have a look here http://www.clubgti.com/forum/showthread.php?t=81658 all the pics have disapeared but it should still explain things.
It can be DIY with a gas analyser but you really need it on the rollers to get it perfect, Stealth seem to be the kings of this... Bassically the Mod involves drilling to expose a hidden screw which allows you to adjust fuel pressure so it doesn't run lean at the top end Best 50 I ever spent on my 16V!
I have heared about the bit drilling to expose the screw to adjust the control pressure, but what is the shimming bit all about, what does that do? What is the system pressure, isn't that controlled by the control pressure?
you dont shim nothing, just turn the allen key to correct the fuel pressure , then check on a rolling road /wideband to ensure the mixture is safe
Its ok I understand now, the link above was talking about setting system pressure under a WUR mod heading so I couldn't understand how a unit with only one pipe in and one pipe out could control the control pressure and the system pressure, but the shim adustment of the system pressure in in the metering head not the WUR.....slightly misleading. Do you screw the screw in or out to richen up the mixture?
cant remember which way...dont go trying it without the pressure gauges and a way to test the mixture under load
I haven't got a pressure gauge, but I have the use of some rollers so I will be checking the AFR as I change it. Just wanted to go the right way to start with so it isn't too lean on the first run. I am guessing the bolt puts pressure in a diaphram so I would think the winding it out should increase to control pressure thus leaning off the mixture.
