Sound: A four-stroke's exhaust note has a lower
pitch than a two-stroke's, probably because its noise-making power stroke occurs
on every other crankshaft revolution. Many times the four-stroke is also turning
at a lower rpm and is therefore not producing the high-pitched scream that is so
common with the two-strokes. This lower-pitched noise may seem quieter, but it
is not.
Without a muffler, .45 cu. in. two- and four-strokes make roughly the same
amount of noise: approximately 108 decibels (dB) measured 9 feet from the
engine. That is loud. With factory mufflers, both engines usually produce
100-102 dB, which is still loud but more common and therefore seldom intolerable
to most clubs.
Four-stroke mufflers are smaller than two-strokes' since the four-stroke exhaust
outlet is smaller. Scale modelers like the smaller muffler because its
diminutive size is less objectionable and easier to work into their realistic
airplanes.
Another commonality is that two- and four-stroke engines usually require muffler
pressure to the fuel tank. Some high-performance four-strokes are equipped with
fuel pumps or engine-driven fuel-pressurization systems that do not necessitate
muffler pressure, but most sport four-strokes are not so equipped. Use muffler
pressure at all times on these engines.
Click on photo to view large image with caption
Maintenance: Two- and four-strokes need
regular attention to keep everything working well, but four-strokes require a
bit more. The main difference is that the valve-to-pushrod clearance must be
adjusted. You must do this before first running the engine, and then again after
the first two hours of run time. Check the clearances every 10 running hours for
the next 50 hours or so; if there is no change, it is usually safe to extend
inspection times to 50 hours.
As does a two-stroke, a four-stroke "stores" a great deal of unburnt fuel inside
the engine after it is shut down. You must run the engine dry of this fuel at
the end of each day. There are two techniques to accomplish this.
Some engine experts favor keeping the glow plug connected and going to full
throttle while the fuel line is disconnected, allowing the engine to run dry.
Others prefer the same procedure but use a high idle instead of full speed. This
is safer and quieter. If you use the idle method, try to restart the engine
after it first quits in case residual fuel remains. But do not overdo it; the
engine has little or no internal lubrication at this point since most of the
fuel is gone.
After-run oil is essential rust protection for a four-stroke. Many good kinds
are available at hobby shops. Some experts prefer Marvel Mystery Oil, automatic
transmission fluid, or a 50/50 mixture of the two. Others like air-tool oil.
You must be careful; the petroleum distillates in these products could damage
the fuel-pump diaphragms or carburetor O-rings in some engines. O.S.
specifically warns against using petroleum products in some of its carburetors.
Pattern pilots fly more in one year than most sport pilots fly in several years.
Based on their extensive engine use, most use Mobil 1 Synthetic Engine Oil or
equivalent as their after-run oil. The synthetic oil has no petroleum content,
will not thicken with time, and seems to prevent rust better than most other
choices, even though it contains no specific rust inhibitors as far as is known.
Whichever oil you choose, use a "glue syringe" (available at most hobby shops)
to inject approximately 10 drops into the crankcase breather fitting, and put
the same amount in the glow-plug hole. Rotate the engine several times and
replace the glow plug. A few more rotations with the glow plug in place couldn't
hurt.
You should be doing this with all of your two-strokes as well, so this is not
extra four-stroke maintenance. The only difference is that the oil is dropped
into the wide-open carburetor of a two-stroke instead of into the breather
fitting.
If YS made your four-stroke, it will not have a breather fitting because the
crankcase is pressurized. In this case, drop the oil into the glow-plug hole and
the carburetor. With the carburetor facing upward, rotate the engine as
described. Never use petroleum distillate oil in these high-performance "wonder"
engines.
