Mountfield HP185: surging/thrumming idle

Hi,

I recently acquired a Mountfield HP185 which starts but runs with a surging/thrumming idle and I'm trying to get to the bottom of the cause.

I've removed and cleaned the spark plug; it was a bit sooty but not too bad, drained out the old gas and put some fresh in and run it for 2-3 minutes. I've removed and washed and dried the air filter. I've adjusted the metal plinth for the idle speed and this seems to have helped slightly but it's not running smoothly. I've removed and cleaned the plastic carburettor too.

What steps would you suggest trying next? Should I run the mower for longer to get the fresh fuel all the way through the line and carb?

Should I replace the spark plug anyway? Also I guess an oil change can't do any harm right?

Any help would be appreciated. Thanks

Link to YouTube video of the mower running. Any help would be appreciated.

The mower has had an oil change now too.

https://youtu.be/t23SUtvQu70

Surging is nearly always because of insufficient fuel for the volume of air.
So either the carb main jet is blocked or dirty, float level is too low, fuel lines restricted, gas tank not venting or an air leak between the carb and the mower or blocked bowl vent.
with the engine running spray around the carb ( but away from the air filter ) liberally with some WD 40 or similar FROM A TRIGGER PACK. NOT A SPRAY CAN.
If the engine speed changes or it suddenly blows a lot of smoke you have an air leak.
Run the engine without a fuel cap to check for fuel tank venting

Thanks for the reply. I cleaned the carburettor out again earlier, taking a bit more time and it's now running as it should.

One thing I noticed is that when I press the priming bulb there is a tiny spray being released out in the direction of the fuel line so there appears to be a leak somewhere. I couldn't see any obvious leaks though but I suspect it's coming from the panel the priming bulb is mounted to.

There is a gasket in there which you may have damaged when removing the carb.

Is there? I didn't see one other than the red rubber gasket where the (plastic) carburettor meets the air filter baffle.