Its a mystery to me

I very much doubt there are any mechanical problems, this is probably two issues with ignition and fueling, The first was likely to have been fuel restriction followed by a plug fouling up due to choke staying on. Taking the motor apart will just delay getting to the problems.

Very frustrating, check mag earthing brush, and pick up track,a bit of breathing thro the mag will oil the points too. Check fuel cap vent,tank and carb filters and by pass outlet in carb (engine side of slide), fouled plugs cannot now be reliably cleaned as insulators absorb stuff as manufacture changed to non glazed a few years ago. Water in fuel can now also kill plugs for good. New fancy plugs now tend to come with resistor built in,no good with a mag. Running out of ideas!, Flog bike and Buy a Rudge!, My Ulster runs so clean that plug looks new, and thats a worry , Its the total opposite to my 99 and they are both resisting all my efforts to carburate properly .The 99 can switch from weak to rich on a daily basis. I don't trust the fuel to be consistant ,getting paranoid now.

You have all the symptoms of fuel starvation. As Roberts says, check the vent in the fuel filler cap first - it's surprising how often this can be the cause. Often blocked by polish. Then make sure fuel is getting from the tank to the carb. Does the petrol tap have an in-built filter? How about the filter where the fuel line enters the carb. Undo the bottom nut on the carb. Is it full of crud or water? Turn the petrol on. Does fuel pour out of the carb? If fuel is getting that far successfully, take out the jet block and check the main jet for a blockage. Once you have done all that, the bike should run. If not, then contemplate spark plugs etc.

This sounds like it coould be an ethanol issue. A while back one of my bikes was running fine and the next time I took it out it ran like a dog needing choke all of the time. I stripped the carbs and they looked OK but on close inspection I found tiny specks of black 'rubber' in variuos nooks and crannies - this black stuff was remnants of the petrol pipes that had dissolved by the ethanol in the fuel - it managed to get through the filters in the feed 'banjo's'. I bought some new ethanol-resistant tube (from Hitchcocks Motorcycles) and all has been OK since.

Previously mike_chizlett wrote:

Many thanks to the rapid response team.

My initial thoughts where fuel until one cylinder dropped out but both plugs are sparking when laid on the engine.

I have cleaned out the carb and blown through all the holes although I didnât take the main jet out for inspection so Iâll try that but after reassembling the carb and putting it back on the engine Iâm getting a nice flow of fuel when tickled.

Iâll check the filler cap, replace the fuel thatâs in the tank and get some new plugs as my next move.

One thing I did do before my Sunday trip was to top up the half-filled tank with a gallon petrol from my can which would have been less than a week old.

Whatâs frustrating is that with a single carb itâll run on one side but not the other

Cleaning the carb is not enough for the pilot jet, this needs drilling out with a #78 16 thou drill, mount it in a WD40 red tube and using your fingers drill through the jet, a 25 pilot jet is 17 thou, the 16 thou drill cleans it out without resizing it bigger. Blame the ethanol for this now being needed after fuel is left in the carb. A blocked pilot jet will cause difficult starting, no idle and bad running until 1/4 to 1/3 open throttle unless you use the choke.

I agree with John, it sounds like a pilot jet to me. Do as he suggests, or just put a new one in, and run it with the tap off until it stops, at the end of each journey.

I think that the initial problem with bad running which becomes better with choke indicates a too lean fuel mixture. Most probable cause is dirt in any jet. Less likely in this case is air leaks between head, manifold or carb. The second problem with loss of one cylinder is most likely a ignition problem. Possible causes are already discussed. But you shall remember that the usual method of checking spark on loose plugs on the head says nothing on the ability to produce a good spark in the compressed air/fuel mixture in the cylinder. This needs a very much higher voltage.

I think that I have heard or read about carbon deposits from the brush causing spark going inside the magneto instead of the spark plug.

Previously Mikael Ridderstad wrote:

I think that I have heard or read about carbon deposits from the brush causing spark going inside the magneto instead of the spark plug.

Don't forget to clean the earthing brush as well as the two pick-up ones.

"Then realised that both bike and mower had had petrol supplied from the same source" Surely not the paraffin can!