Eighteen months ago I wrote a little article about how to adjust the brakes on a lawn tractor because I couldn't find the information myself when I searched. It's turned out to be one of the most appreciated articles on the site, and I still get "thank you" notes on it regularly. In that spirit, here's a similar article for owners of Tecumseh engines on snow throwers (aka snowblowers) and lawn mowers.
The same thing has happened again. I found very little helpful information on this problem when I searched, so I'm writing it up.
Let me be clear first -- I like MTD products. They tend to be on the lower end of the price scale, but I find their designs are wonderfully innovative and simplified. They don't take an expensive product and cheapen it with bad parts. They find simpler ways to get the job done, and often they're really great solutions. I wish I'd kept my old MTD mower instead of replacing it with a Deere, but that's another story.
I have a Tecumseh 8hp engine on an MTD snow thrower from about 1993 (link to rebuilding project). Over the years, I've had to tear down and clean out the carburetor a couple of times due to the weird crap we've got in gasoline now. One year the whole thing was full of blue crystalline material. Once or twice it was just accumulated corrosion and there's always a little dirt.
Starting this fall I've noticed a small amount of fuel leakage and it's been running rough. It didn't leak much, but that's because a long time ago I'd added a cheap in-line fuel cut-off valve that I use when the unit is not in operation. I normally shut off the fuel at the valve and let it run until the carb is empty when I put it away to help keep the carb clean. Since I don't like to drip gasoline (and more important, I don't like to drop toxic MTBE) all over the place, I wanted to fix this leak as quickly as possible. I knew I'd have to take the carburetor off to fix this leak, so while I was at it I decided to clean it out and do the overhaul kit. It's not terribly difficult and as long as the carb is off the engine anyway you may as well spend $11 and replace the little seals and needles that wear out.
Here's where I ran into trouble.
I did the work, put it back together, and it still leaked. If anything, it leaked more. Fuel would seem to gather at the bottom of the fuel bowl (keep in mind, the picture is upside down, the fuel bowl hangs below the carburetor) and it would drip about one drop every ten seconds as long as there was fuel in the carb. Since the fuel is very clear and the bowl is that silver color, it was extremely hard to see where it was actually coming from.
I assumed for a long time that my leak was at the fuel inlet needle adjustment bolt at the bottom of the fuel bowl, or else the drain assembly (which is just a spring holding a seal in place). I must have taken it apart 20 times, make sure the seals were good, put it back together and added fuel -- only to find the leak was still there. What should have been a job of an hour or two at most, took all of one day and into the next morning. I searched on the internet for other people having the same problem, and the most I found was a comment from someone saying "sometimes they leak after you rebuild them." That's not an answer.
I finally found the problem by accident. I was draining the carb (again) to remove it (again) and happened to be looking very closely at it while pressing the primer bulb to get as much fuel out as I could (with the fuel line shut off). Under the air pressure from the primer bulb, I saw a very thin stream of fuel shooting out the bottom of the fuel bowl -- right near the nut, but not actually from there. Inspired by frustration, I cleaned out the bowl again and took it into a dark closet where I shone a very small but very bright LED flashlight into the bowl -- and discovered a couple of tiny pin-hole leaks right at the curve near the middle of the fuel bowl. Apparently time, vibration, and corrosion from the toxic mess they call gasoline now had thinned the metal bowl so much that these tiny pin-holes -- too small be seen without help -- could form.
So, the moral of this long story is -- if your snow thower, lawn mower, or other Tecumseh powered machine is leaking fuel, especially after doing a simple carb rebuild, it may be a similar problem
. When you clean out carb, you're actually making the problem worse by making the hole bigger. Keep in mind, these tiny holes are so small, that testing it by putting water in the bowl didn't help. Water has surface tension sufficient that without pressure it didn't drip.
Once I knew about the problem, I coated the outside of the bowl with some two-part epoxy, put a little crazy glue gel on the hole on the inside (both two part epoxy and cyanoacrylate are fairly resistant to gasoline) and re-assembled the unit. Leak stopped. Finally. I've ordered the replacement bowl and drain spring assembly for about ten bucks and will put them on when it warms up a bit. In the mean time, the seal I made should hold.