strange idle 450sel 6.9
 

hi guys..i have one question.about my 6.9 engine100..its the idle .lets put like this,if the engine run at say 620 rpm the idle is fell like u pull out one ignition cable out and like the engine is running on7 cyl.but the thing is there are no missfires and spots and nothing at all the sound in the exhaust system is perfekt no misses nothing just the feeling when it idles and if turn it up to more than 900 maby 950 around 1000rpm its tot vibration free .but when i put the drive in and the rpm going down the whole car is feeling bouncy. so there is between around 620 and 900 -950 under 620 say 500 perfect and over the last speed perfect to. but there is some modification to the engine.but the thing is just as when i hade K-jet fuelinj. now i have sequential..autronic and distributor less ignition.and supercharger from autorotor....but the idle problem is still the same as before.any idea what it maight be.okay i can put in neutral and the thing is perfect but i know i still whant to know what it can be after all.