MegaTech R/C Real World Helicopter Series Uživatelský manuál stáhnout pdf (Strana 68)

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 elimi-

nate 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 manufac-

turing processes could be lodged somewhere within the meter-

ing jet tube. Remove the carb from the engine and carefully dis-

assemble 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 abra-

sive 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?"

thing, so install them as is; the grease will simply wash out dur-

ing 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 natu-

rally. 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 pur-

chased 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 combina-

tion 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 occa-

sion, 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 pro-

moting 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.

Mr. Gibbs, the seals on your replacement bearings won't hurt a

114 MDDEL AIRPLANE NEW5

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