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 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?

No signs of markings inside the bore, the markes you can see in the photo are oil streaks.

There are two ways we can go now: just assume the rings are shot, and take out the piston; or put the cylinder head back on temporarily and test the ring leakage. You don't need to put the air cowl back on, we won't be starting the engine.

The test consists of pulling the starter cord and memorising what it felt like, then squirting some engine oil in through the spark plug hole, putting the spark plug back in, and pulling the starter cord again. If it suddenly has a lot more compression after you squirt in the oil, it was leaking air past the rings when you did it without the oil. If it leaks, the oil will find its way into the sump before long, so you can repeat the with-and-without oil tests as often as you want. Don't put a huge amount of oil in, or you'll simply fill the combustion chamber with oil, which raises the compression ratio and makes it feel as if it was leaking when you didn't put in the oil.

Because your symptoms are a bit odd, I recommend putting the head back on and doing the oil test. It can ruin your day if you take the piston out and discover the rings are better than new.

I haven't had exactly that happen to me, but I've done something similar but worse. I once laboriously put the engine back in an old PA Vauxhall after replacing the rings - a horrible job, the in-line 6 cylinder engine had to go in tilted so the crankshaft was vertical, without the gearbox attached, then you had to tilt it back to horizontal and lower three spacers in between one of the engine mounts and the chassis rail and pick them up with engine mount bolts, one at a time, in zero space, unable to see what you are doing. The oil pan (sump) couldn't be removed without taking out the engine. I'd done all this, fitted the gearbox, and was just starting on the wiring when my father walked up to me with something in his hand and said "What's this?" He was holding the main oil feed line from the oil pump, which goes inside the sump - I'd missed putting it back on before I fitted the sump. I had to take the engine out again. Not a good day.

Thanks again grumpy, I'll get onto to the test and report back when its done.

Ok test completed, and the results are as you explained, it suddenly has a lot more compression after I put in the oil.

That is a relief: it means there is definitely leakage past the rings, and the piston has to come out - no risk of unnecessary dismantling. That requires removing the engine from the mower. The most difficult part of that is freeing the drive shaft, by sliding the clutch along the shaft toward the chain-case on the left side of the mower. The right (engine) side of the clutch is usually held by a captive clamping bolt, which just needs to be loosened, and can't be removed anyway because it's captive. The retention of the left (chain-case) side of the clutch is obvious and less tricky, though at this moment I don't recall what kind of screw/bolt it is. You'll need to disconnect the speed control cable at the engine end. The engine is held down by four bolts, which are easily visible.

Sometimes the clutch is rusted onto the shaft and it's a bit tricky to get it to slide along. Soaking the shaft/clutch hub where one slides into the other with solvent is the first step in that process. If the shaft is rusty it will need to stand overnight to let the solvent take effect. Various solvents will work - Penetrene or WD40 for example.

I think I can handle all that, I'll get the new rings during the week as well as a gasket kit. I'll keep you updated on my progress although I may not make a start until next weekend.

Thanks Peter. The B&S overhaul manual has all the information you need, but you can get any help you want here, and of course we'd like to know the details all the way through - it is not only an interesting case, but your fully original, thirty year old mower is so pretty and new-looking that it is photogenic.

Thank you for all your help so far, its been very much appreciated.

It hasn't been a one-way deal Peter - if you keep us informed, ask any questions you wish, and post photos as you sort out your mower, you'll be making an important contribution to the Outdoorking archives. That can be useful to a lot of people for a long time to come. We've only been involved in half of one step in the process so far, and would certainly welcome being part of the whole thing, including your set-up adjustments on the mower itself.

Hi grumpy, well said, and a good outcome thus far.
As this is a continuation of an old thread and Peter has decided to rebuild the Briggs, this topic is now complete, so I will close and lock it.
Peter, for your next post re your engine, can you please start a new thread in "Questions on Briggs and Stratton Engines".
If a question arises re the mower chassis itself, please post in "Questions on Lawn Mower Frames" Scott Bonnar cylinder mowers.