RE: The Complete Saito Notes by Bill Robison
Sealing the intake stack – tubing
You will select a tubing diameter that will fit into the recess where the Teflon seal was fitted for the choke plate, and cut it to length to be a snug fit between the carb body and the flange of the stack. The picture is an old FA-80, the white ring is the Teflon seal. Its outer diameter will be 11 mm or 7/16" depending on which rack you visit.
And a small o-ring around the outside of the tube would probably make it as good as you can get. I'll try the outside o-ring on my next one.
Even more on intake stacks, fuel selection, and mess on the plane.
All that oil on the airplane? Two things to start. First, while 20% oil is in “The Book,” 17-18% is more than enough. In fact, the nature of a four stroke engine is such that once the oil level is established in the crank case going down to 10% would probably be OK. I am not recommending you try it, might be expensive. Just as an example of low oil, Technopower says use a MAXIMUM of 5% oil after break in.
The second thing. Does your engine have an intake stack? A lot of the oil could be back spray from the carb, the stack will decrease this, if used with a Bru-Line fine mesh air filter it will be eliminated. Trapping the fuel that is otherwise lost will also increase your flight time on a given amount of fuel.
Now. Is the YS 20/20 fuel “Better” than Omega 15%? Up to you. You will get a small increase in power with the 20% nitro, at the same time you’ll also get an increase in the oil on the plane along with higher fuel consumption. To me the extra power is not enough to justify the higher price, less mess and better fuel economy are the frosting on the cake.
Diagnostics and tuning
If the compression is good and the bearings aren't noisy, don't take the engine apart.
If the compression is good and the bearings are noisy then don't pull the piston out of the cylinder when you take the cylinder off the case.
Bad compression and noisy bearings, then you have permission for a complete tear down.
Seeing the picture of your FA-120, it really can't be that old. It has not only the AAC cylinder, it also has the high tappet guides and the late type cast muffler just barely shows. I doubt it could be more than five years old, or around that at most.
It would not surprise me, if you showed the right side of the crank case above the mounting lug, to see "120S" in gold letters on a black background.
It's possible you're misjudging the engine. Remember the FA-120 is the runt of the litter when you get into the big blocks, it's the smallest of all, it's bigger siblings now go all the way to the FA-220, and even the FA-180 is not much bigger than the 120. So you might just be expecting more than it has to give.
Fuel burn: I see you have the intake stack on your engine. Adjusted properly you should get at least 13 minutes on a 16 ounce tank, and that's assuming full throttle the whole time. With "Average" power settings 16 ounces will give 20 minute flights. Truly, it's all in the carb adjustment.
Don't fret about the leakage from the push rod tube, you probably won't see any when you get the engine dialed in. I would suggest a close inspection of the carb and intake pipe, check carefully for any leakage there. Set the valves, my recommendation is set them both to 0.002" to get the best performance.