Not sure how effective this will be, but ive been putting my L28 back together and didnt know what a few things were on the carby. anyway, reading the service manual i found on the carby has a BCDD unit/valve (Boost Controlled Deceleration Device). It is on the far LH (australian passenger) side of the carby, and has a 1 wire solenoid on the rear. Nissan manual says that when you are coasting with the throttle mostly closed, there is a high vacuum created and the carby does not supply fuel correctly (it actually sounds like it runs a bit lean). This causes a incomplete burn in the cylinders and there is a bit of excess polution that comes out. so to counteract this, when there is high vacuum, the BCDD puts in more fuel (and maybe air) to fix up the fuel ratio and make a clean burn. the solenoid deactivates this system at idle, based on a speed switch of the speedo (park/neatral switch on autos). i reckon if you hook constant 12volt to the solenoid, the BCDD will always be disabled and you may save some fuel.