Magnum Engines in RC Combat
I've been using Magnum engines in RC Combat since the 2002 season. I find that they are generally
powerful and reliable engines, however, the carbs require some specific fixes to make the engines run
reliably. Not surprisingly, the fixes are almost exactly the same for both the .15 size engines used in
A class and SSC, and the .25 and .28 engines used in 2610 and B class. The carb designs are very similar.
The carbs are actually designed fairly well, however they are not built as well as they could be. As a result,
they will leak air, and shed parts in the air if nothing is done to prevent these problems.
- In the two pictures, notice the two screws coming up from the top of the carb. The screw on the left
with the but on the bottom is the carb barrel retaining screw. The one on the right with the spring is the
throttle stop screw. Back both screws out, and apply a generous amount of thread lock.
- When putting the throttle stop screw back in, make sure the throttle barrel can close completely.
Combat rules and safety require that the engine be able to be shut down from the transmitter, so it's desirable
that the carb close all the way.
- When putting the barrel retaining screw back in, put it in just far enough for the carb barrel to operate
smoothly. If it is too far in or too far out, the barrel will bind, or will not move though full travel smoothly.
- Next, remove the high speed needle, and place a small gasket of standard fuel tubing over the needle. You can
see in the pictures that it gets compressed between the needle and the housing when the needle is screwed in. This
fixes air leaks around the needle valve.
- Place a bit of large diameter fuel tubing over the low speed screw. As you can see in the pictures, the low
speed screw actually sticks out just a little from the housing around it. The fuel tubing extends past
the end of the housing. This causes the fuel tubing to actually come in to contact with the low speed screw.
The tubing prevents the low speed screw from moving due to engine vibration, and it prevents air from leaking
in to the carb.
- The Magnum .15 carb mount on the crankcase is a bit weak up front. It is possible that a crash will cause the carb
to be pushed back and break out the front of the carb mount on the crankcase. To prevent this (or fix it after it has
happened), apply a generous fillet of JB Weld. You can see the application in the second picture, just in front
of the carb.
- The Magnum .15 carb is mounted with a draw bar arrangement. Before mounting the carb, back the nut off, and apply thread locking compound.
- The Magnum .25 and .28 share the same carb. The carb is held in with two 3mm screws. Toss out the screws that come with
the engine and replace them. Ace hardware stores cary a screw that is a perfect fit. These screws fit better and are slightly longer, giving
a much more solid mount.
- When installing the carb, be sure to push down hard to compress the O-ring and avoid air leaks.
- On the .25 and .28, be sure to apply thread lock to the carb mounting screws, and snug the screws down.
- The Magnum .15 comes with muffler mount screws that are garbage. Much better quality cap screws are available
from places such as Microfasteners.com. They are 2.5x25mm.