mis firing at low revs

My C180, UK, has been fine for a year or so, its 5 years old, and next service due in 3k miles. Its started backfiring a little and missing, as if its only firing on 3
cylinders at low revs, between 400 and 1000 rpm. Once its got going its fine again, unless I'm in 4th gear or so, and revs are down at 2000 and I put my foot down a little too hard, and its misfiring again. Any ideas ? I thought I'd cahnge the plugs, as I have always done that sort of thing myself before, but, had a look, and I cant find them, I guess they are covered in somewhere.
toad, Worcester UK
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To lean an air / fuel ratio. The fact that it runs poorly at idle and low rpm suggests a vacuum leak - perhaps from a split or disconnected vacuum hose or loose intake manifold.
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thanks for the reply. Any idea how to fix it? or is it a garage job.
toad

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I HAVE OWNED 12 MERCEDES BUT HAVE KEPT A 86 420 SEL THAT HAS THE SAME PROBLEM PERIODICALLY.PART OF THE PROBLEM IS WITH YOUR GAS(WATER)--PUT A CAN OF DRY GAS IN TANK--RARELY A FOULING PLUG,BUT HAS BEEN AN INJECTOR--HAVE LOCAL SHOP SCOPE THE ENGINE---CAR NEEDS TO BE RUN MORE FREQUENTLY--SPKSR
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I dont thiink it is the gas, ( petrol here in the UK ) , I run about 1000 miles a month, and fill to the top about twice a month.
I'll get a garage to check it out. many thanks, great ng.
toad

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Check the spark plug... usually it is a bad spark plug wire causing misfire... but can also be your distributor... check them all.
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Had a look in there, but cant find the plugs, are they hidden under some panel somewhere?
toad

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