Craftsman 17.5

[BurkPub]

I just experienced something new tonight.

I sent the boys to mow a rental property, while a was going to work on a project in the garage. One of the boys said,"Dad you gotta come and look at this." He said the battery seems to be up but the engine won't turn over. So I go look at it. I gripped the flywheel screen and tried to manually turn the engine. It would turn over, but reluctantly. And when it did turn over I heard a sloshing noise. I took off the air filter cover and saw that the carb was full of gas. So I took the spark plug out and gas ran out the spark plug hole. With the spark plug out (and safely out of the way) I hit the starter and blew the excess gas out of the cylinder. I dried the spark plug as best I could and put it back in. It sort of started, but with a huge cloud of smoke. It would run for 20-30 seconds and die. It would start right back up and repeat the process. I decided it must be flooded so I took the gas line off and plugged it. (a 1/4" x 3" bolt works fairly well to temporarily plug a gas line) I thought I would run the carburetor dry and reconnect the gas line. To my surprise, it ran just as good without gas as it did with gas. But it was blowing oil out through the exhaust.

I have a theory of what was happening and why it did what it did, but I want to know if a pro comes to the same conclusion as I did. No, I won't share my theory just yet. The mower is a Craftsman model 917.273370, with a Briggs and Stratton 17.5 HP OHV engine.

[Eli]

The first thing I've got to say is CHANGE THE OIL before you run it more.  The oil is likely full of gasoline which can do irreparable damage to your engine. 

Now, for what caused this.  This is actually a fairly frequent problem.  Carburetor runs over, gas overflows into the carburetor venturi, runs into the cylinder, and seeps past the piston and rings and into the crankcase where it eventually mixes with the oil.  This is one reason why a mower with the gas tank below the engine is a better design because if the carburetor float leaks, it it will not run over since the source of the gasoline is below the carburetor and thus gravity will keep it leaking gasoline into the cylinder and crankcase.

When the gasoline gets into the combustion chamber (the cylinder) it is not unusual for the engine to not turn over since the gasoline can fill up the cylinder and effectively stop the piston from moving up to top dead center.  The following is the recommended way to solve this problem:

1) Turn off the inline fuel shutoff (if the mower is equipped with one), disconnect the fuel line or at least pinch the fuel line off with a locking pliers (aka, visegrips).

2) Remove the spark plugs.   

3) Next, either attach the spark plugs back to the plug wire and make sure the spark plugs are away from the spark plug hole, or better yet in this case, just leave the spark plugs off the wire. 

***Caution:  If the spark plugs are plugged into the plug wire make sure they aren't anywhere close to the spark plug hole when you spin the engine with the ignition key since the spark at the spark plug could cause the gasoline to ignite and cause a fire!!  The safest way is to just leave the spark plugs off the plug wire while you spin the engine.***

4) Now using the ignition key, turn the engine over like you were trying to start the engine to blow the excess gasoline out through the spark plug hole.   

5) Remove the engine oil drain plug and drain the engine oil into a container of your choice so you can properly dispose of it.

6) Replace the oil plug and refill the crankcase with the recommended quantity and grade of engine oil. 

7) Determine and fix the reason the carburetor overflowed.  If this is something you are not comfortable doing take it to a small engine and power equipment repair shop to have them do it.  Most likely the reason for the carburetor overflowing is dirt in the carburetor.  This is especially true if the mower just did this all of a sudden.

8 ) Once the carburetor problem is fixed, turn the fuel shutoff to "on" again (or reattach the fuel line if it was disconnected or remove the locking pliers if that is the case).  If the carburetor does not over flow with gasoline anymore you are ready to go to the next step.  If it still over flows, shut the the fuel off again or disconnect the fuel line, which ever is your preference.  If it still leaks, go back to step # 7.

9) If the problem has been fixed and the carburetor no longer leaks, reinstall the spark plugs.  Reinstall any parts and shields you have not already reinstalled. 

10) Start your engine and mow away!!

The reason to change the oil are as follows:

When the carburetor overflows and the gasoline leaks into the cylinder this can cause a great deal of damage.  First of all, the gasoline in the cylinders washes off any residue of oil in the cylinder making the cylinder dry and greatly increasing friction.  Plus, the gasoline seeps into the crankcase where it mixes with the oil.  This mixture does not properly lubricate the engine anymore since it does not "stick" to the surfaces like it is supposed to.  And the gas will basically "wash" the oil back off where ever the oil had been.  Thus the cylinder does not get relubricated, the bearing surfaces of the crankshaft, connecting rod, and main bearings do not get proper lubrication.  This will in short order cause the engine to lock up from lack of lubrication.

The "sloshing" sound was most likely from the excessive fluid and thinner composure of the fluid in the crankcase, both of which are caused by the gas being mixed with the oil.  The smoking could be from one of two reasons, excessive gas coming into the cylinder or the oil/gas mixture pushing through the breather cover and into the carburetor which would explain why it still ran after the carburetor ran out of gas.  The oil that was mixed with the gas that was pushed into the carburetor through the breather would explain the smoking. 

[JohnB]

Well, we changed the oil. (It certainly smelled of gas and was very thin, with virtually no lubricity) The boys mowed the lawn as instructed, but forgot to shutoff the gas like I told them to. So, needless to say it happened again. Next time I change the oil I want to try to fix the problem in the carburetor if possible. I can't afford to to put fresh oil in it every time we use it.

I was surprised to hear that this is a frequent problem. I had never experienced anything like it. Sure had my head spinning for a while. Thanks anyway.

[Eli]

Indeed, a shut off valve is a great idea anyway since you never know when dirt gets in the carburetor. 

And yes, this is a frequent problem.  Not as in every other mower or even every 5th mower, but this is not the first time I have heard of this problem.