Riding lawnmower engine occasional blue smoke

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RonaldB

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I have a Craftsman riding lawnmower with a Briggs & Stratton engine, which has a few quirks.

Last summer it would cut off after about 45-50 minutes of mowing, like it wasn't getting enough gasoline. A few minutes of rest would cure the problem, and it would run fine after that.

This spring after the normal oil filter-oil-spark plug-air filter-fuel filter tune up, it now starts blowing out blue smoke after about 30 minutes of mowing. The first time this happened I thought I had blown up the engine, but after about 20-30 minutes of checking everything, it fired up normally.

The engine seems to run the same as last summer, same power etc., no weird noises.

I can't figure out what is going on. If there's a head gasket leaking, wouldn't it spew out blue smoke all the time?

Any help would be appreciated.

First time poster, so if I missed any required information, please let me know.

Thanks!
 
A few questions. What oil is in it and how old is the oil? How old is the mower engine? Is the engine fan shroud in place and free of debris. Are you running the engine at wide open throttle? ( You should be).

Sounds like a ring is sticking/worn , or that its getting really hot and the oil is so thin its burning.
 
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Hot stopping can be one of the toughest things to track down, as quite often by the time you can get to diagnosing the problem it has cooled enough that the problem is gone. I have seen it be as simple as the spark plug, or an intermittent problem with the electronic ignition. Or, possibly a speck of dirt floating around in the carb bowl. As far as the smoking goes, probably just getting worn rings. Don't ask me to explain why, but it WILL make them smoke intermittently. Lastly, head gaskets don't make an engine smoke, as there shouldn't be anything but compression in the cylinder. Although come to think of it , maybe if it is an ohv engine?
 
Too high an engine oil level?

Is it "positional" sensitive when it smokes. Like nose down, left side down tilt, right side down tilt, etc?

I have noticed many of the OE engines are sensitive to overfilling of engine oil.
 
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make sure that the cooling air intake is free of leaves etc. In addition; a lean mixure will cause over heating. I use a 10w30 oil in my 20HP B&S engine.
 
first is it ran by a fuel pump or gravity fed to carburetor. if gravity fed you are leaking gas past carb diluting oil. two choices on that cheap fix gas shutoff valve in line, second choice rebuild carb, can also be solenoid at bottom of carb. I would bet on it being one of these problems, blew engine up before dur to fuel dilution in oil.
 
Originally Posted By: Corvette Owner
Does it have a fuel shutoff valve? If not check oil level, if it is over full, gas is getting in oil.

Install a gas cutoff valve, and use it.


+1
 
See if the gas cap is still venting. My JD would not start, took gas cap off just to see if it was venting. It fired right up. The fuel had eaten the insides of the cap up and would not allow it to vent.
Make sure the crankcase vent is not plugged up also, which would make it smoke like you are describing.
 
Oil is Sears house brand, SAE30. I replaced it a few weeks ago, but the oil itself is probably from last year.

Engine fan shroud is free of debris, that may have been the problem last year (it was clogged up).

Yes, running the engine at full throttle, except when I get off to move a branch or something.
 
Originally Posted By: old1
Hot stopping can be one of the toughest things to track down, as quite often by the time you can get to diagnosing the problem it has cooled enough that the problem is gone. I have seen it be as simple as the spark plug, or an intermittent problem with the electronic ignition. Or, possibly a speck of dirt floating around in the carb bowl. As far as the smoking goes, probably just getting worn rings. Don't ask me to explain why, but it WILL make them smoke intermittently. Lastly, head gaskets don't make an engine smoke, as there shouldn't be anything but compression in the cylinder. Although come to think of it , maybe if it is an ohv engine?


You're the second person talking about worn rings. What rings are they? Cylinder rings?

I don't think the engine is an OHV, it's a Briggs & Stratton Model #31P977.
 
Originally Posted By: MolaKule
Too high an engine oil level?

Is it "positional" sensitive when it smokes. Like nose down, left side down tilt, right side down tilt, etc?

I have noticed many of the OE engines are sensitive to overfilling of engine oil.


I don't think I overfilled it, but will check when cold (didn't look like a problem when engine just stopped. The stalling last year seemed positional, nose up, but the smoke seems to come without warning. I'll keep an eye on what position it's in next week.
 
Originally Posted By: expat
Originally Posted By: Corvette Owner
Does it have a fuel shutoff valve? If not check oil level, if it is over full, gas is getting in oil.

Install a gas cutoff valve, and use it.


+1


Is the cutoff valve to prevent gas getting into the oil system when it's not in use? It doesn't have one, but that's an easy addition.

But how would that explain 30-40 minutes of flawless operation, and then a big cloud of smoke...?
confused.gif
 
Originally Posted By: Eddie
make sure that the cooling air intake is free of leaves etc. In addition; a lean mixure will cause over heating. I use a 10w30 oil in my 20HP B&S engine.


I think it's pretty clean. The oil is SAE30, as the manual states that multi-viscosity oils will result in increased oil consumption. Although this wouldn't be the first time a manual is wrong...
wink.gif
 
Originally Posted By: crgstvrs
first is it ran by a fuel pump or gravity fed to carburetor. if gravity fed you are leaking gas past carb diluting oil. two choices on that cheap fix gas shutoff valve in line, second choice rebuild carb, can also be solenoid at bottom of carb. I would bet on it being one of these problems, blew engine up before dur to fuel dilution in oil.


I thought I'd seen a fuel pump on it, but running through the parts list, I can't find one, so I'm guessing it's gravity fed.

I'll try the gas shutoff valve, but like I asked above, how would that explain the 30-40 minute flawless operation and then a sudden burst of smoke?
confused.gif
 
Originally Posted By: oldhp
See if the gas cap is still venting. My JD would not start, took gas cap off just to see if it was venting. It fired right up. The fuel had eaten the insides of the cap up and would not allow it to vent.
Make sure the crankcase vent is not plugged up also, which would make it smoke like you are describing.


I'll check both the gas cap and the vent, but it starts right away. Thanks.
 
A sudden puff of smoke could be gas burning from a stuck float or high float in the carburetor. Hit a mild bump or make a tight turn and boom, puff of smoke.
 
Had the same problem with my Riding lawnmower with a 19hp OHV Briggs Engine. Every time I engaged the blade I would get a puff of blue smoke also I could smell burning oil after the engine was running for a while. What I found is the head gasket had a small problem between the cylinder and where the push rods go thought. I understand this is very common for these engines.
 
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