Briggs & Stratton 35 Classic engine, blue smoke + oil residue out of exhaust + Petrol in oil

[doug9694]

With this carb setup it's really hard to get gas back into the crankcase oil, i'm wondering if you might have a fuel cap that the vent is plugged over pressuring the tank forcing gas out and into crankcase, just my 2 cents.

In my experience that happens with small trimmers and blowers, ect. The opposite is what I see. The engine running creates a vacuum and quits from fuel starvation.

 

[doug9694]

Could be valve guides of valve gummed up with carbon, not allowing it to seat properly

Gray oil is from water. Maybe some was in the tank or can used to fill it. Also if left in the rain or spray.

 

[Boggie123]

Hi LochlanR.
Did you get to the bottom of this? I had my mower (a Hayter Hobby 41 with a Briggs and Stratton classic 150cc, similar to yours) on its side for a while too. I was swapping the engine into another chassis and had removed the fuel tank, but not the oil. Now, with the engine fitted into the restored chassis and serviced, I started the engine and it smokes like hell (it was running fine before the swap) and there is a little oil coming out of the exhaust. Does tipping these engines on its side somehow get oil into the exhaust port (and in your case, also fuel into the oil)? Did the smoke clear after more use?
Thanks, B.

 

[PGB1]

For what it is worth, I have a very old Briggs & Stratton Classic 35. I'm no small engine person, but the blue smoke I've had.

Cause One:
Once I loaned the machine it's on and the borrower was kind enough to change the oil- overfilling it. After getting the oil to proper level, it took hours and hours or run time for it to stop with the blue smoke.

Cause Two:
Another time or two or three or more with blue smoke and oil in the fuel it turned out the be the diaphragm between the tank and the carburetor. Ethanol in the gasoline damages the gasket, it wrinkles and then fuel floods into the engine. Eventually it gets to the oil & dilutes it. When diluted, the now too full oil gets burned with fuel causing blue smoke. Expect many hours of run time to clear the smoke.

The gaskets are about a dollar on places like eBay and a few dollars locally. Changing it is perhaps a 20 minute project. I change it annually as habit.

Paul

 

[Boggie123]

Thanks Paul.
In my case, it was oil in the exhaust silencer (muffler). I took it off, started the engine and ran it for a couple of minutes. No smoke. So, I flushed the silencer with petrol, then hot soapy water, thoroughly dried it off and, seeing as it was off the engine, prepped it and applied a coat of high temperature paint. I refitted it and started the engine. No smoke!

What seems to have happened is that when I tipped the mower on its side by about 45° (exhaust side down) to remove the engine as part of the restoration process, oil flowed into the silencer (possibly via the open exhaust valve?), even though I had drained the engine via the sump plug. From now on, I will only tip the engine backwards (spark plug up) as oil can't get into the exhaust or inlet/carb that scenario.

One note of caution regarding running an engine with high oil level (e.g. to burn off the excess) - A mechanic friend some years ago told me that high oil levels can mean that the crank bearing journals and conrods are splashing about in the oil, and cause lots of air bubbles in the oil. These are then sucked up by the oil pump pickup pipe and with prolonged use, can cause engine wear damage. He was talking about an engineer's report on a car engine failure, so I am not sure how that relates to single cylinder vertical mower engines, but they do have a sump where the crankshaft spins, so it is probably true for them too. I would recommend lowering oil level by sucking some out through the filler hole, with a pump or even a thin turkey baster, rather than running the engine for quite a while to burn off the excess.

 

[PGB1]

Thanks Paul.
In my case, it was oil in the exhaust silencer (muffler). I took it off, started the engine and ran it for a couple of minutes. No smoke. So, I flushed the silencer with petrol, then hot soapy water, thoroughly dried it off and, seeing as it was off the engine, prepped it and applied a coat of high temperature paint. I refitted it and started the engine. No smoke!

What seems to have happened is that when I tipped the mower on its side by about 45° (exhaust side down) to remove the engine as part of the restoration process, oil flowed into the silencer (possibly via the open exhaust valve?), even though I had drained the engine via the sump plug. From now on, I will only tip the engine backwards (spark plug up) as oil can't get into the exhaust or inlet/carb that scenario.

One note of caution regarding running an engine with high oil level (e.g. to burn off the excess) - A mechanic friend some years ago told me that high oil levels can mean that the crank bearing journals and conrods are splashing about in the oil, and cause lots of air bubbles in the oil. These are then sucked up by the oil pump pickup pipe and with prolonged use, can cause engine wear damage. He was talking about an engineer's report on a car engine failure, so I am not sure how that relates to single cylinder vertical mower engines, but they do have a sump where the crankshaft spins, so it is probably true for them too. I would recommend lowering oil level by sucking some out through the filler hole, with a pump or even a thin turkey baster, rather than running the engine for quite a while to burn off the excess.

I'm glad it worked out & thanks for sharing what the cause was!

Thanks, too, for advising about over-filling the oil.

Happy Mowing!
Paul