Thank you as always for the useful replies.
I have checked the exhaust port which seems fine and I also ran it for a minute with no exhaust – still the problem persists.
I took off the carb and the main jet did indeed look blocked – cleaned it out and I could see light through it and thought that would solve it, but still no change.

Yes, I was going to ask about the govenor, as I read about this and noticed it when setting the points. Will try it under load (I have a big patch of stingers and brambles to try it on) and let you know if there is any change under load.

Thanks, Ben.

Just to update.
I took the carb off one more time to thoroughly check everything again and all seems well. Unfortunately, when re-connecting the copper petrol pipe I managed to strip the thread where it connects onto the carb.
I also changed the spark plug (used the once from my mk 1 clifford – has same engine as the allen scythe) and it seemed to run and rev much better despite petrol leaking out of the stripped thread!

So this brings up a few questions.
1. could a wrong or ‘bad’ spark plug stop the engine from revving properly?
2. What spark plug should be used in an Allen scythe – the one that it came with had k-17 on it, but the ground electrode looked very worn and thin.
3. What is the ‘fix’ for a stripped thread in the carburetor?

Many thanks

I tried using the carb from my mk1 clifford rotavator, but it still doesn’t rev properly. I also tried putting the machine under load; it’s like it gets quite close to going into that purring mode, but not quite. It does cut the brambles ok, but just doesn’t quite sound right.
I also looked at the govenor, but that seems to be fine. Would the points gap affect the revs? Does anyone know what the points gap should be? Any other ideas as to what the problem could be would be appreciated.