The 4K & the sierra carbies are both made by aisian?, the main jets and other bits and pieces are interchangeable.
The size of the 4k throats measure as a little bigger than the sierra version. My guess is the smaller throat size in the sierra gives higher air-speed past the venturi so you get a better mixture and better response/ torque at lower rpm. From memory most of the 4k's had a simple weighted second throat opening, where the sierra run a vacuum operated secondary, so getting the secondary to open at the right time might be difficult.
If you are willing to put up with the sierra carby and spend the time, they run well off road, can't quite get steve's to run upside down yet, but am getting closer .. .
If you going to rip the carby off because you want more go, check these couple of regular probs before throwing it out..
- the lever that pushes open the second throat doesn't touch. straighten the lever with a pair of pliers so it just opens the second butterfly- only needs to open the second throat so you can fit a 2mm drill down the side of the open butterfly, if you can fit a 3mm (1/8") its too big.
- when you unwind all of the idle screws make sure that the two butterflies fully close. Most sierra carbies seem to have butterflies that aren't centered, so the throats never fully close, giving poor idle speed & mixture control. loosen the screws that hold the butterflies, gently move the shafts until the butterfly is centred, (it goes fully closed) then tighten the holding screws again- simple
- Check that the accelerator pump is actually working through the range it is supposed to be working. the factory measurement is 3.2-4mm movement of the accel plunger -Measured at the top of the carby body. I have found this measurement leaves a dead spot at the end of the first throat movement - often felt as a flat spot half way through second gear acceleration.
To adjust this, bend the rod that moves the accelerator arm.
Best results found so far are when the accelerator pump moves the during the entire primary buterfly movement (approx accel pump movement at carby top -5-6mm). Any more accel pump then this, and internally you raise the pump plunger out of the fuel bowl and you may get inconsistent fuel delivery at initial accellerator pedal movement when the car is doing big angles..
- lastly check that the gaskets have holes in them where they should - there is a vaccuum passage that needs to be open on both side of the crescent machined in the sandwich plate to the manifold, with this blocked by gasket, you will get some weird stuff happening.