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B&S 18.5 Intek won't start, backfiring through carb and exhaust

Discussion in 'Mechanic and Repair' started by WhiskeyTF, May 21, 2007.

  1. WhiskeyTF

    WhiskeyTF LawnSite Member
    Messages: 2

    A little history to start, this motor has only 70 hours on it. I just put it back together from fixing a seized rod journal. After a few hours of mowing the yard I was out front finishing up cutting the ditch (sidehilling) and the motor stopped abrupbtly with a bang and a clatter, after teardown I found the rod had seized to the crank and snapped. I've replaced both and cleaned it out, the wierd thing is I'd recently checked the oil and it was fine but I only found about half a quart in it when I tore it down, not enough to splash when it was on the sidehill. I also found the headgasket was blown into the pushrod cavity while I had it apart, mabye that's where the oil went? Plug was clean and dry, it wasn't smoking at all.

    It started perfect the first time after reassembly and after a quick adjustment to the governer (had to loosen it to get the crank and silly AVS weight doohicky out of the case) I had it purring along for about a minute. I shut it down to finish bolting on the cowl and fuel pump, and now it won't hardly run at all. It's getting fuel no problem, I've already tried pouring extra gas in the intake to no effect. It is also backfiring both through the exhaust AND through the intake. I've checked the valves are ok and adjusted, it is generating plenty of compression, spark is strong. Opened the case back up and verified timing marks align and everything in there was still kosher. I also went through the carb and blew out all of the orifices, no gunk or anything in there.

    What the @#$@# could it be? I'm about ready to roll this damned POS mower off a cliff. I'm going on 3 weeks of growth now, going to have to borrow the neighbor's tractor and brush hog. Help! :realmad:
  2. Restrorob

    Restrorob LawnSite Fanatic
    Messages: 11,029

    When you adjusted the valves did you rotate the crankshaft to position the piston 1/4" past TDC compression stroke ?

    Back firing through both carb. and muffler is a sign of valves not closing fully.
  3. khouse

    khouse LawnSite Bronze Member
    Messages: 1,465

    Did you get the flywheel key back in there? Just another thing to check. Restrorob is right about the valves. I thought I would throw a secondary cause at you.
  4. WhiskeyTF

    WhiskeyTF LawnSite Member
    Messages: 2

    Yep, as soon as I posted this thread I had an epiphany and went out to check the key as it was literally the only thing I hadn't revisited. I knew I put it in there, but gut feeling... pulled the bolt and found the flywheel about 90° out, key sheared. Not sure how it happened as I'd cranked the bolt down good with an impact. I sacrificed a square shaft screwdriver to the gardening gods and am back in business.
  5. khouse

    khouse LawnSite Bronze Member
    Messages: 1,465

    probably should torque that bolt and stick a new key in there

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