B&S 31C707 engine smokes


Old 05-24-10, 01:58 PM
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Join Date: May 2010
Location: united states
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B&S 31C707 engine smokes

I have a Murray Model 425005x8B with a Briggs and Stratton motor on it. The codes off the motor are 31C707-0175E1-02120-17ZD. Anyhow its a 31C707-175E motor and from what I can dig up it appears to be a 04 model.

Anyhow...the starter died out on it and I ordered a new one. The mower sat for 1 week before I got tired of waiting and pulled out the brushes of the stater and cleaned them up enough to get it to turn over. When I did this I instantly filled got white/dark gray smoke from the muffler and the motor wouldn't idle and died after 3 minutes or so. I let it sit for a few minutes and took the spark plug out but smelled no oil or gas on it. Threw it all back together and smacked the starter to get it cranking. Again it fired and filled my front yard with white/gray smoke. Ran for a total of 5 minutes with me cycling it from idle to full throttle.

FInally it just got so bad, I had a neighbor come running over ready to call the fire department. Needless to say that never happend. I checked for raw fuel in the oil and dont detect an over bearing smell of it. Looking further into it I noticed oil spilling out of the muffler to engine extension tube. Pulled the muffler and tube off and proceeded to yank the valve cover off and the cylinder head. I inspected the top of the piston and hand cycled it through its stroke and see nothing wrong with the bore...you can still see the cross hatching in it. The valves a not bent and had mild carbon buildup on them.

I'm a automotive tech by trade and these little engines are much different from the ones I am use to. My question is could it be a worn oil ring on the piston? or am I over doing this and could it be something as simple as a bad float in the carb?
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Old 05-24-10, 04:25 PM
indypower1's Avatar
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Check the oil level and smeel the dipstick. If it smells like gas, you have gas in the crankcase most likely caused by the needle not seating in the carb.
Old 05-24-10, 04:35 PM
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Last mower I had that blew White Smoke had a blown head gasket. It was really hard to see the crack in the gasket but after changing only the head gasket it ran with no further problems.
Old 05-24-10, 04:41 PM
Join Date: May 2010
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Drop the float bowel and dump the oil out of it...it sounds very much like this thing tipped over the oil ran up through the valves until it filled the muffler and carb... you're gonna have to put up with a bit more of that white smoke untril all that oil burns off the inside of the muffler...instead of playing with the throttle to keep it running ( and fouling out the plug in the process) try using Gumout as starting fluid, and everytime the rpms drop give it another squirt until the smoke clears and she'll run on her own
Old 05-25-10, 12:13 AM
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Check the oil level...if it is overfull, smell the oil in the dipstick tube...does it smell like gas? If so, the carb is leaking. If the oil level is fine, it doesn't smell like gas, and the engine hasn't been tipped, I'd say the head gasket is blown between the cylinder and the pushrod chamber.
Old 05-25-10, 09:41 AM
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Join Date: May 2010
Location: united states
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So I took the motor off the mower last night and brought it into my shop to look it over more. I drained the oil, and noticed its consistance is close to black water and there is a smell of gasoline to it. I also inspected the head gasket and can not visually see anything, even with a magnification glass.

OH the motor/mower has never been tipped or anything along those lines....totally flat surface and zero inclines in my yard.

Think I'm going to soak the carb to clean it out and get a rebuild kit for it, along with a new headgasket and v/c gasket.
Old 05-28-10, 10:52 AM
Join Date: May 2010
Location: U.S.A.
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A problem inherent of all OHV motors, (that is simply solved by making sure the fuel valve is turned off) ,is fuel running through the carb. down through the valves , and mixing with the oil...If the piston happens to be up you can crank the engine no problem...if the piston is down ,the cylinder fills also and when you go to crank it, the valves close ,the piston can't push the liquid fuel anywhere and you have... Hydraulic lock...

This phenomenon happens most often when transporting a piece of equipment with an OHV engine in the back of a truck or on a trailer, where the float and metering pin turn into a little pump due to the vibration of transportation ... a ride around the block with the fuel valve open is enough to do it...Land scaper finishes up a job with an aerator or thatcher ect loads it up to go to the next job and starts freaking out on his employees for seizing his brand new piece of equipment... seen it a thousand times All ohv's have a fuel shut off...it's there for a reason... use it !

Now before you go rebuilding anything simply clean out the carb, change the oil and fire it up...Unfortunately I know of no way to get all the oil out of the muffler other than burning it off while running it...But from the sound of it that should be the cure

PS this can also happen with the piece of equipment just sitting undisturbed if for some reason the metering pin did'nt fully seat Beer 4U2
Old 05-29-10, 05:50 AM
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Join Date: Apr 2009
Location: land of lincoln
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once you get the mower running again you mite think about changing the oil again and filter if it has one if you dont you will be putting a new cam in the motor (ASK ME I KNOW) put fresh oil let run 5 to 6 min then drain well make sure you get all the gas resdew out of the motor

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