B&S 12.5 I/C won't start

Discussion in 'Mechanic and Repair' started by JeffLC, Jun 6, 2007.

  1. JeffLC

    JeffLC LawnSite Member
    Posts: 6

    Model 289707 Type 0154 01 Code 9411214E

    I replaced the connecting rod on this motor. Although I've never been inside this type of engine, everything seemed pretty straightforward. When I reassembled the motor, I made sure to line up the dots on the timing gears, get the governor back in the right place, etc. When I first started it, it ran well (a little white smoke) for about 30 seconds, then quit. It has not started since. Searching this forum, I found some other things to check:

    Spark: It has spark

    Flywheel: The key is in place and the crank / flywheel keyways are lined up

    Fuel: I've tried pouring a little fuel down the carburetor and putting a little down the spark plug hole. Nothing.

    I observed a couple other things that seemed a bit odd. When the timing dots are aligned, the piston is about an inch down in the cylinder. I would have expected it to be at TDC. When the motor is cranking, every 5-10 seconds, it will 'catch', then sneeze back through the carburetor and blow some vapor out. Thinking the valves might be out of time, I removed the muffler / pipe and cranked the engine while putting my hand over the exhaust port. Every few revolutions my fingers would get sucked against the port like it was opening the exhaust valve on the intake stroke.

    I'm stumped. I pulled the motor back apart and everything seemed to be in the right place. Any help would be appreciated.

    Jeff
     
  2. Restrorob

    Restrorob LawnSite Fanatic
    Posts: 11,022

    Sounds like you have some valves that are not closing fully, Worn valve faces/seats worn to the point the stem to tappet Clarence is too small.
    Could also be worn valve guides.

    Did you have the valves out ?

    If not, That could be the next place to go.
     
  3. khouse

    khouse LawnSite Bronze Member
    Posts: 1,465

    Have you tried a new plug? If the valves are correct as to Restrorob's suggestions you may need to check the timing marks again. Did you compare the new and old rod for length. I don't know how or if a different rod will hook up - but I thought I would throw it out there.......
     
  4. JeffLC

    JeffLC LawnSite Member
    Posts: 6

    I pulled the valves and cleaned some crud off one of them. The other one was pretty clean. I also cleaned the valve seats. I did not see any wear marks or damage on either the valves or seats. I also installed a new spark plug and confirmed the ignition system is sending fire to the plug. I double-checked the timimg marks before reassembling it. Sucker still won't start, and has the same weird things happening during starting. It 'hangs' every few seconds, then spits vapor back out the carburetor and also tries to suck my fingers in when I put them over the exhaust port. That seems like it would come from the exhaust valve not closing all the way, but I would expect to feel air blowing out on the compression stroke if that was the case. Any other suggestions would be greatly appreciated.

    My next step may be to replace the motor. If I go that route, how do I find one that will bolt in the same location, or should I just plan on redrilling the motor mount bolt holes in my mower frame and just get a similar size motor? The mower is about 13 years old.

    Thanks.

    Jeff
     
  5. khouse

    khouse LawnSite Bronze Member
    Posts: 1,465

    If the valve lash is correct then it must be the mechanical valve timing or lazy (stuck) exhaust valve. Take it apart one more time and take a digital picture of the timing marks and post it. Do you have a service manual for this engine?
     
  6. Restrorob

    Restrorob LawnSite Fanatic
    Posts: 11,022


    That's just it Khouse, There is no mention of checking valve clearance.

    We know this needs to be done to either fix the problem or eliminate that being the problem.

    Jeff, If you don't know; The piston must be positioned 1/4" down the cylinder past top dead center and a feeler gauge inserted between the valve stem and tappet. Intake should be .005 to .007 exhaust should be .009 to .011, That is with the valve springs removed and holding the head of the valve in place with your thumb while checking.

    If they are too tight you must grind a little material off the valve stem, But go SLOW and don't take too much off and exceed the max limit and grind them as flat as possible.

    If you did check the clearance what did you get ?
     
  7. topsites

    topsites LawnSite Fanatic
    Posts: 21,653

    This is something I've never understood, they all talk about TDC and how important it is,
    no doubt, but WHICH TOP part of the stroke is it?

    On a 4 cycle engine, these are the four things that happen:
    Intake, compression, ignition, exhaust (and this repeats itself).
    And that goes DOWN UP DOWN UP (in the above order).
    Notice the two UP's...

    So, are there not TWO instances where the piston will be at TDC:
    Once at the very end of the compression stroke and right before ignition,
    and again at the very end of the exhaust stroke and right before intake.

    So, could it be you got the wrong tdc, or is this not important?
    Wouldn't it affect which valve opens when?
    Would this explain why your fuel gets shot back out the carb and the exhaust is sucking?

    If this happens to be the solution, would someone care to educate repair manual makers over this, and get these fools to explain this crap to the layman once and for all, rather than just saying "Well it has to be tdc huhuhu" or is it just assumed that if you're this smart then you're smart enough?

    Because above explanation aside, I don't get it because how do I know?
    As of now and for many years, if a repair involves TDC, I don't touch it.
     
  8. Restrorob

    Restrorob LawnSite Fanatic
    Posts: 11,022

    Well EXCUSE me Mr. Topsites, I tend to leave a word out once in a while. Must be Ole age catching up or something. :cry:

    ALL valves are adjusted on COMPRESSION stroke !

    This is how my sentence should have read; The piston must be positioned 1/4" down the cylinder past top dead center compression stroke.

    Sorry for rocking your boat !
     
  9. JeffLC

    JeffLC LawnSite Member
    Posts: 6

    A picture of the timing gears is attached, with the dots aligned. I assume the 'dot' on the cam gear is the little dimple and not the big hole about a third of the way around the gear.

    I don't have a service manual. I've never had one of these apart this far until a few days ago. On a positive note, I've gotten really good at removing the motor from the mower and getting it apart. I can do it in 5-7 minutes now.

    As for the valve lash, I did not check it when I had the valves out, but since the the motor was running fine until I replaced the connecting rod, I don't have any reason to believe valve length is an issue. I bought the mower new and have never had any trouble with the motor until now. Also, I inspected the end of the valve and both ends of the tappet when I had the valves out and there was no noticeable wear. There was also no evidence the valves had receded into the head. Since it was such a pain to get the valve spring retainers back in (I don't have a valve spring compressor), I'm trying to avoid pulling them back out. If it turns out I really need to check the lash to rule that out, I will go by the auto parts store and get one of their loaner valve spring compressors tomorrow.

    I've always understood TDC to be on the compression stroke when both valves are closed.

    Thanks for all the help.


    Jeff

    P1010032.jpg
     
  10. Restrorob

    Restrorob LawnSite Fanatic
    Posts: 11,022

    The timing marks are fine, Why did you replace the rod ?
     

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