I'm worried about my O.S. .40 FX. It runs very hot and the oil in the
exhaust is coming out dark black. I acquired it second-hand, so there's no
warranty coverage on it.
I've been running SIG fuel through it, 10% nitro and 20% pure Castor oil.
The dark black exhaust has been coming and going. When I first ran the
motor a couple of months ago, I thought the exhaust looked too dark, but it
lightened up after a few tanks. Now I'm getting really black exhaust again.
I noticed that the engine still seemed very hot after it had been shut off
for several minutes, as well.
I have an O.S. .46 FXi with much cleaner looking exhaust oil that cools off
after resting for just a couple of minutes. The .40 FX is a completely
different story. Any suggestions on what the problem (if any) might be and
how I might service the motor to correct it would be appreciated!
The OS 40 FX is my favorite engine. I'm very much afraid you have
ruined yours. Let's go through your symptoms and their causes.
The engine is running very hot, because it isn't being cooled properly.
Cooling comes from two sources: the lubricant in the fuel and the air
flow over the engine. Let's discuss the lubricant first. If the engine
is run at a very lean setting, then it will get very hot, since it
isn't getting enough lubricant. The lubricant helps carry away heat
plus it reduces friction. The cure for the second problem is obvious.
I suspect the fuel and/or the needle valve setting is your problem. The
first thing I would do is get some fresh fuel. Make sure that it's at
least 18% lubricant. In general, some castor oil content is a good idea
to protect your engine against lean runs, but you don't need 20%. BTW,
the black exhaust is the Castor Oil getting cooked!
Next take out the needle valve and make sure there isn't any blockage.
Also make sure nothing is blocking the fuel lines. Any fuel restriction
will cause leaness. Next replace the NV and back it out 1.5 turns from
a seated position and start the engine. Let it idle and examine the
exhaust. Run the engine up a few times and note the exhaust. It should
have a thick cloudy mist which is a sign of being overly rich.
Now turn the NV in two clicks and run the engine up. Each time detect
the change in RPM. We aren't striving for the optimum NV setting but
just a point where the engine runs well enough you can fly. Now fly the
plane and see how it performs. If the engine is still too hot, you're
going to need a rebuild. The black exhaust should have disappeared with
a lower Castor content.
If everything is OK, you can tune the engine. Continue leaning it out
until it just stumbles and then back the NV out five clicks.
Regarding a rebuild, OS parts are fairly expensive, and I believe that
you'll be better off getting a new engine. With the new one, thoroughly
read the manual and follow the break in instructions - no flying out of
"Black" exhaust is rarely cooked castor, also occurs with straight synthetic
= aluminium alloy rubbing on aluminium alloy e.g. slightly movement in
vibrating muffler, use of starter motor pushing crank pin back against
backplate. Cooked castor is usually dark brown.
No 1 cure is to make a gasket from cardboard. Press a piece of grey card
from a cereal packet or similar against exhaust port and use hole punch to
cut the bolt holes and a row of holes across the exhaust slot - cut a
diagonal corner to corner across the slot. Bolt muffler to motor then use
sharp knife to cut excess card around the exhaust header = easier than
trying to cut a thin edged gasket before fitting. When motor starts, the
excess card in slot will blow out into header, swell with oil and prevent
the "Blacks". Another point of rubbing will be the prop drive washer when
prop overtightened, especially if the drive washer is not a firm fit to
For confirmation, refer to many of the links on my web page under motors,
Alan's Hobby, Model & RC Web Links
AT may have a hobby shop, but in general, he doesn't know what he is
talking about. Yes Castor oil is normally dark brown, but you're
running your engine in an extremely lean state with a fuel with
extraordinarily high castor content.
To prevent the leakage at the muffler-engine flange, just use some
I might also add that a carb teardown to look for dirt might be a good
idea. OS carbs are pretty easy to disassemble, but the have some tiny
parts which will be impossible to find in your rug.
Yellowish in it's unburnt state. After it has been through a model
engine at the high heats they cause, it comes out a varying colors of
brown depending on how hot your running your engine. Almost black is way
too hot. Way too hot is excessively lean.
Running straight castor lube is a good clue. The engine is running
hot because the cylinder wall is coated with a varnish composed of
cooked castor oil. The varnish effectively insulates the piston
from the cylinder wall and cooling fins, so the whole heat transfer
mechanism designed into the engine gets screwed up. It's a
self-sustaining cycle, as the heat causes more varnish buildup, and so
on. Pull the cylinder and give it a good scrubbing with a plastic
scotch-brite pad. If that doesn't remove the brown residue and leave
a bright chrome surface, use some fine steel wool and be sure you
abrade the cylinder wall circumferentially, not up and down. Do the
same with piston, which will probably be browned on the skirt, and not
near the crown. You don't want to remove any metal near the top of
the piston, so try not to abrade it. Reassemble (after washing with
hot soapy water), and use a fuel that has a lube blend of castor and
synthetic, with emphasis on the latter. Castor is a good thing in ABC
engines to prevent damage if you have a lean run, but just a few % is
AT is completely correct. In order to turn the excess castor to black, you
would have to be running the engine lean to the point of melting the
aluminum. Castor can easily survive sustained temps in excess of 800 deg F.
Your engine can't.