E30 Running rough in 4th and 5th only

[cdruve]

I have a 1983 E30 M10 engine with the Bosch K-Lamda continuous injection system, all original. The car idles nicely with the occasional hic-up and accelerates and drives very nicely through first second and third gear, when you put the car into fourth or fifth the problems start. When you put the car under load in 4th of 5th the car accelerates well and performes normally but as soon as you reduce the load on the engine to coast the enging runs very roughly, if you then change back to third the car runs smoothly and normally again. There is no rough running in 1st 2nd or 3rd under any driving conditions. I have checked the ingnition system fully and it is fine, any ideas what to check next?

[MrM3]

what rpm does that engine start running roughly at? are you sure its a problem in the engine and not the drivetrain?

[cdruve]

It does it at all levels of RPM and the drive train is fine as i have just overhauled all of that. I have also had all of the injectors cleaned so it is not them either.

[gotKRAUT]

Suck Squish Bang Blow....the four strokes...check compression air fuel and spark...if those dont check out and you fix em and the problem persists then i suggest ripping the motor apart...but i am honestly thinking that you will find it in the forementioned..