O.S. Engines Pre-2000 Pumped Engine Carbs - Manual (Page 2)

If an engine that is broken-in will not come into the RPM range shown, you may have to try a different propeller.
Being slightly over or under the range is O.K. Modelers at high elevations may not be able to get their engines to
reach the target RPM range with the suggested propellers because of the low air density. High altitude will cause
significant power loss.
Once the full-throttle RPM and mixture are set, the idle mixture can be adjusted. Slowly retard the throttle. Once
the engine is below ½ throttle, the idle mixture adjustment will have effect. When the throttle is retarded below 1/2,
use the idle mixture adjustment to keep the engine running properly. It should NEVER have to be moved very far
from the center position. Once the idle RPM is reached, the engine should be able to idle indefinitely, with an
occasional clearing by “goosing” the throttle a bit. Idle RPM ranges are approximate, and will vary with propeller,
fuel and the type of exhaust system being used (muffler, tuned pipe, etc).
Engine
Idle RPM Range
.46 engines 2,600 to 2,800
.61 engines 2,600 to 2,800
FS-120 Surpass 2,400 to 2,600
After the idle mixture is set to give a good idle, run the engine slowly to full throttle and then back again. If the
transition is rich, lean the idle a small amount. If the transition is lean, richen the idle a bit. The engine should be
able to accelerate smoothly from low to high throttle, and then smoothly come back down as you move the throttle.
If you can operate the throttle smoothly or quickly, everything is set correctly.
After the carb is set, fly the model and carefully monitor the engine. As the propeller unloads in the air, the full-
throttle mixture may change a bit, and the high-speed needle may need to be adjusted a click or two, richer or leaner.
Once the carb is set, use this procedure to start your engine:
1) DO NOT change any carb settings.
2) Open throttle to FULL.
3) Choke as necessary (starter battery is NOT attached at this time).
4) Rotate engine backwards to clear any possible hydraulic lock.
5) Close throttle to idle, attach the starter battery, and fire it up.
6) Allow the engine to warm up about 20 second, at 1/3 to ½ throttle.
7) Advance to full throttle and check RPM.
Readjust the high-speed mixture only if the RPM is not where it should be. The high-speed setting should change
very little, if at all, from day to day. If the weather has had a major change (hot and humid to cool and dry), or you
make a change in fuels (10% to 15% Nitro), or change something else that is major (from a muffler to a tuned pipe,
prop size, field elevation, etc.), then your engine will need to be reset. Otherwise, you should not change the carb
settings much at all. Of course, you must continually monitor your engine so that it’s not being run too lean in the
air.
If, after all of the above, you cannot get your engine to operate in the proper RPM range, and you are not operating
at high field elevations your engine is probably in need of repair. At high field elevations you may have difficulty
with low-speed adjustments. You will probably have to live with a higher idle speed. The carb does not have a lot
of adjustment range at the low end, and therefore you may not be able to lean the low end enough to get an
extremely slow idle. If, not matter what you do, you cannot get your engine to run properly, first check a few
things: Make sure you are using fresh, good-quality 2-STROKE FUEL with 10% or 15% Nitro; a good-quality glow
plug (R/C long for 2-cycle engines, or O.S. “Type F” 4-cycle plug for the FS-120); and that there are no problems
with the fuel system (no debris, no air leaks, etc.). If all else fails, you should have your engine serviced. It may be
defective, damaged, or worn, depending upon circumstances.