I just put a ka24de into my 1990 240 sx about a week ago now there is smoke coming from the exhaust and the rpms fluctuate. does anyone have any ideas

 

What color is the smoke? Are you losing power? Losing water? Losing oil?

 

Was the KA24DE in good condition?

it could be dirty fuel injector

 

black smoke - running rich
white smoke - burning oil
blueish smoke - coolant

sounds to me like the timing could be off also. make sure all your electrical (ignition) is correct, and that the grounds are correct

 

That's wrong. White smoke with a sweetish smell is burning coolant. Blue-silver smoke is burning oil.

 

i'd have to say headgasket is blown... if it's smoking, and the rpms are flucuating, the it's loosing compression in one or more of the cylinders.. that would be my guess..if the timing is correct and the smoke is blueish of course.

 

yup, my bad little mixed up there...
Blue smoke is oil burning, white smoke means that youre burning either some sort of contaminant, like burning coolant, which would indicate a blown head gasket and black smoke is from fuel...

 

It also depends on how much smoke... if it is a significant amount, it is possible its a blown head gasket.

 

i talked to the mechanic he said it is the fuel that had been sitting in the car for 4 months because the car was not running due to my previous engine being totaled. ( the smoke is black)

but i have another question when i get to 3,500 rpm or over 55mph the car starts to shake . i cant go over 3500rpm without the car starting to shake a lot i have to swith through the gears pretty quickly