Small Engine Saturday: B&S 550 Tune-Up Edition

Wherein I discover: “Holy crap, how is this thing still running?”

Three years ago, I acquired a “non-running” Bolens (MTD) 21" push-mower with a 158cc Briggs and Stratton 550-series (5.5 lb-ft) motor for free. He used it maybe 5 times and couldn’t get it to start again. With a small yard and power-company rebates he decided to buy a battery-powered mower and give away the Bolens. I took it home, gave it some fresh gas, cleared/re-primed the carb and it started right up. ¯\_(ツ)_/¯ I’ve been using it ever since on my half-acre lot with basically zero maintenance. It has been running a little rough, so I decided it was finally time for a proper tune-up.

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I purchased a new spark plug, new air filter, and a new blade at Lowe’s. I’ll get it all online next time, but I wanted quick returns if something was the wrong fit this time around. I also didn’t have a 13/16 spark plug socket, so I grabbed one of those too. Oil and funnel were already on hand.

The blade isn’t really part of the engine tune-up, but the original one was in such bad shape that it wasn’t so much cutting the grass as just beating it into submission. It also helps to get the blade off to access the oil drain plug on most of these mowers. While you have it off, it is best to sharpen or replace it. I have a sharpener, but this one was beyond hope for any more sharpening.

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The cosmetic shroud doesn’t technically need to come off, but it makes it easier to get to the air box and I had a hex-socket driver in the right size laying around, so off it came.

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Also, this should go without saying, but first things first - disconnect the spark plug before messing around underneath a lawnmower! Spinning the blade under there by hand or with a socket can very easily start it.

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I had to improvise to get the deck off of the ground enough to get access to the oil drain. I actually tipped it on its side this morning (after disconnecting the spark plug) to pull off the blade to see what model number I needed. But that won’t work for draining the oil. Two 5-gallon buckets worked as a nice make-shift lift.

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Drain plug sits next to the output shaft. Don’t forget to take out the oil cap/dipstick to vent the crankcase so the oil will poor smoothly out rather than glugging and making a mess.

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The drain plugs on these small B&S motors use a 3/8"-drive square. You just put an extension bar for your socket wrench right in and turn...

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...holy crap that is some nasty oil! That’s what 3 years on the original oil looks like. I don’t think I’ve ever seen oil quite this black. Also, it smelled like shit. SAE30 tends to smell worse than car engine oil in the first place, but this used oil was seriously rank.

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(Edit: Sorry for all the white space here. I have no idea what Kinja is doing. It looks fine in the editor.)

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Yick. Also, note to self for next time: my oil capacity is slightly above 12 oz.

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Got the excess out without making too much of a mess. I still can’t get over how black this stuff was. I can’t believe the motor was still starting at all.

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In with the new!

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This is what the black oil originally looked like.

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On to the spark plug. Wonder what it will look like...

Sorry for potato pic - my hands were too greasy to hold the phone steady
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...yep another reason I can’t believe this thing was still going. I have no idea how this thing was still making a spark.

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Here’s the new one - notice the lack of carbon. I just twisted this in by hand and gave it a little bit of torque with the socket wrench. This and the oil change should really help the motor run better. The new air filter won’t hurt either.

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Old air filter isn’t in the worst shape, but we may as well replace while we’re working. Notice evidence of carb cleaner in a can.

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The underside is worse, but most of that is just grass dust that can be cleaned off. I may wash this one and save it as a backup.

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The new air filter in place. This one requires a bath in SAE30 before installation. Luckily, I had a little left after the oil change to coat it.

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The blade and adapter can be put together loosely before crawling back under the mower. It all goes on with a 5/8" socket.

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Interestingly, the instructions with the new blade didn’t say anything about sharpening before use, so I just put it on as-is. Installation of blades on all of these mowers is similar: you get them hand-tight and they start to spin. I used a piece of scrap 2x4 to hold it in place with one hand and torqued it down with the other. There is probably a torque spec for these, but I just get them as tight as I possibly can.

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At this point it was all back together (except the shroud) and it was time to try it out. Will it start? (NSFW language, sorry. Also, apologies for the portrait video - it was just the easiest way to hold the phone and film this.)

Started on the first pull! But will it cut grass? I gave it a 5-second test...

...and it appears to be working fine. So now instead of mowing the lawn, I am going to go hang out at the pool. I put the shroud back on and put it away until tomorrow when my lazy butt will actually do some yard work.

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Soon.

Hopefully I won’t let this go for 3 years until repeating. The whole deal only took about an hour. Lessons learned: get a bigger bottle for the oil and having a second funnel for the clean oil going in would be nice.

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Also - it is very hard to actually kill a Briggs and Stratton motor.

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