Hi Mike -
Just downloaded the pdf manual from Accel site - very interesting as they still use a version of Calmap albeit a far more extensive version. I liked the early version of Calmap that I used on Roy's initial ECU and this version seems very good indeed, plus the documentation enclosed is very helpful.
Anyway, reading the blurb - I notice they specify (1.4.2) "A vacuum line must be attached between the fuel pressure regulator and the side of the intake plenum"
This would enable the fuel pressure to be raised for boosted applications such as turbo or supercharging, Plus, more importantly, will lower the fuel pressure from 45psi to perhaps 32-33psi at idle where there is a substantial vacuum available on the plenum. This lowered fuel pressure effectively extends the controllable range of the injectors because there is a minimum time that the injector will correctly operate. If this 'pulse' gets too small, the injector will fail to operate as designed and fuelling become erratic. So, by lowering the fuel pressure 'at idle' the ecu can use a larger pulse width (bigger numbers) and therefore better control the fuelling.
IF and it is a big IF, IF your ECU was initially programmed with this vacuum connection expected. Then without it, the fuel pressure would be higher than expected and hence more fuel would flow than required = sooty rich mixture.
Don't get me wrong, I'm not saying ALL engines require this connection, (We don't on Roy's!) BUT the fuel maps do look very different with and without it and in general, I believe it is a useful arrangement.
Connecting one would be worth a try as at full throttle, it would not change your existing mixture, but at idle, would definately be leaner as the fuel pressure is lowered.
Also - do you use a Map sensor? a fluctuating vacuum at idle can trigger acceleration enrichment if the Map sensor/parameters are not correctly configured.
I will print the Accel Manual and have a further read as it is very good in its explanation of EFi in general (would benefit other forum members interested in Efi as a general read) the various trigger types detailed is especially extensive. (flippin heck! - how sad do I sound?!)
Finally - "Hi, Paul. Been to 'Brands Hatch'lately, or do you cruise around the beaches at Brighton? (4 yrs. at RAF Lakenheath)"???? huh?? or do I have a short memory?? /ubbthreads/images/graemlins/confused.gif