RPI have rising rate regs, do these cure that problem? I have also heard of drilling a hole in the flap, I guess this allows some air through to help cure the richness? I am wanting to use one of my flapper syatems on my 4.6 in the 110.
I haven't messed around with the rising rate regs, or improving the flapper system, as I just junked the stock ecu for a Megasquirt anyway. The big problem is that the fuel curve seems all wrong (or was on my motor anyway), and once the AFM was adjusted to give best settings while about 2,500/3,000 rpm, it was rich low down and lean up high. A rising rate reg may help, I thought they were normally used in turbo situations, but may work for this case.
Drilling a hole in the flap will mean the AFM vane is moved less by the air passing through (it was designed for the required air flow of a 3.5, not a 4.6)
Realistically though, if you have soldering skill, the cheapest way to make the 3.5 injection perform perfectly is to throw a megasquirt on, and clamp the fuel reg up a bit in a vise to make it run at 3 bar. This allows you to chuck the AFM (which has a TINY hole through the guts of it anyway), along with cold start injectors etc.
Not an option for everyone though, the DIY spirit has to be pretty strong.
84 Rangie, 3 inch spring lift, 2 inch body, Megasquirted 4.6, R380, rear Maxi, 34x11.5 JT2s. Simex FM installed.