This is a common occurance in the buses I drive. Especially if the bus had been operated in a shuttle route in the city, lots of stop and go. Then taken for an Italian tune up. At night it made a pretty good show when I looked in the mirror.
The only thing that sounds funny though, is that I got sparks when I was full throttle, and you got them when backing off.
Could be that is just when the sparks slowed down enough so you noticed them. 
__________________
Ed
1981 300CD (Benzina)
1968 250 S (Gina) 266,000 miles!
1983 Alfa Romeo GTV6 (Guido)
1976 Jaguar XJS-saved a V-12 from the chevy curse, what a great engine!
1988 Cadillac Eldorado (better car than you might think!)
1988 Yamaha Venture (better than a Wing!)
1977 Suzuki GS750B
1976 Yamaha XS 650 (sold)
1991 Suzuki GSX1100G (Shafty Gixser)
1981 Yamaha VX920RH (Euro "Virago")
Solex Moped
1975 Dodge P/U camper
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