Cleaning the carburetor parts
Use carburetor cleaner and lint free cloths or paper towels to remove all built up brown or green gunk, varnish, and other contamination from the metallic parts. Pay particular attention to the machined passages and metering holes.WARNING: Carburetor cleaner is both flammable and the fumes are toxic. Do all cleaning away from open flames and outdoors if possible. Wear eye protection. The stuff will also eat plastics including some plastic eyeglass lenses.
Take care where non-metallic parts are still in place as extended contact with harsh solvents may degrade their properties (inlet seat and primer bulb, if present). Low pressure compressed air may be used to blow out passages but only use this on the fuel line from inside the carburetor body - else you may end up with the inlet seat clear across the driveway never to be found again.
DO NOT use wires or metal instruments to clear any of the passages as their size is critical.
The small hole in the hollow bolt on the bottom is most critical. Make sure it is cleaned down to the shiny brass and that this hole is unblocked and fully open:
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| | _ Hole in nut (approximately .025") - use carburetor cleaner
| | | | and wooden toothpicks to clear it out down to shiny brass.
| | | | DO NOT use metal wires!
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If you are absolutely sure there is no hole in the bolt (some models may forgo this), check further up on the central tube - there may be a tiny hole about 1/2" from the bottom. There has to be a hole somewhere for the gas to be sucked up through the carb!
I first use carburetor cleaner inside and out with cotton swabs to remove all traces of gunk from the inside. Use as many as needed till no more discoloration shows up. Then, use the broken end of a wooden toothpick or popsicle stick to clear the .5 mm diameter hole in the side. In severe cases, this hole may not even be immediately visible due to the varnish and gunk buildup.
If this hole is narrowed or clogged, the engine may start but then die in a few seconds. Gas enters the reservoir in the nut slowly or is forced in by priming but the normal suction cannot replenish it quickly enough.
Fine steel wool may be used on the float hinge pin if it is rough or there is evidence of rust but do not use anything abrasive on any of the other parts. Persistence with carburetor cleaner and cloths or paper towels should prove sufficient.
Inspect the inlet needle and seat. The needle should have sharp uniform edges and no visible damage to the conical tip. Any damage half way down the conical part - where it actually contacts the seat - will result in leakage and flooding. The seat can be removed if damaged by pulling it out with a hooked wire - careful - you do not want to scratch the body! If removed, do not reuse but install a replacement. The new seat goes in groove side first (lubricate with a drop of oil) and can be pressed home with a blunt rod.
If the throttle plate was disassembled, clean these parts with carburetor cleaner. Use a cotton swab to get into the bearing surfaces in the carburetor body.
DO NOT attempt to disassemble the carburetor beyond this point - the pressed in main fuel nozzle is precisely fitted and is not removable. The welch plug (pressed in disk) should not be removed unless you suspect contamination in the primer chamber (if any).
Carburetor rebuild kits are available and are economical where almost any parts need replacement.
(From: Jim Williamson (Willjim@gte.net).)
If soaking the carburetor in cleaner:
When you remove each part from the dip tank rinse it with warm/hot water (as hot as your hands can reasonably stand). The parts that have passages - force water through the passage. This does two things: (1) rinses the internal passage of the cleaner and any old junk (2) gives you a VISUAL check that water is coming out the other end of the passage. The visual check is the key here - you could use compressed air to rinse the passages but you don't see the exit stream. On a clean passage the exit stream will be nice and solid indicating no particles hanging up in the passage.