Get off my lawn.

Friday, June 01, 2007

Mower Surgery

I had a little trouble with my mower this evening.

It's a Honda, the kind that always starts the first time you pull the rope. I've had it for about 5 years, and I had noticed it wasn't quite the same smooth-running quiet Honda Accord of a mower it was when new. Over the past couple of seasons, I had noticed the engine had taken on kind of a metallic "ping" sound at cruising speed. (I had put a "Type R" sticker on my mower based on the advice of a teenager in a Honda Civic, who said that doing so would add horsepower. I figured the pinging noise was just the extra horsepower making its presence known.) Not only that, but the life of a mower at my house consists almost totally of being ridden hard and put away wet. I change the oil sometimes, and try not to run it into walls, but other than that, my mower's not exactly something I pull out and wax every weekend. Besides, it's very easy to start. I figure if you can start a mower on the first pull, it must be in pretty good shape.

Except that tonight, it didn't start on the first pull, or any successive pull, up to about 100 of them. I pulled and pulled, and it just sat there, not even trying. There are few things as annoying as a pull-start engine that won't start. It's not like you can just sit there cranking it with a key, going "Come on... COME ON!" like they do in movies. You have to work at it, which is a pain in the butt.

Not only would it not start, but it was also making strange belching noises and puffing white smoke ever so gently out the exhaust pipe. I checked for spark, which it had. I checked for fuel supply, which it had in spades. When I removed the air cleaner and tried to start it, it was blowing air out the intake, and sucking it in the exhaust. Once, a ball of fire shot out of the carb. The only thing left that I hadn't looked at was the valve timing, which I reasoned must be hosed up somehow, since the engine was basically pumping air through itself backwards. "Valve timing" sounded serious, kind of a "take the engine apart" kind of thing. But for some reason, I felt like screwing with it.

I found something out on the internets that described the procedure for checking the valve timing. As it turns out, it's really easy: Take 3 nuts off the top cover, pull the cover off, spin the flywheel until the arrow on it points at one of the 3 mounting bolts (to arrive at TDC), remove the spark plug, pull the cam cover off, and make sure the two alignment marks on the cam drive sprocket are lined up with the edge of the block. It took all of 2 minutes to take it apart and check.

I noticed that the timing marks on the sprocket were on the opposite side from where they were supposed to be. So I slipped the belt off, spun the sprocket 1/2 turn to line the marks up, put the belt back on, and then fretted about the fact that I had made a 180-degree adjustment to something that couldn't possibly have gotten that far out of whack on its own. But cycling the engine through its 4 strokes by hand and watching the valves and flywheel, it looked like it was working the way it's supposed to work. Everything happened in its proper sequence when I looked at it in slow motion.

While I had it apart, I checked the valve lash adjustments. The nominal clearance on a small engine is about .005". The exhaust valve on this one was more like .125"! At TDC, it was hanging there loose, and would make a clanking noise when you moved it with your finger. I realized where the loud pinging noise had been coming from. The poor rocker arm had been beating itself to death.

So I adjusted the valves, bolted everything back together, (another 2 minutes), pushed the throttle up, and pulled the rope. A big cloud of black smoke came out the exhaust pipe, it spun up, and ran like a top. It was also quiet, like it was when it was new. I ran it around the yard in the dark, and it sounded like it was mowing (I couldn't see anything, but I'll know tomorrow).

I just told Craig and Doug today that I don't like tinkering with engines, but I really kind of enjoyed doing this.


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