I've uploaded this in case it helps anyone with a garden machine tool having a Ruixing H142R carburettor, fitted to some Ryobi models (and other "cheap" makes such as Homelite and Sanli use Ruixing carbs). The brushcutter was difficult to start when warm, and after several hours use it ran too slowly to be usable. On searching the internet it seems that many other people have the same problem - I believe the fuel/air mixture is preset too lean.
On this machine the idle (marked L), and fast running (marked H), mixture adjustment screws require a special tool known as a "pacman" style adjuster - obtainable for about £15. However, it is possible to cut slots in the adjustment screws to adjust them with a flat screwdriver. I did this using a mini cutter disc fitted to an electric drill, having removed the carb. from the machine. I adjusted first the L, and then the H mixture screws, and although not perfect the machine now starts better, and runs at a usable speed. Instructions on how to adjust the mixture on 2 stroke engines are commonly available. It may be illegal for users to tamper with this adjustment in some countries.
Having adjusted the mixture the engine does usually work acceptably most of the time.
I should say to please take care if you're working on engines - 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 March 2016:
The Ryobi tool has worked to some extent for a couple of years, but has been very reluctant to start, or sometimes impossible. On two occasions I've found that a piece of debris has located or "grown" in the spark plug gap - see photo below (I couldn't determine what it was):
I've stripped down the caburetter again, more fully this time - finding what looked like a piece of brush hair in the H (high speed) mixture valve. Maybe it'll run faster with that removed. There was also a sticky, wax like substance in the centre of the metering section "bowl", which I removed. I have probably left fuel standing in the tool for too long on some occasions.
Update August 2016:
The engine of this machine stopped working while using the hedge trimmer. On stripping it down I found that the bolts holding the cylinder onto the crankcase were loose, the piston was badly scored, the crankshaft bearing had disintegrated - in effect a disaster. I suspect the cylinder bolts may have worked loose some time ago, which would give poor starting and running performance.
1. If poor starting and running can't be fixed by working on the carburettor (and the plug, spark, fuel and air filters etc. are OK) - check the cylinder to crankcase fixing bolts and gaskets (check them regularly anyway).
2. Don't purchase "cheap" machines made by Ryobi, McCulloch, Sanli etc. unless they are intended for very light or practically no use!