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

[LochlanR]

I'll take yous through a step by step of what i've done:
- Tipped mower over for about 20 min to replace back wheels. 20 min on each side
- Mower was then left on its wheels while being serviced
- Cleaned fuel tank
- Cleaned carb
- New diaphragm
- Oil was extracted and replaced to the max and not over
- Mower was ran for a while, smoke cleared to a point where it wasn's smoking out my garden
- Cut lawn no issues
- 2 Days later, start it and it puffs big clouds of blue smoke out and continues to smoke
- Oil is now dark grey and smells a bit like petrol

I bought the mower as spares but was listed as only needing a back wheel as it had broken, so I don't know the condition of the internals. I can only assume that the piston rings are worn, so its letting oil past the top of the piston and burning it.

 

[Curtisun]

The carb you showed sits on top of the fuel tank does it not?
Since it does not have a fuel line running to it, it has to use the vacuum to pull fuel up through the tube going down into the tank. So it has to have a diaphragm and that diaphragm has to have a hole somewhere in it leaking by. The only two routes fuel can get into the oil is:
1. Through the carburetor to crankcase vent port.
2. Through the carburetor to the intake through the intake valve to the piston then by the rings.
Since The mower I presume sets level when it gets oil in the fuel it is either being syphoned into the oil by the crankcase or by the mower setting at an angle. It dos no matter if the angle is left, right forward or backwards.
If you have a full Tank of fuel and leave mower setting on an incline it can and will drain back through the carb to the crankcase oil either through the jet that regulates the amount of fuel or through the vent.

 

[Curtisun]

I'll take yous through a step by step of what i've done:
- Tipped mower over for about 20 min to replace back wheels. 20 min on each side
- Mower was then left on its wheels while being serviced
- Cleaned fuel tank
- Cleaned carb
- New diaphragm
- Oil was extracted and replaced to the max and not over
- Mower was ran for a while, smoke cleared to a point where it wasn's smoking out my garden
- Cut lawn no issues
- 2 Days later, start it and it puffs big clouds of blue smoke out and continues to smoke
- Oil is now dark grey and smells a bit like petrol

I bought the mower as spares but was listed as only needing a back wheel as it had broken, so I don't know the condition of the internals. I can only assume that the piston rings are worn, so its letting oil past the top of the piston and burning it.

The flappers if they do not seat good or the diaphragm is not next to the carb then the flappers will allow fuel to leak by if the mower is not setting level. Btw worn piston rings will not matter about fuel getting into the oil. New rings will not stop the liquid fuel.

 

[MartinR]

Just bought and serviced a 35 Classic engine on a push mower, Started it and there was alot of blue smoke, although I did have to replace two back wheels so I had it on its side for about 20 mins. When i got it the oil looked to be over filled quite a bit so made sure to change it to the right amount before starting it. After starting, it took ages to burn off the oil until it eventually cleared. Used it to cut the grass and left it for a couple days. Just went to start it now and it shot out a cloud of blue smoke, its been kept upright and not on its side, checked the oil, its gone very dark gray after one use and smells a bit like petrol has mades its way into it.

What could be the issue? I assume it to be the piston rings or something relating to that.

When you put the mower on its side did you have the carb on the upper side? With the oil as you describe it you need to change it ASAP.

 

[stihlmania]

Just bought and serviced a 35 Classic engine on a push mower, Started it and there was alot of blue smoke, although I did have to replace two back wheels so I had it on its side for about 20 mins. When i got it the oil looked to be over filled quite a bit so made sure to change it to the right amount before starting it. After starting, it took ages to burn off the oil until it eventually cleared. Used it to cut the grass and left it for a couple days. Just went to start it now and it shot out a cloud of blue smoke, its been kept upright and not on its side, checked the oil, its gone very dark gray after one use and smells a bit like petrol has mades its way into it.

What could be the issue? I assume it to be the piston rings or something relating to that.

Have you run it for sometime to see if smoke goes away? Also a second oil change I find is necessary to clean up oil.

 

[PGB1]

My 35 classic did the same as yours, LochlinR. I changed the carburetor diaphragm (the top gasket looking thing in your photo) and it solved the problem for about 4 years. When it did it again, I changed the diaphragm again & the problem hasn't yet returned. It's been another 4 years. The diaphragm may last longer if you buy a real one. I bought eBay clones.

I'm by no means a mechanic, but examining the old versus new diaphragm showed that the old was wrinkled. I suspect alcohol in the fuel caused that.

I hope this helps,
Paul
PS: There's no place on the 35 Classic to put the shut-off that Born2Mow mentioned, but I have them on all of my gasoline equipment that has float bowl carburetors. They're a great aggrivation saver. I also have in-line tees with valves on seldom used equipment so that I may readily drain the fuel from the tank after the engine cools.

 

[Curtisun]

My 35 classic did the same as yours, LochlinR. I changed the carburetor diaphragm (the top gasket looking thing in your photo) and it solved the problem for about 4 years. When it did it again, I changed the diaphragm again & the problem hasn't yet returned. It's been another 4 years. The diaphragm may last longer if you buy a real one. I bought eBay clones.

I'm by no means a mechanic, but examining the old versus new diaphragm showed that the old was wrinkled. I suspect alcohol in the fuel caused that.

I hope this helps,
Paul
PS: There's no place on the 35 Classic to put the shut-off that Born2Mow mentioned, but I have them on all of my gasoline equipment that has float bowl carburetors. They're a great aggrivation saver. I also have in-line tees with valves on seldom used equipment so that I may readily drain the fuel from the tank after the engine cools.

Your suspicion about the alcohol is correct it will degrade the diaphragm over time and about it not having a fuel line. This carb does not have a float but uses the jet to regulate flow. I believe one of those two flappers allow the excess fuel to return to the tank and the other acts as pump valve. If either leak by or you have a hole in that diaphragm it will put fuel into the oil while it is running or setting at an angle. I do not remember having a classic in my shop, but I have had some with the same carb.

 

[thesilentone]

The carb is mounted directly on the tank, (on the side of the engine) and is a diaphragm pump. No float valve, no place for a tap !

Sounds to me like the breather valve is not working properly, take it off and check.

 

[Curtisun]

There is one other thing that comes to mind about these type carbs. If memory serves me correctly, they are made of high temp polystyrene plastic and when plastic gets old it can crack. If this carb has a crack in it were the fuel flows, then that could also be the problem.

 

[first pull]

Just bought and serviced a 35 Classic engine on a push mower, Started it and there was alot of blue smoke, although I did have to replace two back wheels so I had it on its side for about 20 mins. When i got it the oil looked to be over filled quite a bit so made sure to change it to the right amount before starting it. After starting, it took ages to burn off the oil until it eventually cleared. Used it to cut the grass and left it for a couple days. Just went to start it now and it shot out a cloud of blue smoke, its been kept upright and not on its side, checked the oil, its gone very dark gray after one use and smells a bit like petrol has mades its way into it.

What could be the issue? I assume it to be the piston rings or something relating to that.

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