13b Bridgeport with 48mm Weber IDA going rich between 5500-7000 rpm why?
 

Hey guys, 13b Rx4/5 engine running carbon seals used in a rally car, in the off season we decided to give her a total over haul/freshen up, all new seals, springs, lightened the rotors, side clearance, FD Rear oil reg, new oil pump etc as well as adding a moderate size (4mm thick) CNC cut bridge to the existing intake ports (as from the pics which was rather large Stage 2/Extend Port, with way oversized exhaust ports courtesy of the previous engine builder)

Once we got her up and running and run in enough, off to the dyno we went, and strangely although the bridgeport made more power than the stage 2, it has this weird flat spot (caused by rich conditions as observed by the AFR/lambda meters) that starts at 5500rpm but clears at 7000rpm as you can see. Nothing we tried improved it…which is a problem as in a rally car that’s is where you want the most power and most torque but due to the rich conditions we are down in that area.

On the dyno sheets Thick red line is the new bridgeported engine, thin red line the old Stage 2/extend port.

In terms of Carb tuning we have NOT changed a thing from the previous tune…
48mm Weber IDA
42mm Chokes
F11 Tubes
205 Mains
120 Airs
70F10 Idles
.70 Acc Pump
6mm Gross Fuel Inlet Jet with 4 PSI pressure

Running 98 Octane Pump gas premixed @ ratio 150:1

Timing is at 24 BTDC Leading ~20 BTDC Trailing
Exhaust is 2x slip jointed 2” primaries straight through collected just before the diff into muffler.

The ONLY thing that has been changed apart from the bridgeporting and timing slightly increased, is adding a straight through resonator (stainless packed) on each primary 2/3 of the way down.

In terms of AFR's I know it would be helpful if they were on the dyno sheet but they werent included on the print out, but I can assure you that at WOT @ 7000-8000rpm they were pretty much spot on, which surprised me as I thought we would be needing bigger mains given the additional porting but it seems not...?

Any ideas on what would cause this rich area and advice/tips to remedy would be great.

Brendon