the disk area.

2. The disk can move back and forth.

The engine still turns over freely and does not seem like there is any broken parts internally.

Did the smoking and spewing oil begin suddenly or did it happen gradually? If it isn't a stuck breather valve, it sounds like an abnormal amount of blow-by, which gets into the crankcase past the rings or through the piston, and is more than the breather valve can cope with. How does the compression feel, when you pull the starter? Different from a four-stroke rotary mower?

If the problem developed over a period of months or years, it sounds like you need new piston rings. If it happened suddenly, something has broken somewhere.

It happened suddenly, the compression feels fine and is different to my four stoke mower.

How is the compression different from the other mower?

The four stroke you can feel it starting up or kicking in, the B&S is just turning over.

That sounds like a description of hardly any compression pressure. I'm guessing that it has lost compression due to terminal wear, or broken rings, or a cracked piston.

When it runs, complete with smoke, and oil running out, what is the engine performance like? Does it run like when it was a rip-snorting young one, or like an old horse trudging to the glue factory?

I shut it down pretty quickly after the smoke appeared. The engine can be turned over pretty easily by hand with only little resistance.

Peter, I have to ask you this question to ensure I haven't overlooked something that could change the whole complexion of the problem. You didn't, by any chance, add a lot of extra oil to the sump just before all this happened did you? Overfilling with oil could result in oil flooding through the breather system. The classic cause of overfilling is to fill a dipstick engine with oil as if it were a level plug engine.

If the oil level is where it should be, and you have hardly any compression, it sounds more like a cracked or slightly perforated piston crown than anything else, though badly broken rings could do it. Unfortunately that puts us back into the prospect of overhauling the engine: dismantling, removing piston, cleaning and inspecting piston and rings plus cylinder, replacing defective parts, and reassembling. This isn't terribly difficult stuff, but it does require some basic tools and getting yourself quite dirty with black oily stuff.

Please clear up the oil level question, and let's see if anyone else has a better diagnosis than mine. If not, I think you are going to need some spanners.

No oil was added prior to the problem.

Oil level is where it should be, I don't mind doing a minor overhaul or getting a little dirty but if it's going to be a major issue I'm pretty much back where we started.

I've got noting to lose at this stage I suppose by removing a few more bits and looking further.

That's more or less the way it looks to me. If I were you, at this point I'd be removing the cylinder head and taking a look at the top of the piston.

Here is the page in the B&S engine overhaul manual:
http://www.tpub.com/content/recoveryvehicles/TM-5-4240-501-14P/css/TM-5-4240-501-14P_150.htm
You can learn more by using the "back" and "next" buttons on the top of the pages. You might like to read ahead a bit, and always check the manual before you do something to your engine.

The first step will be to remove the air cowl, to expose the cylinder head. It is secured by four small hex-head screws. Then the cylinder head is held by 8 hex-head bolts, of two different lengths. The long ones have to go back into the long-thread holes when you finish the job. As soon as you have the head off we'd all like to see pictures of the top of the piston, and the underside of the cylinder head.

Incidentally if the piston is broken, something unusual must have happened to cause it. One of my interminable anecdotes: I had an OHV Morris Minor long ago in my youth, and I was driving it one night when a rattle suddenly developed in the engine, and almost simultaneously one cylinder stopped firing. I declutched and switched off in less than 5 seconds, probably a lot less, and rather dolefully towed it home. Next morning I removed the cylinder head. Things were much better than I expected. All that had happened is that a nut welded to the inside of the oil-bath air cleaner had come unwelded and dropped through the manifold, past the inlet valve, into the cylinder. It had immediately hit the side-electrode of the spark plug, closing up the gap and stopping it from firing. I discarded the nut, regapped the plug, and put the head back on, even re-using the same copper-asbestos head gasket. Adjusted the tappet clearances, and drove away no worse off. If it had taken a bit longer to hit the ignition switch, however ... At the time I thought it had probably dropped a valve and smashed a piston, so I was a happy lad.

Nothing appears to be broken but there is certainly some buildup.

It looks slightly oily, but that is to be expected since it has been inhaling oil recently. It does not appear to have been inhaling much oil, just a whiff of it. That amount of carbon is quite minor - on the low side for such an old engine. The piston crown seems in good condition. The next potential problem to check is the piston rings, but they seldom fail suddenly. I suggest you rotate the engine so the piston is at the bottom of the stroke, and look at the cylinder bore. It can be in excellent shape if the rings are just worn out, but will probably show signs of scoring if the rings are broken. We need to find out as much as we can before considering taking the piston out to look at the rings, because that is a bit harder.

And so grumpy, the plot thickens....Lol!!

Yes Deejay. It all fits for worn-out rings except that the crankcase pressure increased suddenly rather than gradually. I haven't thought of any explanation except ring failure though. Maybe someone else has a theory?