Mower mechanical help please

owd potter

I have a Champion petrol rotary mower powered by a 3.5 hp Briggs & Stratton engine that is perhaps 15 or so years old now and recently has begun to run erratically. 
I have done a couple of cuts this year so far and mower has started reliably enough, usually on first pull, but has thereafter seemed sluggish running at idling speed only although with occasional brief spells during which revs will rise to a normal rate and fall back to idle shortly thereafter. 
Changes of the fast or slow setting of the throttle lever (choke?) seems to make no difference, and the engine does not respond. Engine also seems to emit dark grey puffs of smoke occasionally and smells gassy or perhaps of fuel.
After completing half a cut today mower cut out, from lack of fuel I thought (although tank not empty). after refuelling I could not restart the engine.
Unfortunately, I have been remiss on servicing as mechanics is not something I'm particularly adept with, and would appreciate any suggestions of simple things that I might check or service to get restarted and running properly again.
TIA
Owd

steephill

How old is the fuel? If it is from last year then it should have been treated with a fuel additive to aid storage. If the fuel is fresh then it is probably a fault with the carb, the diaphragm rots out so needs to be replaced. It might even be cheaper and easier to buy a new carb.

owd potter

Thanks @steephill
fuel is fresh. 
Are there other simple things that I should check and rule out in a process of elimination before attempting to find and then remove and then strip a carb...eek!!

padmeister

Replacing the diaphragm on a Briggs and Stratton is quite simple and took me about twenty minutes. You should only need a socket set or spanners. If you google you’ll find a few videos showing how on YouTube and the part only cost me approx £2 from eBay.

Take a look at the videos and see whether you think you can tackle it. Worth a try before you go into anything more expensive.

Johnnyp

Take off the fuel tank and carb (could  be all in one unit or separate from each other depending on age of engine ) and strip it down being careful how each part came off and lay out on a rag or tray so you don't loose any parts ,as previous person said change the diaphragm and give all the tubes ,holes in carb  good blow out to clear of any crap that may have built up over the years . There may also be an adjustable screw for the air/fuel mixture , YouTube how to adjust this as this might solve your problem BEFORE you tackle the carb strip down and save you a lot of hassle.  Good luck 👍

owd potter

@steephill @padmeister @Johnnyp
Many, many thanks you guys for your suggestions and tips.
I have all the enthusiasm in the world to have a go at fixing this, cos well, tinkering with oily screws, taking things apart, finding and solving problems is one of those inexplicably compelling things to do. 
I am usually reticent to get involved with mechanical things, taking stuff apart that I might not then be able to figure out how put back together, but your encouraging comments have convinced me it's worth a go.
Brilliant ! 

Pete.8

Have you tried just cleaning the sponge air filter?
My mower does the revving stuff too, cleaning the air filter sorts it.

owd potter

Err no, not yet, but this sounds like a simple enough 1st step.
many thanks @Pete.8