Craftsman lawn tractor burning oil

bkenobi · Oct 18, 2015

I have a second hand Craftsman lawn tractor from the 90's (AFAIK). The tractor has always used a bit of oil but the last season it has started to use significantly more. When I started it yesterday, it ran ok (small amount of smoke) but after a few minutes (around the time I engaged the mower deck) it started blowing a massive cloud thick enough it was hard to see through. The motor still sounded fine, so I continued working. I ran for 1-1.5 hours until it started acting like it was out of fuel. I topped off but that didn't fix things. After some basic diagnostics, I found that the plug was fouled.

In the past, I've had a little smoke when cold but it runs fine thereafter. I started checking oil level more regularly and realized that it was probably being run low, so I've been checking/topping off every time I use it. Currently, I have to fill the oil reservoir prior to running it or if I run for an extended period of time or else I'll have basically no oil.

I cleaned up the motor, performed a compression test, and replaced the spark plug a few months ago. At that time, I had good compression. I checked the compression today and I still have 70 psi. As I mentioned, after it stopped running, the plug was completely fouled with a crusty buildup. There are 3 areas that seem to have some oil build up outside the engine.

Small amount on lower half of valve cover.
Some oil/dirt build up around the oil fill/drain tube. This is also where the split in the engine (sump/cylinder) is located, so could be a leak in the gasket.
Some build up on the block near the throttle cable/carburetor (governor bolt?)

There is no evidence of any leak under the tractor, on the frame, or on the mower deck. It appears to me that since there's no significant leak/puddle (nothing compared to what I put into the engine each time) that the oil must be getting burned especially since the spark plug was so crusty and I'm always running with a bit of smoke.

I'm not sure what I should be looking for at the moment. I spoke to a local repair shop and they suggested that it could be a head gasket leak, but if I have good compression, that doesn't make sense. I would think it would be more likely a leak in the valves if compression is good. When I pulled the valve cover off, there was a puddle of oil at the bottom of the compartment. I turned the motor over and there was some spray of oil that appeared to be sourced from the lower push rod, but that could just be splatter from the puddle.

Mower details:
Craftsman 917.270723
Serial # 071999B 003832

Motor:
Briggs Inotek 17HP OHV turbo cooled vertical shaft
Model 311707
Type 0132-E1
Code 990714ZE

davesl708 · Oct 18, 2015

Briggs does not publish compression numbers on their small engines but 70 psi seems low. Would expect 90+ psi. You could have bad rings or valve guides.
Any idea of the hours on the engine?

Are the spark plugs the right temp range?

bkenobi · Oct 18, 2015

It took several cycles to get to 70ps. I'm not sure if that's typocal or not. It does hold pressure for at least 1 minute.

How would I check spark plug temperature?

davesl708 · Oct 18, 2015

bkenobi said:
It took several cycles to get to 70ps. I'm not sure if that's typocal or not. It does hold pressure for at least 1 minute.

How would I check spark plug temperature?

Your manual should specify what plug should be used. I would speculate you have low compression and that indicates worn rings or valves/valve guides.
Cold plugs do not burn as well and will foul out quickly.

bkenobi · Oct 18, 2015

I replaced the plug with the same as was installed which is the same as the manual specifies (Champion RC12YC). The manual specifies a gap of 0.030" while the plug is factory set to 0.032-0.038". I wouldn't have thought that would make that much difference, but I'll get a plug gapper tomorrow to get it set correctly.

I have a bit of an update from this evening. I drained the oil I installed yesterday and had maybe 15 minutes of run time on it. I was only able to capture 20oz though it should have 3.5 pints/1.75 qts/56 oz. The fluid was black as if it had been in there for ages. I only had 1 qt so I couldn't completely fill it up but I figured that would be enough to test it for a minute or so. With low oil, I can run it fine with no load. If I try to engage the deck, it quickly stalls. I'll get more oil tomorrow but I wouldn't have expected that low oil would keep the engine from running for a few seconds.

davesl708 · Oct 19, 2015

I am convinced you have low compression. The oil is getting dark due to fuel blowing by and getting into the oil. The fact that it cannot handle a load also indicates low compression preventing the fuel from burning completely resulting in the smoke, oil consumption and loss of power.

bkenobi · Oct 19, 2015

If that's the case, then using an inspection camera should confirm worn cylinder walls? I was thinking of renting one to confirm prior to tearing the engine down. If it's bad cylinder walls, I'd probably be better off trying to source a new motor I imagine.

davesl708 · Oct 19, 2015

The wear will be in thousands of an inch. You will be able to see scarring on the walls if the engine was ran to low on oil but not the amount of cylinder wear.
You will have to pull the head to measure the cylinder wall to see if it is out of spec.

May be a good idea to pull the head and inspect the piston, cylinder walls and valves/valve guides. This will also let you see if it is a blown head gasket. Any way looks like pulling the head is the first step. You will need a new head gasket when you put it back together.

bkenobi · Oct 19, 2015

Sounds good. I found the Briggs manual and parts page for my engine:

Results

Is there any kind of video or guide that you could recommend reviewing prior to tearing anything down? I've worked on a couple small engines, but I'm not an expert. It looks like there's quite a few covers that have to come off and perhaps even the carb in order to get the head off. I just don't want to tear things further down or damage something unnecessarily. If not, I can just take it extra slow.