How to Assemble a Model Aircraft Engine Carburetor

Now that you have your carburetor taken apart and cleaned, it needs to be reassembled.

The last article did not discuss how to clean the carb, so I will provide some guidance here.

• Do not use anything inside your carburetor that can scratch, gouge or otherwise damage it.  Use toothpicks, bamboo skewers and cotton swabs.

• I use denatured alcohol for cleaning carb parts.  After they are clean, I put the parts in a basket such as used for making tea and rinse them over a pot.  That way if anything falls out of the basket it will not go down the drain.

• Dry all part thoroughly and immediately to prevent rust.

• As soon as the parts are dry, oil everything.  I like sewing machine oil because it will not damage O-rings or other types of seals.

• Keep everything sterile.  Once you start cleaning, crud may come loose that can clog the jets in the carburetor.  Be sure to remove all dirt and lint.

• When reassembling, do not turn the needles all the way in.  Thread them in a couple turns and wait until the carb is fully reassembled to reset them. This will prevent damage to the spray bar if they are turned in too far.  More about this later.

OK, let's put it back together...

Assembling the Carburetor

Note:  Information contained in the instructions supersedes information presented here.  Be sure to review the parts diagram and instructions that came with your engine before assembling the carburetor.

	A light oil such as sewing machine oil is a good choice for lubricating and protecting carburetor parts.
	The first item is putting the spray bar back in the carburetor body.  In this case there is a flat on the spray bar that keys it to the body. The spray bar is oiled well and then pushed through the body until it keys in place.
	The high-speed needle ratchet and nut are replaced.  This nut also retains the spray bar. I have also threaded the fuel nipple back on the body. It is important that you develop a touch for how much to tighten these items.  The carburetor body is soft aluminum and easily stripped. Items should be snug, but not over tightened.  The nipple has a seal that will spread slightly as the nipple is tightened.  You do not need to completely flatten this seal.  Just snug is good.
	The low speed needle is threaded back into the carburetor barrel.  This needle engages the spray bar as the barrel is closed.  If it is threaded in too far and then the barrel is forced closed, it is likely the spray bar will be damaged.  For now, just thread it in a couple turns.
	The spring is replaced on the barrel.  The throttle stop screw retains the barrel in the carburetor body.
	Put the barrel in the body and rotate it so that the slot in the barrel is toward the back and up and so that you can see straight through the carb opening (full throttle). Insert the throttle stop screw.  If it does not engage the slot you will know it because it will jam the carburetor barrel and prevent it from turning.  Be careful with this step so that you do not gouge the barrel. The stop screw should be threaded in so that it allows the barrel to fully close.  Move the barrel from fully open to fully closed.  If it jams at either extreme, adjust the screw so that the barrel can move smoothly. Tighten the lock nut.
	The throttle arm and retainer nut are replaced.  With this particular carburetor, the low-speed needle needs to be threaded in far enough to allow the nut to tighten. Once the nut was tight, I unscrewed the low-speed needle until it hit the retainer nut.  Again, this is to prevent the needle from damaging the spray bar. Next, I closed the throttle fully and turned the low-speed needle in until it just started to feel snug.  From there I turned it out 2 full turns.  That will be the starting point for the needle when I run the engine.  Undoubtedly this setting will be too rich and require further adjustment but the engine needs to be run to determine this.
	The high-speed needle is replaced.  It is threaded in until it just becomes snug and then turned out 2-1/2 turns.  That will be the starting adjustment for this needle.

Again, further adjustments need to be made after the engine is run.  There is no way to determine what the actual settings should be without running the engine.

This is why you should not disassemble a carburetor unless you need to.  Once the needles are set properly, it is foolish to disturb the settings unless absolutely necessary.

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Copyright © 2004 Paul K. Johnson