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Lawn Boy F-series Carnage

5380 Views 9 Replies 8 Participants Last post by  Jimbird89
I know you guys love pictures, so I took some good ones for you. A guy brought in a 1991 Lawn Boy Commercial self-propelled mower. He said something in the recoil was bad because the engine would turn over sometimes, and other times it would lock up. Well, it wasn't the was the connecting rod. Depending on where the broken pieces floated around in the crankcase, it would either lock up or spin free. I tore it down for a rebuild today and it was NOT a good sight. I can't really salvage anything from the engine. I don't know why the rod broke, but I think it was an extreme case of hitting a solid object. The blade adapter was broken off the collar, the self propel pulley was split in 3 pieces and the center hole was much too large from bouncing around without the set screw being tight. There was a major blowout in the crankcase from the rod breaking. Bent reeds. The exhaust ports were carboned up so bad that it couldn't have been running well before it let go. The piston was scored, but came right out of the bore very easily and the rings were in good shape and free in the grooves, so I doubt it seized at any point. The rod bearings smelled like burnt popcorn. I am going to use another block, crank, and rod. All other parts will be new. I am the only guy in about a 75 mile radius here that will works on on old LB's, so the people that come to me generally want stuff fixed regardless of the cost. The dealers are beginning to shy away from the 2-cycles. It's a sad thing, so I have to keep the old machines going.

Anyway, here's the pictures. Be sure to look at the exhaust ports from inside the cylinder.


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The only thing I salvaged from this short block was the crank. It ended up not being bent. It's actually in good shape...somehow. It's a shame the jug half of the block exploded like that. The bore was almost perfect. The broken ends of the rod in this engine were actually polished smooth...not jagged and broken. No idea what happened in there, but it had to sound really bad. I've had a few blocks break like this, but as someone mentioned, usually it's the other half of the block. I had a Duraforce come in last year with a broken rod. It popped the carb mounting flange out of the block completely. The carb was bolted to the flange just sitting on the deck hanging by the fuel and primer lines. I have no idea what kind of mix was run in this F-powered machine. The guy that currently owns it bought it from a lawn service several years ago, and it has a LOT of hours on it. My Quietflite was run on SAE30 at 16:1 for 52 years and has nowhere near the carbon crap buildup as this '91! I had another partial shortblock from a '94 that I put together for this '91. Piston and rod from a V-series, original crank, new bearings, new reeds, new crank seals, etc. Should be good for at least a few years. I see far too many broken rods in F and E engines. Anyone every bust a rod in a C or D-series? They seem to hold up much better. Possibly due to the non-clamshell block design and less flex? I have a buddy who's a machinist and loves to make everything out of stainless. I wonder if a stainless rod would cause an imbalance in the engine if no weight was added to the crank counterweights?
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The more I work on these machines, the more I like the older engines. I learn the weak points of each design and quite frankly, the C and D engines don't really have many downfalls. Of course the F is easier to rebuild and has less components, but they just don't hold up to the abuse that the older ones will. A friend of mine who was a Lawn Boy mechanic from 1971 up until a few years ago was telling me how much he loved the C engines. He said the most common piece he had to replace on a C-powered mower was the deck. People would regularly hit a rock and toss it through the deck and not even bend the crank. He would just take the engine off, plop it on a new deck with a new blade and off it went...until the next rock. With 2-1/2 hp and roughly 4.5:1 compression, it's pretty hard to hurt a C-series. They were known to hold up well at 4,000 rpm much better than the D-series when used in snowblower applications. I have never heard of an F living very long at those revs.
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