Saito FG-20 4-Stroke

BY:Mike Hoffmeister

Why to Buy
First, the FG-20 features an all-new, purpose-built Saito pumper-type carburetor, which is much more compact than a butterfly type pumper carburetor, making the engine easier to fit into a wide variety of aircraft types.
Also, with its efficient ignition system and a compact, purpose-built pumper-style carburetor, the FG-20 is lightweight enough and small enough to fit into most airplanes designed for .60- to .90-size two-stroke glow engines. The FG-20 will spin a variety of propellers, is easy to start and burns fuel at an extremely miserly rate. With the rising cost of glow fuels, it is more appealing than ever to choose an engine that runs on regular pump gas (with two-stroke oil pre-mixed). Fuel consumption rate is so low that your model can now use a 6 oz fuel tank yet still get flight times up to 15 minutes. The smaller tank offsets the added weight of the ignition system and gas carburetor. Alternately, you can use a larger tank in the model and get ridiculously long flight times!

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Break-In and Performance Testing

I mixed up a gallon of fuel, using regular-unleaded pump gasoline, with 5% (6.4 fluid oz.) of Evolution two-stroke oil. This gave the required 20:1 mixture as specified in the directions.

The directions suggest using a 15x6-in. propeller for break-in, so I started out with an Evolution 15x6 composite propeller. I would also later test the engine with a total of eight propellers, with results from all eight included in this review.

After securing the engine to the test stand and double-checking everything, I turned over the engine a few revolutions by hand, with the carburetor at half-throttle and my finger fully choking the carb inlet, until I saw fuel in the hose all the way up to the carb, then turned it a couple more revolutions. Then I switched on the ignition and used a Sullivan Dynatron starter to start the engine, and the engine fired right up. I kept its rpm below 4000 for about 10 minutes, as specified in the instructions. I had to make some adjustment to the high speed needle valve as it was set a bit rich even for the break-in running.

I then started to run the engine up to high power for short bursts, and after another 15 minutes I started to tune the carburetor progressively more toward optimum power settings. The instructions include a clear, step-by-step approach at tuning the carburetor. The engine easily held 1800 rpm idle without stumbling upon quick throttle opening, and it also held steady rpm at full-throttle.

Setting the new purpose-built Saito gas carburetor is a bit different than glow, and it is also different than dealing with a traditional Walbro® pumper-style gas carb, such as what is used on the Saito FG-36 and most two-stroke gasoline engines. With gasoline, when you deviate from the optimum air-fuel ratio, power quickly drops off whether you are on the lean side or rich side of optimum. With glow-fueled engine, there is a wider range of air-fuel ratios where performance is maintained. I found the high-speed needle to be quite sensitive to movement vs. a glow-fueled engine, and if you turn it 1/8 of a turn, for example, it is quite a large adjustment. While the instructions say to tune the high needle to run 200 to 300 rpm on the rich side of optimal, I found that I got better results setting only 100 rpm rich from optimal. If I set it richer, the engine would not clean out as quickly after quick throttle opening, even with the low-needle optimized. I did not find this to be a problem, but it is different than my previous experience, so it is necessary to keep an open mind regarding optimizing the settings vs. how you may do it on a glow engine. I found that in all of my testing, the high-speed needle settings were from two to four clicks out from the one and a quarter turn setting. With this carburetor, one click actually makes a noticeable difference!

The low-speed needle also took some time to optimize, as I did some experimenting to see how it felt with a variety of settings. Rather than just trying to very quickly achieve a best setting, I actually recommend doing some experimenting just to learn the response of this carburetor and adjustment to needle changes. I believe that some initial experimentation will give the user a better feel for what fine-tuning adjustments are needed to get the carburetor just right. Once optimized, I found the engine to idle extremely well and consistently, and it would take throttle well also. After quickly closing the throttle from several seconds at wide-open, I notice that the rpm will quickly drop to 3500 or so, hang there for a second or two, then drop back to the 1800 to 1900 rpm setting. At first, I mistook this as an indicator that I had the low-needle set incorrectly, however after testing other settings and consulting the Horizon technical folks, I learned that this is actually a sign that the carburetor is set correctly. A bit different than what I was used to, but it worked extremely well.

The engine sounds great, with a couple of the 16x6-size propellers, 12 lb of static thrust was achieved. After the initial break-in and fine-tuning of the carburetor, the engine started very quickly and responded well, and with no surprises. With the larger propellers, I found that I needed to set high speed needle slightly richer than with smaller propellers, but the difference was very slight, with only two to three clicks of the needle separating the smallest and largest load propellers. I did not change the idle mixture, but did have to increase the carburetor opening slightly to maintain idle rpm, when higer load props were used.

I measured the current-draw of the ignition module, when running on a 6-volt regulated power source. At 1800 rpm idle, the current draw was 0.32 amp, and at any power setting above idle, the current draw is 0.37 amp. This is a VERY efficient ignition module, and allows use of a smaller ignition battery pack compared to some other gas engines on the market. Saito recommends 1000 mAh minimum, which would give two plus hours of engine run time with sufficient battery power to spare.

With gasoline vs. glow, there is less cooling effect of the fuel, as glow fuel flow rate is much higher and the latent heat of vaporization of the alcohol-based glow fuel is greater — fuel is greater, providing a cooling effect of fuel vaporizing. These factors can tend to make a gasoline engine run with higher operating temperatures than a glow equivalent. For these reasons, it is recommended to assure that you have adequate cooling air flow, both to and from your engine, and also to use a metal engine mount to help wick some extra heat away from the engine crankcase. I had no issues with operating temperature whatsoever – these are just some best-practices to keep in mind! Also, while I have yet to install the engine in an airplane and fly it, I did get some advice from Horizon which I would like to pass on. While the carburetor is a pumper-type, it is (like a glow engine) affected by fuel tank position. It is best to keep the centerline of the fuel tank roughly at the same height as the carburetor needle-valves, and to keep the tank close to the engine with the lines as short as you can make them within practical constraints. This will minimize tuning effects of aircraft orientation of fuel level in the tank.
I also checked the sound pressure level using a digital meter, and measured 88 dB (with A-weighting selected) at a distance of about 10 feet, behind and to the left of the propeller. The sound is pleasant and much of the noise emitted is from the propeller. For future reviews, I will improve my sound level measurement method and standardize measurement location and height so I can start to provide some more useful figures.

Engine Hardware Layout
Disassembling your FG-20 is discouraged, but I wanted to lay out the parts for you, and also to get a feel for how the engine comes apart and goes together. The parts fit and quality is superb throughout. The base gasket tore upon disassembly, but I had a spare available so no problem there. To remove the propeller drive hub, you have to be careful to first remove the tiny set screw with alignment pin, as it passes through the split in the collet, and seats into a small pocket milled into the crankshaft. This is what maintains the orientation of the drive hub to the crankshaft, which is critical since the ignition trigger magnet is mounted in the drive hub (see photos). Once this pin is removed, it requires an appropriately sized gear puller and some moderate heat on the aluminum drive hub to remove it. Other than these steps, the rest of the disassembly/assembly is the same as the glow version, and was quite straightforward.

About the only engine maintenance required for the FG-20 is to occasionally adjust the valve clearance. This is documented in the instruction manual, with illustrations, and also a small kit of tools is included with the engine. As the engine breaks-in, the valve clearance can change slightly and re-adjustment of the clearance is necessary to maintain optimum performance. If allowed to get too tight, the valves may not fully close, allowing leakage and loss of compression. If valve clearance gets excessively large, there will be an increase in valve train noise, and power loss due to the valve not opening as far or staying open as long. This is nothing new for the FG-20, and is common to all four-stroke RC aircraft engines. Also, see the separate how-to article in this issue for setting valve clearances.

The Saito FG-20 performed flawlessly during all testing, however it did take some experimentation and “re-learning” regarding how the engine responds to carburetor setting adjustments and tuning. With this in mind, setup and tuning is straightforward and the engine is the most fuel-efficient power plant I have ever tested in this size range! Fuel cost is super-low, run-time per tank of fuel is twice that of a glow engine, and the lesser oil usage results in far less oil and residue being expelled from the exhaust and crankcase breather vs. a glow-powered engine. Even the ignition module is super-efficient, sipping only a third of an amp from the ignition battery pack. After seeing it perform, and hearing the sweet sound on the test stand, I can’t wait to fly the FG-20 in one of my models.

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