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RPM
Using the needle in the carb helped, but not much. Frustrated, I
gave up; the Cub has been hanging in my garage ever since!
After reading your article in the November 2001 issue of Model
Airplane News concerning proper break-in, I purchased a test
stand, hoping to figure out the problem with the 1.48, but I didn't.
Before I ran the engine on the test stand, it had about 20 ounces
of fuel through it. I was finally able to get the engine running at
a rich four-cycle, but it took seven (!) turns to the rich side on
the needle valve. This seems excessive. I've tried the old trick of
using a small piece of fuel line around the needle valve to eliminate
air leaks, but that didn't work either. When I go from seven
to six turns out, the engine runs at peak rpm; there is no
adjustability. I've considered replacing the carb but have no idea
what to replace it with."
Tom, it's a shame that you've had all this trouble with the MDS
1.48 2-stroke engine; it seems like a good match for the Cub.
The first thing that comes to mind is, what size fuel line are you
using? You didn't say in your letter. Is it the large variety or the
medium? If the latter, I suggest that you change to the larger
size. Fuel-flow rates can exceed 2 ounces per minute at a rich
"four-cycling" mode of operation with this engine, and that's a
lot of flow for medium-size fuel line.
Another possible cause of the problem is a restriction within
the carburetor itself. A piece of flashing or a chip from manufacturing
processes could be lodged somewhere within the metering
jet tube. Remove the carb from the engine and carefully disassemble
it, looking closely at the jet and idle-needle
assembly/interface. With the primary needle valve removed,
blow air or force fuel through the spraybar with a fuel
bulb/syringe to see whether it behaves as if it's restricted. Try
running a piece of small diameter music wire, or a pipe cleaner,
through the spraybar.
Last, make certain that the carb is tightly sealed to the
neck of the crankcase; even a small air leak can lean a
fuel mixture in a hurry.
NEW BEARINGS FOR A MOKI
D. Gibbs emails: "I have a three-year-old Moki
1.8. It starts very easily, runs very reliably and
has plenty of power. In mid-flight the other
day, however, the engine stopped. The front
[ball] bearing had failed, and a small piece of
steel from the failed bearing had lodged at the
very front of the crankcase housing, between
it and the crankshaft. This caused only very
minor damage to the housing, just behind
the front bearing, and didn't damage the
crankshaft at all. Using 600-grit wet/dry abrasive
paper and some mild detergent mixed
Ball bearings for crankshaft
with water for lubrication, I was able to support—front and rear.
completely clean up the inside of the
housing. The large, rear ball bearing seems fine, but I figured
that while I had the crankcase housing unbolted from the
crankcase, I would change out both bearings. I had previously
acquired replacement bearings from a local supplier, but unlike
the unsealed originals, these are sealed. The new bearings are
good-quality units from SKF. Would it be all right to replace the
old bearings with the sealed units? Does it make a difference?"
Mr. Gibbs, the seals on your replacement bearings won't hurt a
114 MDDEL AIRPLANE NEW5
thing, so install them as is; the grease will simply wash out during
engine operation. If you don't like the slight frictional drag
that the seals produce, simply remove them. Pry them from
their seats with a tiny slot-type screwdriver (such as those used
by jewelers), or use a hobby knife with a no. 11 blade. Don't
touch (and possibly damage) the balls in the process (unlikely,
since you'll be careful!). You can then wash out the grease with
a solvent; I like to use lacquer thinner because it doesn't attract
rust-producing water from the air (caused by hygroscopic
action, as occurs with alcohol). Allow the bearings to dry naturally.
Don't use compressed air to hasten drying; dirt in the
airstream could contaminate the bearing assembly. Lubricate
them with Marvel Mystery Oil and install them in your Moki.
OLDER FOX ENGINES
Mr. B. Jackson of Burlington, Ontario, Canada, writes: "I purchased
four old Fox engines, including an early .78 with an
exhaust baffle coupled to the carburetor. Two Fox .36s and a Fox
.29 completed my find; all engines are new in the box! When I
told my fellow club members that I was going to use one of the
.36s in a Balsa USA Stick 40, they told me that Fox .29s and .36s
were hard to start, the carburetors were difficult to adjust, and
they really vibrate—especially in a lightly constructed model
such as the Stick 40. They also informed me that the Fox .29
and .36 require a minimum of 15-percent nitromethane in the
fuel. Can you tell me how much of this information is fact and
how much is fiction?"
Mr. Jackson, it sounds as though your Fox .29 and .36s are
equipped with cast-iron pistons and steel cylinders. This combination
required several hours of patient break-in before the iron and
steel were "bedded-in" and heat-cycle stress relief was achieved.
The break-in fuel was very important to the "iron Fox"; it required
a minimum of 25-percent castor oil (preferably 28 percent, with
a low nitromethane percentage—about 5 percent).
Although the Fox instructions were clearly written,
many modelers ignored them and tried to fly the
engine right out of the box; this never worked and
gave rise to the problems described by your club
members. In my opinion, this was not the fault of
the manufacturer or the engine. Properly broken in,
Fox engines are great runners.
In reference to Fox carburetors, I'd like to note
that Duke Fox produced more carburetor designs
than any other manufacturer during the '60s and '70s.
Another prolific producer of carb designs, SuperTigre, had to
take a back seat to Fox in this arena. On at least one occasion,
Peter Chinn (past engine columnist for Model Airplane
News) was diligently trying to explain the workings of a
recently released Fox carburetor while Duke Fox was promoting
a newer version in an ad in the same issue!
With proper break-in and fuel (5- to 10-percent nitro is
plenty), Fox R/C .29s and .36s are easy to start, smooth running
and very throttleable—if the owner follows Duke's detailed
instructions. As an added benefit, a properly maintained "iron
Fox" will run and last forever.
Klotz Special Formula Products Inc., 7424 Freedom Way, Fort
Wayne, IN 46817; (800) 242-0489; fax (219) 490-0490;
www.klotzlube.com.