Briggs & Stratton 12.5 I/C issues

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I used to fix lawn mowers in my high school mechanics class almost every day, this one stumps me though. The engine is on my neighbor's riding lawn tractor. 12.5 hp vertical shaft Briggs & Stratton I/C, probably from the late 1980s. When he brought it to me it wouldn't start. I replaced the plug with the correct Champion and set the gap. It does have spark. I also changed the oil with Rotella T5 10w30, and put on a new air filter. I took the Walbro carb apart and cleaned it. Next I put in fresh gas and changed the fuel filter. The engine ran roughly and was hard to start. I've cleaned the carb 2 more times, even soaked it in carb cleaner overnight. It runs a bit better but still doesn't start very easy and runs slightly rough at full throttle. I even ran a thin wire through all the jets when I had it apart. The carb looks very clean. I tried adjusting both the jet screw and the idle and it didn't do much. Any ideas? It runs perfectly smooth at idle and 3/4 throttle.
 
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if the flywheel moved then it would have problems all the time after it moved. And there's no reason for the key to ever shear. if you're confident it's not a carb problem or intake vacuum leak problem, pull the head and check the valve seats. I've had the valve seat come loose and spin around in its recess in the block, and that would miss and pop all the time. On any gas 4-stroke the problem you describe where it high rpms you have a miss if it's not fuel/carb related and not ignition related, then it's usually a valve problem. In this case it could be bad seats, and it may even be the cam I know B&S have used plastic cams in small engines whether yours has one I don't know. And I don't have hands on knowledge with the exhaust valve relief mechanism, but maybe look into that. I don't think it would be that though since you're fine everywhere except full throttle. I would consider pulling the head and giving it a wire brushing to remove carbon from around the valves.
 
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jeepman3071

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I just looked at it again and it seems like there is a lot of oil coming from the front of the engine, like in the head area. Would a blown/leaking head gasket cause these issues? It is still pretty hard to start even warm.
 
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Originally Posted By: 1 FMF
if the flywheel moved then it would have problems all the time after it moved. And there's no reason for the key to ever shear.
I respectfully disagree since I have had this happen to my own equipment and experienced the same symptoms as the OP. It was a partially sheared key, which threw the timing off ever so slightly...you can see it with a timing light on those that have marks. The only other indication I had of a sheared key or being off timing is the engine runs hotter...which most would not even take note of anyway.
 
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IIRC, a partially sheared/bent flywheel key would retard the timing. It would start OK, maybe even idle half way decent. Some would turn the idle speed up and forget they did so. Would be most noticeable at higher speed.
 

jeepman3071

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I've dealt with completely sheared keys but never a bent one. I will check that tomorrow. This motor is definitely tired and has seen some rough use. The guy has come to me a few times after it "ran funny" and it had almost no oil in it. The rest of the mower (1980s Simplicity) is in great shape. He said that if I can't fix it he might just get a new mower. I'm thinking a newer engine might be a better fix, since the rest of the machine is in good condition. The frame and other components of this rider are better built than our 2005 home depot special John Deere. I'm thinking a rebuild or new motor and the machine would outlast a home improvement store mower.
 
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You can get a suitable replacement engine for that machine fairly cheap. Try Small Engine Warehouse. They have tons of engines in stock and probably one that would drop right in. That Simplicity tractor, if in decent condition, is a gem worth spending a few bucks on.
 

jeepman3071

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Originally Posted By: boraticus
You can get a suitable replacement engine for that machine fairly cheap. Try Small Engine Warehouse. They have tons of engines in stock and probably one that would drop right in. That Simplicity tractor, if in decent condition, is a gem worth spending a few bucks on.
That is exactly what I did. Ordered a 13.5 hp briggs from Small Engine Warehouse. Upon talking to the owner, he was interested in keeping the machine instead of getting a new one, and with a new motor, it will be as good as new. thumbsup I'm going to service the mower deck, change the gear oil, and clean the machine up for him. On a side note, what is a good way to break this motor in? Should I just let him mow the lawn a few times then change the oil?
 
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For break in, I'd run it at lower speeds with no load until it warms up. When it's up to operating temperature, I'd put it to work at full throttle for five minutes or so then slow it down to half throttle for a minute or so then back up to full throttle for five to ten minutes then half throttle again. I'd do that for about an hour then just go to work with it. Varying the load is good for seating the rings and running in the bearings. Run the engine for the five hours usually recommended by the manufacturer then change the oil. If running in warm conditions, I'd go with any quality HDEO 15W40 and change it according to manual instructions or once a year. No need to go over the top with numerous oil changes for break in. That's absolutely unnecessary and completely wasteful.
 
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