Engine UNEVEN ENGINE RUNNING

I have an MTD Mini Rider 76SD (420 cc) ride on mower bought and used here in France. I am told by my very reliable specialist repair shop here that the motor is in fact supplied to MTD by Briggs and Stratton.

Normally the engine runs smoothly but recently (and perhaps co-incidentally just after the repair shop replaced the worn out blade drive belt) it has started to run very un-evenly. It sounds like the motor almost stops then picks up again. However when I engage the blade and begin mowing it seems much better. When the blade is then disengaged the un-even running returns again.

I bought the mower new in March 2013. I had a similar un-even running problem in August 2015 and eventually replaced the carburettor which solved the problem - after previously having drained and replaced the petrol with new, and also replaced the fuel filter. Surely I shouldn’t need a new carburettor every few years should I?

A friend has suggested that the afterfire solenoid at the bottom of the carburettor might be part of the problem. He said that I might consider trying to disable this by cutting off the needle type piece in the solenoid that blocks the fuel flow to the carburettor when the key is switched off, so that the fuel flow is NEVER blocked. He says these solenoids can sometimes become troublesome. There seem to be no adjustments on the carburettor itself. I have just looked again at the OLD carburettor and have practiced this cutting off operation and I’ve attached photos of this. So do the experts think this is a good idea or not? The access to the carburettor is dreadful – you have to dismantle all whole top covers and a metal bit around the bottom of the carb in order to remove it, so I don’t want to have to all this again if there are other solutions. I can’t just cut the wires to the afterfire solenoid (which is possible in situ without all the dismantling) as no power to the solenoid would result in the needle always being up and thus permanently blocking the flow of fuel, so the motor could never run.

Also can you advise me whether the exhaust system needs any routine servicing – the sort of thing that I should do myself? The user manual says nothing about this. I ask because the exhaust sounds load when the motor almost stops, but then again I suppose that it to be expected. Annually I always replace the spark plug, clean the air filter, and do oil change with a new oil filter.

All helpful suggestions would be gratefully received. Thank you.

From the picture I can say that is not a briggs engine, but the MTD made in China engine. You are having the same issues as you did couple of years ago with a partially blocked jet in the carb. More specifically the blockage is on the idle circuit of the carb. Either going to have to remove and clean the carb, and install a carb repair kit, or replace the carb.

What he said above.

Sonic clean the carb, drain the tank, lines, and look for sand from your gas pump. Between the sand and ethanol, every carb that sits for more than a month has to be cleaned out.
We tell our clients, if you have to use pump gas, strain it at the pump with white coffee filters.
You will see the gunk that goes thru your car engine, yet gets suck in your mower carb.