Stihl MM55C Tiller Fuel System Rebuild

 Ethanol claims another victim.  Small engines generally do not like Ethanol.  Cheap pump gas usually contains 10% to 15% Ethanol.  Ethanol eats the seals inside these small carburetors.  So, if you use cheap pump gas, and leave it in your small equipment continuously then eventually you will have a fuel system breakdown, and have to repair your small gas powered equipment.

In this case it is a Stihl MM55C Tiller.  It is a good Tiller, but I don't use it very often.  Generally I use it a lot in the springtime, then it sits there the rest of the year.  Well, I left fuel in it over that time.  One springtime when I went to use the Tiller it wouldn't start, and wouldn't crank either.  I try to pull the cord, and the motor wouldn't move at all.  Well, it had a hydrostatic lock.  There was liquid fuel in the combustion chamber, and it wouldn't let the piston move up.  So, I pulled the Spark Plug out to relieve the pressure, and it spewed liquid fuel out of the Spark Plug port.  Then I realized that a seal had failed in the carburetor, and was letting liquid fuel get into the engine.  Big Puppy Sigh...

So, this is pretty typical of small engines.  The equipment is kind of old, a few, or more years.  I had been thinking that I had been using "good" gas, Super, 92 octane, but it still has Ethanol in it.  Small Engine Manufacturers usually will have a compatible fuel available for their equipment.  Stihl makes Ethanol Free 2 Cycle Fuel for their equipment.  It is expensive, but is it worth the time, and energy needed to repair a damaged fuel system?  So, the Carburetor is well gunked up due to use, and years of sitting.  Well start pulling the fuel system apart.

Stihl makes small engines very well.  They have a clean design, and are simplified as much as possible.  They take service well, and I can get parts fairly easily.  There were three screws in the back cover, one screw in the fuel tank, one screw in the air cleaner, and two nuts held the carburetor on.  All the fuel lines are press fit, and there is some tricky part shapes that make everything fit together when the screws are in.  It is refreshingly simple.

The Fuel Tank is pretty icky, as usual.  I bought all the fuel system parts, so we'll be replacing the fuel lines, fuel filter, and the carburetor.  Generally I like to get a rebuild kit that has all the replacement parts for the carburetor, but in this case the only option was a replacement carburetor.

The Fuel Tank itself needs to be cleaned, not just to get the gunk off the outside, but also to flush out the old gas inside, and any particulates that have got into the Fuel Tank.  So the Fuel Tank get a full kitchen dish washing with soap, and hot water.  We want to totally flush out the Fuel Tank, and then let it dry out completely before re-assembling the Fuel System.

When re-assembling the Fuel Tank you can use some Silicon Spray to lubricate the press fitted Fuel Lines.  A little Silicon Spray will help the Fuel Lines to go on easier, and then the Silicon Spray will dissipate into Silicon Hoses with no problem.  It is certainly looking much better at this point.

Installing the new Carburetor is making my Small Engine look a little more complete.  I salvaged the exterior gaskets from the old Carburetor.  They seemed to be in marginal shape.  I used a little more Silicon Spray when I installed the Fuel Lines on the Carburetor.

 Something I noticed is this Carburetor doesn't have a Choke Valve in the Carburetor.  The Choke Valve is integrated into the Air Cleaner Assembly, and is all plastic.  I'm not sure of the reason why it is like this, but I'll assume it is another way to simplify the design, and possibly reduce cost.  It is neat, anyway.

Once it was all back together I double check my work, then gassed it up, and gave it a krank.  I spent a little time tuning the mixture screws, and my little MM55C is working again.  Lets go rip some dirt...