1996 3500 Dually
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the first thing i would check would be the timing gear case see if it leaking any oil from around the fan it the 12 valves had an issue where a dawl pine would come out of the block fall in to the bottom gear and break the case and the case is not cheap and if you buy the truck that would be the first thing to do would be take that plate off and drill and tap a hole in front of the dawl pin put a bolt in front of it and it will never fall out making you have this problem if u look on here or google it you will find everything you need to know about it there are a lot of threads about it
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the p pump stops fueling after 2250 rpms so thats what your feeling all the 12 valves do this is what u need is a 4k govener spring kit witch you will stop fueling at 3600 rpms but your stock valve springs will start to floot at 3200 rpms so as long as you stay under 3200 rpm you will be fine and if not you need to buy 60 pound valve springs but in less your sled puling or drag racing there is now reason to go over 3000 rpms and also u need to grind and slide your plate and do the afc houseing mod if u look on hear you can find threads about it or google it and it will find them for you it is really simple also if you want more power find a 215 hp p pump and swap it i think they where only on 98 12 valves but might be wrong