Easy solution, pour in about 1/2 a pint of good quality paraffin, the type you can buy in petrol stations. It is the main component of Redex cleaner, and is really cheap. About 1/2 a pint to each full tank, it will gently clean off gunk and muck. Don't put much more than 1/2 a pint in as it can be a bit harsh on rubber seals in the fuel system. I used to tip a bit in every 4th tank full on my old Alfa 155 and 156. It used to smoke a wee bit for the first 10 miles or so after it went in, then it stopped. Worked particularly well on the 2.5 V6 in my Alfa 155. It really helped out the speed of starting on that beast!