It’s been pretty dry around these parts lately, and I haven’t had the mower out in several weeks. The mower, you may recall, is an old Sensation with a 4hp Briggs and Stratton engine. It’s been kept in the shed since 1983, and though pretty dirty most of the time, it was still running well when I last used it. The grass was starting to look like it needed a trim, so I hauled the mower out and filled it up with gas. I was expecting a little trouble since the last time I used it, I had run it out of gas (which I usually try to avoid), as I knew there was some grass and debris in the gas tank. About 10 pulls on the rope later I realized that I had a little more work to do than usual to get her going again. Everything felt normal on the pull, you could hear the normal chugging of the piston, it just wasn’t firing up.

I have my little mower maintenance rituals I do every season, like changing the oil, checking the plug and cleaning it up, cleaning the air filter and oiling it properly. I like to sharpen the blade if needed, and I check the linkages and throttle spring. Once in the past I was burned by a cable that had gotten loose and was allowing the mechanical kill switch (basically a ground on the coil) to activate even with the deadman bar held down properly, so I usually check that first. Another time I simply didn’t have enough gas in the tank (it has to be high enough in the tank to start properly), so I usually pretty much top it off to be sure. None of my little rituals helped me this time, and I was getting frustrated.

I was never much of a gear-head, although I always wanted to learn more about engines. When the mower I’ve used for almost 25 years wouldn’t start, my first thought was not: “Time for a new mower”, which in today’s throw-away society is the generally accepted immediate response to a situation like this. I knew that this was a (relatively) simple engine, and that it was in good mechanical shape. It *had* to be something stupid that was causing it to not start. I didn’t want to abandon a good machine knowing that somebody with some experience, who spent just a little time looking at the problem would be able to get it going in a matter of minutes. Besides, it was a challenge.

You can see some pictures of the mower on my flickr page.

A neighbor of mine is good with old tractors and engines, but he was away for a couple weeks. Another neighbor offered her mower but I really wanted to get this machine running again and use my own mower. There isn’t much sunlight remaining at this time of year when I get home from work, and each night I would race home, grab my tools and try one fix after another before it got too dark to work. A couple of nights I worked in the garage, but with all of the junk we have still to rid ourselves of stored out there, and the overpowering smell of gas and carburetor cleaner, it’s not the best place to work.

Since I don’t have a lot of experience to draw on, each night I spent several hours on the internet pouring over texts and forums having to do with small engine repair, general troubleshooting methodologies, and specific Briggs andStratton hints and diagrams. Although there are a lot of moving parts involved, the basics of a small engine like this are pretty simple: Mix gas with air and ignite it in a chamber with a spark. The first thing to do is make sure you are getting a spark to the combustion chamber, then make sure you’re getting gas there too.

I wasn’t sure if I was getting a good spark, so I pulled the plug connector off and tried to watch for a spark to the plug tip as I pulled the recoil. I couldn’t see a spark, but I wasn’t sure it this was because I was shaking the mower, the distance across the gap was too great, or if it was just too light out. This led me to an investigation of the coil (called the armature on my Magnetron model) which is supposed to produce current as the magnets on the flywheel are spun past the conductors in the coil. I used my handy multi-meter to check the resistance across the main lead to the crank case - I got the requisite 5K Ohm which means the coil is probably OK (at least it wasn’t an open circuit). I checked the ground wire off the coil which leads around to the kill switch in the front of the mower for continuity and it was good, and it didn’t appear to be shorting out on anything. Finally, I replaced the spark plug and tried again. In the dark now I could clearly see a good flickering spark jumping over to the spark plug. I didn’t pull the plug and check for spark from the threads to the case because I didn’t have a good way to hold the plug, and I didn’t feel like getting shocked. The ignition was working, we had a spark - so why wasn’t the engine firing?

When I pulled the spark plug, it was wet. I was afraid that the residue on the spark plug tip was oil which would mean that oil had entered the combustion chamber somehow, but it smelled like gas. Was it flooding? If there was gas in the chamber, why wasn’t it igniting? I got some starter fluid and sprayed that in the chamber, still nothing. I had Cathy pull the rope while I put my finger over the spark plug hole, there was definitely good compression, I was afraid it would suck my finger into the hole! Frustration was starting to settle in good.

Three times (once each night) I syphoned out the gas, and removed the carburetor from the gas tank, disassembled the carburetor and attempted to clean the various cavities, jets, venturis, and pipes. My engine uses a ‘pulsa-jet’ carburetor type with an automatic choke. The carburetor sits directly on the gas tank separated by a vinyl diaphragm. The top of the tank is machined with several cavities, some of which end up with vacuum in them due to the presence of the diaphragm, probably for the purpose of pulling fuel up from the tank like a fuel pump. Two venturis (tubes with a narrowing at one end) also act to create suction to send fuel wherever it has to go, and speeding it up a bit (possibly vaporizing it a bit too?). Each time after assembling I would try to fire it up again with the air cleaner off. After the second attempt, I happened to notice that the action of the choke plate (a yellowish plastic butterfly valve which regulates the amount of air the engine gets) was not as springy as I thought it should be. I took special care to clean off the pivot points of the choke plate, and I inspected for scratches in the choke throat. The whole thing was pretty clean, and although I did manage to get some dirt out of there, I really didn’t find any varnishy gunk which could possibly have gunked up the works. One of the tips I had read suggested sticking something into the choke throat to hold the plate open (allowing more air to enter the mix). Doing this I managed to get the mower to fire for the first time! It ran poorly, chugged and fouled the spark plug with all the misfiring. Blueish smoke (not very much of it) was coming out of the muffler. The euphoria at having gotten the machine to actually turn over was quickly tempered by the knowledge that I couldn’t mow the lawn with a machinists ruler stuck in the air intake and chugging blue smoke everywhere. The last time I disassembled the carb I paid more attention to the position of the diaphragm, thinking that my last two attempts had created leaks against it because of poor positioning. This time I also disconnected a rod that connects the choke plate to the diaphragm and acts to hold the choke plate closed through the action of a spring on the underside of the diaphragm. I had left it all in place the first two times, but read that I needed to “set up” the choke plate properly, by holding the choke plate open, and connecting that rod, while jiggling it a bit. I also removed and cleaning the fuel intake line, discovering a bad gasket on one end (still not fixed). Once everything was back together again, I filled up the tank with NEW gas.

I set the needle valve all the way closed, then backed it out 1 and a half turns as specified in many of the troubleshooting guides.

The gas wasn’t old, only a few weeks ago I had gotten a fresh 5 gallon can of unleaded (as required) and had mowed with it once or twice with no problem. But, there had been some really humid conditions in the intervening time, even though we got no rain, and I was beginning to suspect that perhaps the gas can hadn’t been sealed properly or something stupid. I got a new can at Home Depot and filled it at a different gas station with Premium unleaded. Thus filled, I still had to manually hold open the choke to get it started. It was running crappy again, but once or twice it got up to speed! For some reason I decided to hold the mower in a position with the front wheels raised, and it started running better. It ran for a few minutes like that and I slowly started to lower it, little by little. Eventually it was sitting horizontal, and running well! I couldn’t risk it dying on me, so I quickly mowed the front and back yard with no problem at all.

The moral of the story here is to try fresh gas first next time.