Intermittent Oil Burning Bamp; S engine

CRACK !

the cold engine crack is too small to let the oil out, after the engine warms up, the crack expands and BAM ... oil leak.

Just a guess...

I've seen this condition once many years ago. The function whereby the oil control ring(s) scrape oil from the cylinder walls and return it to the crankcase doesn't work.

This could be caused by a number of different problems. In the case with which I am familiar the engine was rebuilt with rings that were correct for the engine model; but the engine had once been rebuilt with all 8 pistons from a later year engine that were not recognized by the amateur mechanic doing the latest ring job. The result was that the oil rings used, wrong for the pistons, did not exert enough force on the cylinder walls to properly scrape off the oil thrown up by the rotating crankshaft and rod bearings. The interesting thing about this particular case was that the engine had very satisfactory power output because there was no incompatibility of the two compression rings. And neither did the spark plugs show any of the oily carbon buildup that we normally see on typical worn-out oil burning automobile engines. It just created huge amounts of smoke from the rapid burning of lubricating oil (a quart every 20 miles).

Other causes in the case of a single cylinder engine could be an oil ring improperly installed or seriously damaged and sticking in the groove when the engine heats up. Or a new ring installed in a groove that still had unremoved carbon buildup from previous operation.

A loss of circularity of the 18hp engine cylinder wall could also create high oil consumption. But the gradual change expected from both thermal expansion of the metal engine parts and reduction of oil viscosity with increasing temperature is inconsistent with the observed sudden onset of oil consumption.

On the other hand an oil ring that continues to function up to the point where restrained movement makes it finally unable to follow the variations in the cylinder geometry can explain the sudden step function in oil consumption.

Perhaps this suggests a possible cause of increased oil consumption for the B&S single cylinder engine. I'm assuming here that the engine recovers proper oil consumption after complete cool down. What other causes might there be? None other has yet come to my mind but I think there are other possibilities I wouldn't think of. Possibly something to do with crankcase ventilation allowing a sudden pressure buildup?

Ed Weldon

Disassembly will reveal the cause otherwise speculation may only amuse

See if you can find and check the crankcase breather tube. if that system gets plugged up it can cause excessive crankcase pressure to push oil up into the air intake and increase oil consumption. Let us know what you find.

h

If I disconnect the crankcase breather tube from the carb while the engine is smoking, it spews oil. When it is connected, it dumps this oil into the carb and causes the smoke.

Good news. Sounds like the lead you need to solve the problem and also highly unlikely that engine disassembly to inspect the oil rings will be needed.

Be forwarned that dipsticks may be incorrectly marked or simply the wrong one for the engine. Best to try to find an instruction or repair manual for that engine and review the specific instructions for oil fill amount and checking level. In the smaller B&S engines I'm familiar with these features are fairly crude, especially in older models. Also be aware that if the engine has been disassembled in the past the work could have been botched or the wrong parts from another engine used in the areas related to crancase capacity (unlikley) or crankcase ventilation.

Ed Weldon

GA Ed. Sounds very much like a bad ring. You might have damage to the ring grooves on the piston as well.

Does it start doing it after you mow on the side of a hill?

My mower will start smoking badly if I mow on the side of a hill or go over very rough ground. It is a horizontal engine. The reason is obvious in my mowers case.

OK, The reason for this is not obvious to me. I have my mower in the shop currently and my mower is smoking badly while on an incline or make a sudden change in direction. What is the cause for this? B&S 18.5 HP single cylinder.

Yes, slopes and going from forward to reverse abruptly, exacerbate or initiate the problem.

Read the operating manual for the lawn mower you are using.

Maybe after 30-60 minutes you get to the close-in work where you have to pass the mower by or under bushes, shrubs, and such (flexible) impediments. It being that you are probably also warmed up by this time, you might not notice when a branch or twig pushes against a control wire/lever and, say, closes the choke.

Or, perhaps, something as simple as a mixture control or needle valve setting being changed due to heat, or mssing spring or part.