All too often, I have found electric starters to be grossly in adequate for bigger engines, especially big four strokes and gasoline engines. Sullivan does make a starter thats made for this use but its expensive, very heavy, and usually requires 2 12 volt batteries . Not to mention having to charge it up. There is a better way
Converting a String Trimmer to a starter.
They are cheap, light weight and reliable. The difficult part is, String Trimmers run the opposite direction of an airplane engine. This is solved by using a gear drive. I happened to have a Magnum Electric starter laying around with a burned up armature in it. So I removed the gear drive and starter cone. I machined a coupler to adapt this gear drive directly to the centrifigal clutch on the trimmer. The next problem was making handles. I studied this for a couple of days looking for the ideal location. Once located, I built the handles. The throttle is mounted on the top handle for easy access using your right thumb for control.
The factory muffler was kept since it was OSHA rated. However, I added some aluminum wool as muffler packing and an extension to re-direct the exhaust downward. This lowered the decibels considerably and kept the exhaust fumes away from the operator.I managed to lower the decibels down to 80 from the original 83db. The engine starts on a single pull every time!
The entire mechanism is a retro-fit meaning that the "original" configuration is still intact and can easily be converted back into a string trimmer at any time. The engine displacement is 21.2cc and is rated at a little over 1 horsepower. This, with the 3 to 1 gear ratio provides ample cranking torque even with a centrifigal clutch. I have a 3" diameter hub for gasoline engines with big prop spinners. I will have to modify it a bit to mount to the Magnum Gear box.
All in all, this was a very easy project requiring more imagination than anything else. I bought the string trimmer dirt cheap and had all of the remaining materials laying around my shop. The ECHO GT200R Trimmer was particurarly suitable in that everything was easy to work with and it's the lightest and smallest of the ECHO trimmers. Echo in reknowned for reliability and durability... so this starter ought to last a lifetime.
I installed the 3" hub on the starter. The 3 to 1 gear ratio may not be low enough to start a really big gasoline engine but a 5 to 1 or lower certainly will. It's all about gearing...
This engine will tach about 8000+ RPM with no load, so if we assume it would drop to around 4000RPM under full load that would make 1333 RPM on the propeller with a 3 to 1 ratio. That's fast, but not much torque! Most gas engines will start nicely at 600 RPM. A 6 to 1 ratio would make 666 RPM at the prop and have massive torque. At some point I will make a 6 to 1 gear drive and try it out...
We tested the starter on a Saito 1.80 and the starter didn't even know there was an engine coupled to it . The starter flat spun the crap out of the 1.80... I then changed the cone to the 3 inch and tried the starter on my Zenoah 74cc twin cylinder. It started it good, but I had to rev up the engine prior to hitting the spinner. The compression on the 74cc engine was tough to overcome from an idle. Next, I tried it on a Zenoah 62cc single cylinder engine. The starter had no problem at all. The clutch on the starter engine engaged well enough to overcome the compression and spun the 62cc effortlessly.
The gas starter is a convenient and powerful setup. With very little modification, I'm certain I can get it to start virtually all gasoline engines well over 100cc. The trick is to raise the stall on the centrifigal clutch which is nothing more than putting stronger springs in it. I have about $120.00 invested in the project. A Sullivan Starter equal in power would easily exceed $300.00 and weigh 40 to 50 pounds including HD batteries.
After lots of starting on all kinds and sizes of engines, the Monster Starter is still working great! I did put a bigger carb on it to boost the power a bit and it's working excellent too. The starter has proven to be a valuable asset to my R/C tool collection. The 3" cone has proven to be the one of choice. It seems to fit the most spinners.