B&S 12.5 HP runs away when first atarted

[gtmotat]

My 1984 Wizard died on me and I was in need of a new to me mower.

I found a Murry Mod. 40708x92a With the 12.5 B&S 286707-0413-01 Motor.

It is in great shape and it has been kept indoors and it's care shows. For a Murry it is a well built mower and it ran when I got it.

It wasn't long until the fuel line showed to be falling apart inside due to the methanol gas. I changed it out and all is well. also got a pretty fair amount of crud out of the gas tank that was floating around.

However when it has sat for a couple of hours or more it runs away when first started. It also blows blue white smoke out of the exhaust. I have to pull the gas lever all the way to slow and it calms down and quits smokeing. This takes 30 to 45 seconds. At point it will mow and drive without any problems until it sits up for a while.

I changed the fuel filter and the line. I have added Sta-Bal to the gas that is now in the system. I will add that I changed the oil to 40 weight because it is 100 + here in Texas right now.

Any Help will be appreciated. George T.

[BOSOX]

Apologies, but what do you mean by runs away? The smoke after sitting for a few days or weeks is oil in the sump getting by the rings into the cylinder. When started it'll smoke like a train until it burns off. If the spark plug faces forward park the tractor with the front wheels a couple inches off the ground. A 2x6 plank or something like that. If the plug is rear facing prop the rear axle. That should keep the oil out of the cylinder. My S82 RER JD has that problem so that's what I do. Good luck.. :trink39:

[gtmotat]

I mean that it runs at high RPM's. More than at full power. It sounds as though it is going to rev up until it blows. Lok I mentioned it slows down and goes back to normal in just a short period of time.

[Tractor-Holic]

I just talked to a friend who got a similar Murray,and he put a 12.5 HP motor on in place of the original 12 HP Briggs--his was winding out too,only if you set the throttle at a bit past idle would it slow down some,and the govenor work properly,anywhere else it was ungoverned!..we discovered a bent linkage on the govenor that was rubbing against the other one going to the carb,after bending them slightly to keep them apart,it was back to normal..

I think what killed the original 12 HP motor was the govenor being so encrusted with sludge and oil/dirt mixture,it was sticking wide open a lot--either that,or his kid learned he could get it to go 10mph instead of 6.5 mph,by "holding that wire rod"..until it THREW a rod!..

I hear the 12 HP and 12.5's seem to grenade more often than other sized vertical Briggs motors,so its a good idea to ensure the govenor works and keeps top RPM's below 3600,even a bit lower probably would be better..

[Walt 2002]

"I hear the 12 HP and 12.5's seem to grenade more often than other sized vertical Briggs motors,"

Well this is a good case of don't believe everything you hear. The most common cause of any B&S throwing a rod is operator error, often gas contaminated oil.

Walt Conner

[ohiocowboy1277]

I have a old noma 4wheel steer mower with a 12.5 briggs I/C that once it starts runs great hard to start and if you shut it off when hot will start hard if at all but other than that runs good and if the hour meter is right it has about 1600 hours on it no knocks smoke or anything and it had the dirtiest oil i had ever seen when i got it along with a spark plug that i could unscrew by hand ! it wasnt even finger tight infact i think it was about 3 threads from coming out by itself lol that was the ticking i heard because of the compression blowing by the plug . been to lazy to clean the carb plus with my luck ill get it running good and it will blow up lol

[Tractor-Holic]

Seeing how the govenor linkage is easily bent on my friends 12.5 HP Briggs,I bet many of them get like his was, and run ungovrened,and eventually grenade--as ANY engine would..maybe the 12 and 12.5 HP's got a bad reputation for "blowing up" when it was the govenor linkage design that caused it more than anything..they probably are no "worse" than any other engine out there..

I have seen MANY older Briggs that looked like the oil was changed 10 years ago,and the air filter NEVER cleaned once--yet they still purr like a kitten!..some engines you just cant kill!..

had a few "high hour" engines pop though,AFTER I decided to be "nice" and change the oil,replace the air filter ,etc,though..probably all the sludge in them was all that was gluing them together,and loosening it up did them in..