An older brush cutter was not revving fast enough. After checked things like air filter, fuel tank and filter, spark plug, thoughts turned to the carburettor – is there dirt inside, are parts worn? It’s also worth checking the adjustment of the fuel mixture. Some other problem areas are carbon debris in exhaust port, a leaking crankcase gasket or seal (engine may have external oil residue), and worn piston ring/cylinder.

It looked like a carb. problem, and because it was an old machine I thought I’d start by fitting a rebuild kit (pump diaphragm, gaskets etc.). Having found the make and part number (stamped on the carb. usually), I found some information about it on the internet and located a source for the kit – on Ebay in this case.

This is the the carb. partly dismantled, a kit of new parts (usually costing around £5 – £10), and the “exploded diagram” – useful not essential. A couple of links to carb. manufacturers are here: WALBRO | ZAMA – much information on these websites. Another web page, “Basic Small Engine Repair“, appears to have useful information including photographs on this type of carburettor.

With the metering and pump parts removed, I used carb. cleaner and air from a foot pump to flush the fuel pathways within the carb. body. I have seen advice not to use carb. cleaner on this type of carburettor, also only to use low pressure air for flushing. The the cleaning fluid was only present for a short time in this case. I had also removed the idle and power mixture screws, which were reset to their factory setting (final adjustments made with the engine running). Moving the mixture screws (if fitted) should not be necessary on newish machines. However it seems that some Ryobi models, and others using Ruixing carburettors, do not start and run well with the manufacturer’s setting.

After several hours of use, the engine cut out at high speed and over-revved or hunted at idle. Luckily this was easy to fix – a blocked petrol cap breather hole resulted in intermittent fuel starvation. It was possible to dismantle and clean the cap components. Other causes could include a collapsing fuel line, or an air leak in the fuel line or carburettor seals.

Please take care if you’re working on machinery like this – there is a risk of a fuel fire, and other precautions should include wearing safety spectacles. It’s important to get both the petrol/oil and the fuel/air mixtures correct to the manufacturer’s specification, otherwise the engine may be seriously damaged.

Update, 2018: What’s under a Welch plug?
There may be one or more of these soft metal caps in a carburettor. They seal areas which have been drilled and/or machined. They can be removed for inspection of hidden areas, but a new one is needed since the old one is normally damaged in removal. They are a press/distortion fit, and a suitably shaped punch is needed to install a new one. This is easy with a circular shape, but a wooden “punch” can easily be made for other shapes. The photo below shows the area under the Welch plug of a Walbro WT249 carb (above centre):
This plug covers the low speed fuel pathway. Light can be seen through three small holes which go into the carb. barrel. These are for the idle fuel “jet” (top), and two lower holes (progression), which carry fuel as the throttle is opened from idle. Fuel is sourced from a fourth hole via a one way valve (brass insert with hole, see below). This valve feeds both the low and high speed fuel paths in this carb, others may have a separate one-way valve for the high speed circuit.

The fuel flow is constricted by the “L” mixture adjustment screw (lower right). It’s only necessary to remove Welch plugs if it’s though a jet hole is blocked or that there may be debris underneath it.

One Way Valves
There may be one or two one-way valves in carburettors like this. They let fuel flow one way and exclude air flow into the fuel paths. They are delicate, and a reason why only very low air pressure should be used to clean the carb.. The valve(s) can get stuck open or shut or be intermittent, in which case the engine may be impossible to start and/or power up from idle. Sometimes new valves are available, but if not a new carburettor is probably needed.