Every manufacturer of machinery uses a slightly different carburettor. Over the years, though, a few simple designs persist, and they all work more or less the same way. Even the most complicated ones share the basic structure of all carburettors. Instead of photos or drawings, we will use word-pictures, in the hope that the thing you hold in your hands will fill in the details by itself.
Because this is not a course in mechanics, and because you are probably reading this in a moment of agitated desperation, we will name and describe things as we progress on the job of looking to clean a carb. Note that setting (‘tuning’) a carburettor is outside the scope of this article, as it should never be needed under normal circumstances. Or did you drop your lawnmower’s suspension, with Mags and nitro? No? Then don’t mess with the carb’s settings.
Let us start by getting together a litre of paraffin, a bowl big enough to soak your carburettor in paraffin, and a little box for all the little parts that you will most assuredly drop and never find again. A pressurised can of carburettor cleaner fluid may come in very handy, or a compressor. Some clean rags; the Greenpets workshop uses cheap toilet paper to clean spills and protect sensitive bits. A paintbrush to clean with. Half a cup of soft acid. Citric acid solution is best, but vinegar does in a pinch. Something to make new gaskets with. See the Gasket Making section.
Clean up. Set yourself up on a flat, hard and clean surface, maybe covered in newspaper.
You will be spilling a lot of flammable and toxic fluid, get fresh air flowing around you.
Make sure you have plenty of light, especially if you are unfamiliar with the specific machine. Sometimes an adjustment screw looks like a mounting screw, and you end up screwing yourself by screwing around with a screw you should have eschewed.
With the paintbrush, or a stick, or compressor, get rid of the gunk packed around your carburettor. Do not touch one screw or bolt or link, before you have put an honest effort into getting the dirt off. The more you get off, the less chance you permanently damage your engine by getting dirt into the piston chamber.
Remove the air filter box. This usually includes loosening the two nuts holding the carburettor onto the block. As of this moment, the danger exists that you will get dirt into the intake, try not to move the carburettor yet.
With the air box out of the way, clean up some more, specifically with an eye on observing every linkage and connection on that carburettor. Everything is important, make drawings if you cannot take photos.
Carefully remove all linkages that prevents you from pulling the carb off the studs. Touch nothing you do not need for this single requirement: Get the carb off the studs, without using any force whatsoever.
The wires and springs are all made of steel, they have a degree of ‘give’. Do not bend and deform any of them, they are all precision parts!
The mating surfaces on a carburettor are absolutely flat and scratch-free. Do not pry a carb off by shoving tools into the packing gap/s.
If the gasket is stuck, soak it in some paraffin, and use a puller. DO NOT SCRATCH the faces! You MUST replace those gaskets, anyway. Do not throw the pieces away, you will need them to reconstruct the replacements, or as sample, should you have a parts shop available.
Keep track of every little part, put it in the box, buying replacement parts will be virtually impossible.
As soon as you have the carburettor away from the block far enough to shove a clean cloth into the intake manifold, do it, without dragging dirt in.
DO NOT SHOVE ANYTHING INTO THE CARBURETTOR.
DO NOT SCRAPE THE INSIDE OF THE CARBURETTOR FOR ANY REASON.
Once the carburettor is in your hand, wash it liberally with paraffin. Wash the area where it was mounted. It is very important that every surface is true and clean when you put that carb back. Wash everything a mile around the carb area. Get all chances of dust away from your workplace.
Your carburettor should come apart in two pieces; the main carb, and a small bowl at the bottom that seals onto the carb with an airtight gasket, usually rubber. Do not scratch, stretch or break this rubber seal. It may be reused, but it needs to be perfect.
The standard Fong Kong machine has a round bowl held onto the carb by the brass jet that screws into the bottom. You may need to remove a small solenoid first, held by two small screws. This solenoid is not a critical part (more in the electrical section) but it does form part of the seal at the bottom, so be careful.
The main carburettor, at the bottom, should show you a plastic bubble, often a half moon, held onto hinge post by means of a simple steel pin. Watch for this pin falling out and disappearing into the same black hole as your number 13 spanner and your favourite blue sock (just the one). The plastic bubble must swivel on the pin, to close the fuel flow when the bowl is full, much like in your toilet. Test this by blowing into the petrol inlet pipe. You may prefer using a short blast from your carb cleaner spray can. When the plastic float hangs, the fuel must flow freely, when you lift it gently to the top, the fuel flow must be cut off completely. No obstructions, no leaks allowed.
Carefully remove the steel pin, and take out the float for inspecting the “needle-and-seat”.
Hooked close to the swivel on the float, you should see a small rod, with a tiny spring behind it, and the end is capped with a pointed rubber bit. You are looking at the “needle”, and the rubber tip seals onto the “seat”, regulating the flow of petrol into the float chamber. If the needle does not perfectly seal onto the seat, fuel keeps coming in, overflowing into the carburettor, flooding the engine and causing a fire hazard. These can be bought from most auto-spares shops, they come in standard sizes. It slides off, don’t bend it, and DON’T SCRATCH THE RUBBER TIP.
Carefully put the carb down in a bowl and cover it with clean paraffin.
Inspect the bowl. Is it shiny bright pretty clean? If you find a sludge, you have probably found the source of all your woes. Rinse the cup with paraffin, leave to dry.
Gently clean the rubber seal with paraffin, and check that it still fits the edge of the bowl. Put both safely aside.
While the carburettor soaks, you may as well start on your gaskets. Make new ones as per the Gasket Making instruction set elsewhere. If you are going to buy some, take the samples with you. The carburettor will not dissolve, let it soak, go get your gaskets.
Looking at the bottom of the carb now, you should see a largish brass insert. It may have a slot for a screwdriver, a big, fat, straight screwdriver. If you can blow cleaner into this thing, and it comes spurting out inside the carburettor with no backflow, you can leave this jet be. If your nozzle sprays back at you, the jet is blocked, a serious problem. Use a large flat screwdriver to extract the jet, and soak in vinegar for five minutes. Blow again, and see if jet is open. NEVER TRY AND BORE THE HOLE OPEN WITH WIRE OR TOOLS. The tiny hole in the end is a precision part, vital for the correct running of your engine.
On the carb intake, choke side, you should see some small holes arranged around the large intake port. Look for the ones with a brass insert, and use the nozzle of your carb cleaner can to blast through there. The spray should exit cleanly into the carburettor throat, via tiny little holes near the one where you see a sharp point stick out the carb wall. That jet should also spray out some cleaner.
Try blow cleaner through every hole you find. Experience and manufacturer-specific manuals tell which ones are important, the rest of us try to clean anything that looks like a port.
With your carburettor shiny clean inside and out, start reassembly.
Take note of the rubber seal on the float chamber, they are long and soft and wiggly, and often fall out of the little groove, to be squashed and damaged forever by the tightening of the chamber screw.
Taking care, replace each gasket with the correct one. Note that the little ports at the intake usually requires the gasket to be specifically shaped out.
It is often helpful to remove all and any panels, tanks or thingamajigs that prevents you from looking at the carb from every angle when refitting it.
Replace all linkages, refer to the photos you took earlier. You didn’t? Take the machine and every single bit part and screw to a mechanic, or get ready for hours of playing at a surreal form of five-dimensional Mechano®. Do not fool yourself by fitting the carburettor first, you will need to turn and twist every little thing a bit to get them back in. Experience is of help.
Once all the linkages are back, hold the carb firmly in place with the bolts, finger tight only, and test the operation of the choke, accelerator and governor linkages.
The temptation to start the motor without filters may be overwhelming. It is very satisfactory to pull the rope and the engine starts beautifully, and you stand back a bit to admire a difficult task well completed, and there the thing sucks a wad of rubbish into the intake and you can hear the tappets clattering as that rag carbonises right on the valves’ sealing surfaces. Or a wad of grass, paper, anything….
Before you put the air box back on, make certain all the gaskets are present and in the correct order and orientation.
REFIT THE AIR BOX AND FILTER/S BEFORE STARTING.
If this does not solve your problem, it may be you need to service the valves. Leaking valves can make it difficult or impossible to start an otherwise healthy motor. See the Cylinder Head section.
One simple reason for failure after servicing the carburettor, is a poor seal on the gasket between the carb and the metal piston intake. If it sucks air there, it might not suck enough fuel/air mixture through the carb.
The most common failure of the carburettor after service, is forgetting to open the fuel tap. Allow a few moments for the float chamber to fill up before you start.