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[Field Marshal Dr. Opti]

Thanks chaps......I've just spent 25 minutes watching a 'carb clean' video on Youtube.

It has been getting harder to start. I noticed in the video that my carb does not have the spring on top that seems to open and close the butterfly.

[Field Marshal Dr. Opti]

https://youtu.be/joDmD0oOlJ0

[Field Marshal Dr. Opti]

They can be temperamental bastids when not used regularly. I doubt it's timing or HT related as it was running when you switched it off. Check that mice haven't nibbled the fuel line, plunger bulb or HT lead. Timing is only usually an issue if you've hit something hard with the blade, which sheers the woodruff key, and then the crankshaft slips out of place to the timing plate.
Normally a good spin with the starter rope is enough to get them going but carb issues are generally the culprit. You also have to be careful which way you tip a petrol mower as the oil will find every little hole in the engine. My wife tipped our Toro on its front to free the blade from clogged grass and then wondered why it wouldn't start again. I cleaned the carb and tried starting it until my shoulder gave up. Luckily, when you remove the top cover of a Toro you can see the crank nut that the rope coil sits on. A socket on an adapted extension, fitted in a decent drill, will start it in seconds if all is ok. Dangerous but effective.
Carbs don't wear out as they are such low pressure fuel flow on these, but many change them as they think it's the cure. I'm yet to replace one, even on mowers over thirty years old. What engine is it, Briggs and Stratton ??  if so, what series is it (Usually on the engine cover) as I need to know what carb it has on it. Some are a resonating fuel diaphragm, and some have a proper bowl. The resonating diaphragm can be manipulated by hand to give it movement, as they go a bit stiff when not used regularly.
Are you using the rope pull in the rope guide on the handle. If so, remove the rope from the handle guide and pull from the engine as you get more revolutions per pull.

It's a  Qualcast 51cm petrol S/P... supposedly with a B and S engine. About 3 years old.

XSZ51C-SD, The carb looks very much like the one in the video.

[YZ250]

.............
It's a  Qualcast 51cm petrol S/P... supposedly with a B and S engine. About 3 years old.

XSZ51C-SD, The carb looks very much like the one in the video.

Briggs and Stratton 625e Series engine then, with a proper carb (albeit plastic) so no diaphragm. All you can really do is clean the carb as per the video. Wire strands are good for this, you don't need any fancy gadgets or jet brushes.  If you have a spark, the engine kill must be working as intended, so no issue there. If it has a kill bar on the handle, tie it up so that you can concentrate on the rope pull better. I have had some success (mainly with 2 strokes admittedly) running a blow lamp across the plug to dry it out/ warm it up. Once you eventually get it going, it should start fine on subsequent fire ups. As said previously, pull the rope directly from the engine to get more revolutions per pull, not through the handle rope guide.
Only other thing I can think of, as it has a carb with a float bowl it has no primer bulb, so I assume it has auto choke. Try to see if this is working ok. It normally operates with a seperate rod connected to the top of the carb, and gradually turns back as it warms up. Shame you can't drill start it, that takes all the hard work out of it, although in saying that the choke still has to work. (Insert thumbs up smiley here when those evil Christmas smileys have gone)
Good luck.

[YZ250]

Also, before you rip it apart, remove the air filter and check the butterfly position. It should be fully closed when cold. If it's not, ping it back closed with a screwdriver and try starting it again.

[Field Marshal Dr. Opti]

YZ250.....thanks for this. You are a man who knows his mowers.

[Nick W]

Blowtorch to dry out a plug is OK. But you can do all six simultaneously on a ring of the gas stove....

[Field Marshal Dr. Opti]

What about the missing spring from the top of the carb that (I think) controls the butterfly. Can I still start the mower if I don't have it?

[YZ250]

Blowtorch to dry out a plug is OK. But you can do all six simultaneously on a ring of the gas stove....

 
Quite right.

On 2 strokes, you can test the push, pop, bang workings by leaving the spark plug in the HT lead, but out of the hole resting on the cylinder, pull the cord or the kick start and watch flames fly out of the plug hole. You can then say, Yep, that's all good so it should start. 

[YZ250]

What about the missing spring from the top of the carb that (I think) controls the butterfly. Can I still start the mower if I don't have it?

Use an elastic band to hold the choke closed just to try it. If it starts, you can then sort out any missing bits.    Check that the butterfly is fully closed before attempting to start it. 

[Field Marshal Dr. Opti]

What about the missing spring from the top of the carb that (I think) controls the butterfly. Can I still start the mower if I don't have it?

Use an elastic band to hold the choke closed just to try it. If it starts, you can then sort out any missing bits.    Check that the butterfly is fully closed before attempting to start it. 

Will do. I have just taken the carb off and I'm getting a perverse pleasure from stinking of petrol.

[TheBoy]

I'm getting a perverse pleasure from stinking of petrol.

When you were a nipper, and dad was filling up with 4 star, did you used to unwind the window so you could smell it?


Obviously, I never did that. Not me. Nah. Well, not often.

[YZ250]

What about the missing spring from the top of the carb that (I think) controls the butterfly. Can I still start the mower if I don't have it?

Use an elastic band to hold the choke closed just to try it. If it starts, you can then sort out any missing bits.    Check that the butterfly is fully closed before attempting to start it. 

Will do. I have just taken the carb off and I'm getting a perverse pleasure from stinking of petrol.

 

I know what you mean.    If I recall, the plastic carbs don't have a drain plug in the bowl like the aluminium type, so if you tip them over petrol pisses out. I've never been able to work in gloves so with all of what my skin has absorbed I've got no chance of a long life. 

[Field Marshal Dr. Opti]

I'm getting a perverse pleasure from stinking of petrol.

When you were a nipper, and dad was filling up with 4 star, did you used to unwind the window so you could smell it?


Obviously, I never did that. Not me. Nah. Well, not often.

Yeah....loved it. 5 star though.

[Field Marshal Dr. Opti]

I have cleaned all the jets ( in fact any orifice I can find) with a needle. It is now sitting in fresh petrol. I shall give it an hour or so before putting it back together. It looked pretty clean but I'm hoping it will behave when I come to start it.