The reason I ask is I've been starting the twin 16 hp on My tractor with a charger/250 amp jump starter, and it has been whizzing it over pretty good, after a while of using the Roper I decided it was worthy to invest into a new battery, so now that I have a 500 amp battery it turns over sluggish, Thank goodness it starts up rather easy otherwise it would be doing just as you have described your friends has done, and has got me thinking more along line of in need of a stronger battery rather then a starter,
can you remember how quickly the engine turns over while using booster cables from a stronger battery?