Whenever I test run and adjust a new gas engine, I prefer to set it up on a sturdy test stand/table instead of on my model airplane. Here is my Zenoah GT-80, 80cc twin-cylinder gasoline engine before I installed it in my Fokker Triplane. The fuel tank (and smoke oil tank) are installed and the throttle linkage and choke linkage are also worked out. But to adjust the carburetor and select the correct propeller, this is how I do it.
I make a heavy-duty, open top box from 3/4-inch pine boards which I screwed together with sheet rock screws. I then added a birch plywood face drilled to accept the engine. I used 1/4-28 cap-head bolts (the same as on the model,) and the throttle linkage is also exactly the same geometry as in the model. This way I could fine tune the servo travel and end-points with my Spectrum DX18 transmitter. The servo and receiver and battery pack are all properly installed in the engine test box as is a 32 oz. Sullivan fuel tank. I installed a 2-line fuel setup and use a T-fitting and a fuel dot for filling and draining the tank. For this engine I am using smoke mufflers from Slimline Products and the engine is very quiet and the performance is excellent. No over-heating at all.
The engine box is attached to a sturdy picnic table with 3-inch-long Deck screws and is very secure. (My wife was not pleased so I recommend an old table you can use exclusively for RC stuff).
The box is also very easy to remove after testing and I keep it handy in the workshop. The birch “firewall” pad can be changed out to match different engines. For testing I used a variety of 24 to 27 inch propellers and I finally selected a hardwood Falcon 26×8 propeller with excellent results. The fuel is a gas/oil mix of 50:1 using Husqvarna Chainsaw 2-stroke oil. It comes in convenient 2.6oz. bottles ideal for mixing with 1 gallon of gasoline for the 50:1 ratio.
The engine has a starter spring attached to the aft end of the crankshaft so the GT-80 is very easy to start without using a big electric starter.
Starting procedure is: 1, close the choke and open the throttle fully. 2, Grab the prop and pull it clockwise to load the spring. 3, At the 2 o’clock position simply release the prop tip and the engine spring spins the prop. After about 5 or 6 tries, the engine will “bark” to indicate it has enough prime in the carburetor. 4, Open the choke, and set the throttle just above idle with trim full open. 5, The engine starts after about 3 more flips and it settles into a nice idle. Lowest reliable setting gave an idle just under 1,950 rpm and the top end without adjusting the carburetor was a smidge under 7,000 rpm. I used a digital GloBee optical tachometer to check the numbers.
After running two full tanks run through the engine and tweaking the carburetor it was ready for re-installation in my Balsa USA triplane. My test flight of the 1/3-scale Triplane (back in August,) was very successful and the engine performed perfectly not requiring any additional adjustment. I really like using this engine test box setup to adjust my engines, especially large gasoline buring beasts. Give it a try. And remember, safety first when ever handling and running any RC engine.
Here the engine is shown installed on my unfinished Triplane. Notice clearances cut for proper cooling.
Test flight day at the flying field. The GT-80 fired right up without a hitch!
After a few flights with various props, I found that a Falcon 26×8 propeller and the Zenoah GT-80 combo was just about ideal for my giant scale Fokker triplane.