B&S 12hp I/C teardown

Discussion in 'Mechanic and Repair' started by kogafortwo, Aug 22, 2009.

  1. kogafortwo

    kogafortwo LawnSite Member
    Messages: 85

    I started tearing into the 19 year old B&S vertical shaft motor from my Snapper RER today. It had been bucking and stalling really bad after warm up and no amount of carburetor repair or other maintenance would fix it. I figured since it must have hundreds of hours of it by now, it was time for a tear down anyway.

    One thing that was out of whack when I took the PCV-valve cover off, was that the intake valve spring was off center and kind of cockeyed. This engine has never been torn down before and ran great right up until this spring, so here are some questions:

    1. How could this happen by itself?
    2. Could it explain the bucking and stalling, like locking the valve open after warm up?

    I will finish the teardown tomorrow and if I find anything really interesting I will post pictures. I want to diagnose my problem to the point where I decide to buy a new engine or get a rebuild kit (about $60 from PSEP so definitely where I'm leaning).
  2. Restrorob

    Restrorob LawnSite Fanatic
    Messages: 11,029

    1. Sure, There's nothing to keep the base of the spring centered fully.

    2. Most likely not.

    Is the carb throttle shaft tight in the carb body ? This style carb was prone to worn throttle shafts.
  3. kogafortwo

    kogafortwo LawnSite Member
    Messages: 85

    Yes, it's tight now that I replaced the shaft, seals, and felt washer along with the governor linkage and spring. That was one of the first things I did but it still did not fix it.

    Also, the needle, seat, and float are new and set properly, and all the small passages are clean. The ignition coil and spark plug are new also. They didn't fix it either. The air cleaner is clean, so is the inline fuel filter. The crankcase breather valve looks clean.

    So now I have to decide if a rebuild job with rings, valves, gaskets and seals is going to fix my problem, even if I didn't figure out exactly what's wrong with the engine.

    FIXDISS LawnSite Senior Member
    Messages: 445

    Does this engine have the old style air filter assy with the 2 screws in the top? One which goes into the carb throat and the other is offset ? If so there is a second adjustment screw under the air filter assy. that will take out the surging/bucking you experience under a load.Also you need to make sure you have no vaccuum leak at the intake manifold at the head. Just my half-fast opinion...
  5. kogafortwo

    kogafortwo LawnSite Member
    Messages: 85

    I finished the teardown today and started cleaning everything up. The two compression rings both have end gaps of about 0.040" at one inch below the deck. This sounds excessive to me.

    Restrorob or fixdiss, do you know the right ring gaps?

    I found Briggs' online spec sheet, but it doesn't list the ring end gaps and neither does their online help. Pretty lame if you ask me, becasue I usually think Briggs is a good company with good products.

    Fixdiss, yes this is the old style carb with the adjustment under the aircleaner assembly. Adjusting it didn't help.

    One more question: I did not bother to test compression in this engine before I tore it down becasue it is sol old (20 years), I figure it needs a rebuild no matter what. So if it did have really bad compression, would that make the bucking and stalling worse after warm up? It did start buring a little oil in the last year or so.
  6. Restrorob

    Restrorob LawnSite Fanatic
    Messages: 11,029

    Compression ring reject is .030".....Oil ring reject is .035".....

    Can you define this "bucking" ?

    RPM's going up and down ?

    Backfiring/popping back through the carb ?

    Misfiring ?

    Check the valves/guides for excessive wear, Grab the head of the valve and try rocking it back and forth. Worn guides can effect running as the engine warms.

    Replace the governor gear assembly no matter what but check the old one, The hole that slides over the camshaft wears and the pin holding the plastic gear in place. These can cause unstable operation and high speed RPM's......
  7. kogafortwo

    kogafortwo LawnSite Member
    Messages: 85

    Restrorob, in order of your questions:

    Bucking is when it starts to skip or misfire under load when it gets warmed up after about 10 minutes. It will run evenly until I hit a heavy patch of grass o anything to make the governor kick in, then it bogs down. If I disengage the deck or let out the clutch, RPM's pick back up and it will run evenly again for a minute or two. This gets steadily worse until the RPMs vary from wide open to dead stop and it quits.

    Yes, it seems to misfire occasionally. I am not sure about the backfiring through the carb. It doesn't sound like it.

    The exhaust valve moves side to side in the bore a few thousandths, maybe 0.010 or 0.15 when the valve head is about 1/2" off the face but the shaft is still fully engaged in the guide.
    The intake valve moves about twice that much side to side.
    By this I mean a tilting movement, rocking the valves in their guides side to side at the head. The valve stems themselves do not have a ridge or any wear marks, except for the discoloration of where the stems ride in the open and not inside the guides.

    Here is something different: look at the picture of the inside of the crankcase vent. There is a wear mark right where the intake valve spring lines up, like it was rubbing against it for a long time. What do you think of this? Is that normal? It is on the right side of of the valve in this picture.

    I want to decide if this engine is a tosser or if I should order the rebuild parts.

    Thanks for your excellent help.

  8. Restrorob

    Restrorob LawnSite Fanatic
    Messages: 11,029


    With that much slop in the valve guides it's a wonder how it ran at all !

    That explains the off centered spring and wear spot on the breather. The drop in power hitting high grass is most likely the hole in the governor assembly I explained earlier, Too much play in the governor assembly will cause it to malfunction.

    To properly repair this engine, The guides will need to be reamed over-sized and guide bushings installed. This requires special tools in which Briggs had their own tool kit years ago. Since this engine is so old I'm not sure if you can still get the replacement guide bushings and tool kit, I'm sure if I plugged your engine model and type numbers in my dealer's site it would show a OHV replacement engine and not even a shortblock.

    You may want to check into a new replacement engine or up-grade to a new unit.....

    Good Luck
  9. mowerknower

    mowerknower LawnSite Senior Member
    Messages: 766

    Is the carb throttle shaft tight in the carb body ? This style carb was prone to worn throttle shafts.[/QUOTE]

    Most likely the cause of the sloppy valves, engines are gonna get air from the easist place possible. If the throtte shaft is loose its gonna suck air(and dirt) through there doing more damage to the throttle shaft, valves, piston and cylinder.
  10. kogafortwo

    kogafortwo LawnSite Member
    Messages: 85

    Back when I used to work on cars a lot, I remember you could get some kind of tool to knurl the inside of the valve guides and raise up a lip of material. The valve stems would fit into them tight and you could get a few thousand miles out of a set of heads rebuilt that way.

    But that was iron, so I don't know if you can do that on one of these aluminum blocks. Plus, woudl it even be worth it?

    Maybe I am looking at a new engine. The rest of the mower and deck are good so I hate to throw it all away.

Share This Page