• Glow Fuel:  I will add more to this section later but for now I simply want to mention that there are two types of engines in this category; 2 cycle  and 4 cycle.  The 2 cycle engines are more like a Suzuki while the 4 cycle engines sound more like a Harley.  I know, I know ..... Suzuki has a Harley knockoff but you know what I mean the ying-ying type vs. rooom-rooom.

• Gasoline Fuel:  My first gas engine came already installed on a 72" Sukoi-26, that I bought at the Weatherford, TX swap meet.  The Bomber Field Group from Houston had 2-3 tables of stuff and the owner of the SU-26 had sent this one along but was not there.  They gave me his name and e-mail address and they told me they thought it was a 35cc Ryobi weedwacker conversion.  My first lesson came when I couldn't get the engine to run and my second lesson came after I contacted the owner.  He told me he had just bought the airplane and had never flown it himself but his son had flown it a couple of times (likely story).  He had no information on the origins or operation of the engine.

At this same time I was administering the estate of a relative who had some acreage over Southwest of Ft. Worth.  I think that every time he had a problem with a weedwhacker he went and bought a new one.  There were five weedwhackers of three different brands (one of the worked).  So I set out to learn something about these small engines and I think I have become somewhat proficient.  In fact I have repaired over a dozen such engines now and thus far I have not been stumped.  Knock-knock- knock.  That was me knocking on my wood desk.  The first thing I learned was every small engine in the world (well maybe not all but certainly most) are equipped with a Walbro carburetor.  Furthermore, problems with these small engines is almost always a fuel supply or carburetor problem.  Especially if they have set around unused for a while.

• My Weedwhacker Engine Experience:  I am referring to my first gas engine as a weedwhacker engine simply because I don't know what else to call it.  There isn't a mark anywhere to identify it's manufacturer or origin.  It definitely isn't a Ryobi, however, it may be 35cc but the one time I had the head off of it I failed to measure the displacement and have wished ever since that I had.  I may pull that head just to measure it some day but it is running so good right now I don't want to mess with it.

1.      First problem:  This engine broke the mold and didn't fit any of my small engine experiences sited above.  I discovered the spark was intermittent and would come and go as I wiggled the spark plug wire where is exited the coil.  This is a magneto type ignition system.  I was led to this problem area by some epoxy glue that had been smeared at the wire exit point.  But of course the previous owner didn't know anything about that.

Solution:  I searched the internet and everywhere else I could think of until I finally found a lawn mower repair guy who was an RC’er.  We went though a whole pile of dead weedwhackers and chain saws and nothing matched.  We finally picked a coil off of a McCullough chain saw.  It wasn’t even close.  I had to fabricate a new bracket to hold the coil but in the end it worked great.  It turned 7,200 rpm with a 16X8 prop.

2.      Second Problem:  6 feet in the air, climbing out after a touch and go the engine quit.  The airplane stalled and I nosed it into the asphalt hard enough to destroy the McCullough coil. 

Solution:  This time I took a coil off of a Weed Eater brand weedwhackers, and low and behold it worked just as well.

3.      Third Problem:  After 50 or so flights I began having difficulty getting the engine to pull gas into the carb even when I choked it.  This would occur after I had let it set for a week or so.  I could pull the carb and blow it out, put it back on and it would be ok.  By trial and error I discovered I could limit the air to just the inlet valve and it would work.  Concluding the inlet valve was sticking I installed a complete carburetor kit but my problem still persisted.  I went to a forum on RC Universe  and got some suggestions, one of which was to try a purge bulb.  My carb had a port for a purge valve but it was capped off.  When I hooked up a purge valve and it wouldn’t pull gas I knew my problem was worse than I suspected. 

At this point my dear wife was encouraging me to go get a new engine.  But I’m in the hobby to scratch my technical itch and to learn.  And, after all what is an engineer if he is not a problem solver.  The more I worked on this carb the worse it got until I could no longer get it to draw gas under any condition.  I could prime it with a skirt of gas or ether and it would run off the prime but die immediately afterwards.  In the end it was http://wind-drifter.com/technical/wg8walbro.htm) that gave me the clue that triggered a solution.

Solution:  In the section on Metering valve they showed the following pictures:

An showed how to adjust the metering lever.  You can buy the gage from Walbro, or you can make your own.  If you download WalbroGage.pdf  you can print out an exact scale image of the gage (be sure to turn off page scaling when you print it).  If it does not print correctly, there is a ruler in the picture you can use to scale it.  The dimensions are also included.  

That little lever looked sturdy enough I couldn’t imagine it could be out of adjustment but when I checked it was at least 1/16th of an inch too low; meaning that the diaphragm would have move a lot farther before it opened the inlet valve.  Eureka, this was surely my problem.  When I bent the lever back to the correct height I was greatly surprised by the little amount of force required for it to bend.  Suddenly I realized my aggressive use of the air gun was my enemy.

When I put it all back together I am now consistently getting 7,800 rpm with a 16X10 prop, and I am at 80 flights and counting.