I'm trying to bring back to life a 1979 245dl with a b21f + m46.
New.

cap, rotor, plugs, wires, cleaned the intake manifold and TB.

The car now stumbles and misses at low rpm's.
This car has the kjet and i have no clue on how to start to diagnose this.
any clues, ideas.

all the best

K-Jet is a high-pressure fuel system; you need to confirm the system pressure by testing.

You may have to replace the electric fuel pump, but there isn't much else to go wrong on these cars.
--
Stef (81 245 B21A SU M46 3.91) 327000km - in the garage for the winter, 81 244 BW55 3.31 on the road)

I had similar problems until upgrading the in-tank pump. First check is fuel delivery at the engine -- you can uncouple the fuel line and run the pumps by jumping the fuses, should get about a quart in 30 sec. as I recall. If that's okay, crawl under the car and check the delivery to the main fuel pump (observing proper precautions, and run just the in-tank pump when doing the second check, the main pump is cooled by the fuel flowing through it so you don't want to run it with the supply uncoupled.) Checking pressure is the last resort, and requires relatively expensive tools.

IPD sells an upgrade kit for the in-tank pump that I used, but as I understand it, you can buy the necessary parts somewhat cheaper, search the archives here for more detail.

Hope that helps ...


--
'81 GLT 245 @ 259K; '83 DL 175K