I have a 1998 John Deere GX85 mower with a 13HP B&S engine (Model#28M707 -- Type 1162-E1 -- Code 980325ZA). Have always maintained a strict tune-up policy with all my outdoor equipment and this past month has brought up an engine issue. For the first time, I had very little oil on the dipstick. Refilled and put in new spark plug (I buy a whole box and exchange out x2 a mowing season since I mow 3 days a week almost in spring and x2 week in late summer/fall). Put in 30W synthetic oil and went to start with no luck, but battery sounded low and was 4 years old. Replaced with better compression, but no start. Pulled spark plug and found the electrode smashed. Rechecked the numbers again and right spark plug. Put in new one and same result. Now I'm alarmed. This time, I barely put in spark plug and just "snugged it int." Mower ran fine until almost done and wife came and got me. Would not start so I pulled plug with smashed end. Had an older set with almost same numbers, but but shorter shaft. Put in and ran fine for about 3 mowings and wife called. No oil on dipstick so I told her to put in a quart of 30W synthetic and fill to top of line. She called back and said she put in a quart and a third, but couldn't get mower started. Came home next day and pulled spark plug. It was dry, black and full of carbon deposits that flaked off. Spark plug was not broke or smashed so I did a compression test and found it to be 89 pounds x3 attempts. Checked my oil and found it to be way over the fill line and my wife swears it was at the full mark and was witnessed by neighbor. Drained oil off to just below full mark and put in new spark plug. Started right up with a hint of blue smoke and smelling like it's somewhat flooded. I also noticed the oil looked old and smelled like gas after she just put in over a quart and a third! I believe my rings are okay, but question if the valves are stuck open with air, oil and gas all mixing together. Wondering if the repair is really worth getting a new one, but this lawn mower has been wonderful. Also remember a couple of times hearing a knocking sound just prior to finishing mowing on those times as well. Sorry for the rambling, but wanted to paint an accurate picture before I right this off as a sunk cost versus investing in a new one.

 

I belive it is time to pull the inspection cover off of that mower and see if your lower end is not worn. sounds like a rod bearing is just starting to go and allowing the piston to come up to much. a smashed plug is either to long or something is wrong internaly with that motor. also it should not use that much oil that fast.

Murphy was an optimist.

 

It sounds like your plugs are too long. You said the new plugs were a different number and slightly shorter. That seems to be the obvious answer.

The gassy oil is caused by a leaking carburetor. This may also be a big part if not the entire reason for the oil consumption. The gas gets in the oil, makes it too thin and it gets into the combustion chamber and burns off. You should be able to clean the carb and replace the inlet needle to correct the problem.

The knocking...it could be from the oil level too low, or carbon buildup in the cylinder from the burning oil/gas, loose engine mounting bolts, etc... If it only does it occasionally, I wouldn't be so concerned. Fix the carb, drain and refill the oil after the carb leak is fixed, and see what that nets you.

 

Thanks for the reply. The "longer" plugs I have used consistently over the years and to put the same plug in and have it destroyed concerned me. I'm glad I had a plug with a shorter stock, but I know it not right. It's borrowing me time for now. For the cheapest route I will clean the carberator and replace the inlet needle as well as change the oil. I knew I had a partial problem with the carberator from other questions in the forum. I'm hoping this work, but I'm suspicious to say the least. Probably looking at another bigger John Deere mower in the near future. Thanks!!

 

Oh, I had the impression you tried a different plug that was a different number and slightly longer when you began experiencing the problem. If you are using the same plug, with the exact same number and length of reach as what you used to use, and the gasket is on the plug, then I'd say speedwrench is right and there is enough rod wear allowing the piston to come up enough to tap the plug. This would also create a knock, to say the least. It should be a more than obvious knock if this is the case.

 

Well, I cleaned the carb and now there is no more gas smell in the oil nor is it showing overfill on the dipstick. I mowed the yard after changing the oil, and yes, I could smell oil burning. I stopped briefly to move a few items, restarted and Iblew black and then blue smoke before going faint blue after a minute or so. The smell did go away after several minutes and almost done. I let the motor cool, checked the dipstick, and I was down a quart already after just changing the oil. It was a little clear, but for the most part, it looked like I needed another oil change. Being the mower is 10 years old and I have a fairly big yard, I think its time to invest in a new John Deere and then sell my GX85 for whatever I can get. I'm on borrowed time before I blow motor.

 

You may be right. The oil consumption could be from a head gasket, but the plug thing indicates a bigger problem. If you're using an RC12YC champoin or equivalent, and the piston is hitting the electrode, then you probably are on "borrowed time".