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How to Disassemble a Model Airplane Engine Carburetor

January 21, 2009



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Airfield Models (http://www.airfieldmodels.com/)How to Disassemble a Model Aircraft Engine Carburetor

The carburetor on a model aircraft engine is actually very simple.  Even so, a lot of people have problems adjusting them properly because they have no idea how they work.  Reading the instructions and studying the parts diagrams helps.

If you are not mechanically inclined, then I suggest you do not take the carb apart.  It is not difficult and does not require special skills, but if you are the kind of person who wedges a Phillips screwdriver in an Allen head fastener, then you probably ought to just leave this item alone.

Most carburetors are similar in construction but not identical.  This is a typical carburetor assembly, but yours is probably different in some ways.  Read the instructions that came with the engine, study the part diagram and use the proper tools.  Do not force anything.  If a part does not come out easily, then it is one of three things:

  • The part in question is not supposed to come out.
  • The part does not come out the way you think it does.
  • The part is glued in place by caramelized oil.  If this is the case, then a little heat will loosen it.  If that does not work, then leave it alone.  Contact the manufacturer or importer for guidance.
 
 

Disassembling the Carburetor

Note:  Information contained in the instructions supersedes information presented here.  Before disassembling the carburetor, be sure to review the instructions that came with the engine. 

A typical model airplane engine carburetor This is the front of the carburetor.  This particular carb is made by O.S. Max and is typical of most R/C carburetors.
Back of carburetor showing location of throttle-stop screw The back of the carburetor.  The arrow points to the throttle stop screw.  This screw has two purposes.

It limits the amount the throttle barrel can close and prevents the throttle barrel from falling out.

Never use this screw to prevent the barrel from fully closing.

You should always be able to kill the engine from your transmitter and using this screw to prevent the barrel from closing will also prevent you from being able to shut off the engine.

Remove the throttle-stop screw to remove the carburetor barrel This arrow points to the milled slot that the throttle stop screw engages.  The angle at which the slot is cut causes the barrel to slide in and out of the carburetor body as the barrel rotates.

The spring to the left of the barrel assists the barrel in moving properly.

High-Speed needle valve assembly On the opposite side of the barrel is the high-speed needle valve.  It is threaded directly into the spray bar assembly.  The needle is used to adjust the amount of fuel that can enter.

The nipple to the right of the photo is where fuel enters the carburetor.

The spray bar is a tube in the carburetor where the fuel actually enters the engine.  Most spray bars have a slot in the tube that atomizes the fuel as it leaves the spray bar.

High-speed needle ratchet is retained by a nut that also retains the spray bar This nut retains both the needle ratchet and the spray bar.  When this nut is removed, the spray bar body is pushed through the carburetor body and pulled out from the other side.

The needle valve should be treated carefully.  It has a precision ground taper and will not function properly if it is bent, nicked or has other damage.  It can be cleaned with very fine steel wool, but do not over-do it.

Spray bar shown inside the carburetor Another view of the spray bar.  This is another delicate piece and should be handled carefully.  Any damage at all such as nicks or bends will almost certainly cause erratic engine runs if the engine runs at all.
Remove the spray bar from inside the carburetor body The spray bar body shown on the same side of the carburetor body from which it was removed.

Although it is difficult to see, there is a hole in the spray bar toward the middle.  This is where the fuel comes out of the spray bar and into the engine.

Again, the spray bar on your carburetor may be attached to the body in an entirely different manner.  In some cases it is a press fit and is not designed to be removed except at the factory.

Low speed needle assembly Most carburetors have the low-speed needle opposite the high speed needle.  This is why the carburetor barrel moves in and out.

As the barrel closes, it moves into the body allowing the low-speed needle to enter the spray bar which leans the idle mixture.

Therefore the low-speed needle affects the mixture from idle to mid-range.  When the barrel is opened farther than approximately half-throttle, the idle needle has no effect the mixture is solely controlled by the high speed needle.

However, the high-speed needle affects the entire throttle range which is why it must be adjusted properly before setting the low-speed needle.

Low speed needle show removed from carburetor barrel The low speed needle removed from the carburetor barrel.  There is an O-ring seal near the head of the needle.

These seals are important so take care of them and replace them if necessary.  Air leaks will cause havoc with your engine and you will never get it tuned properly.

The completely disassembled carburetor The carburetor completely disassembled.  By the way, the O-ring at the bottom of this photo is hard as a rock. This engine is over 20 years old and has all its original rings.

This one must be replaced because it will not seal properly and will probably break with the slightest force.

 
 

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How to Disassemble a Model Airplane Engine
How to Assemble a Model Airplane Engine Carburetor

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Copyright 2004 Paul K. Johnson