Running rich? I thought this EFI on the new engine was computer controlled to a fine point where it would run at the proper fuel ratio all the time, regardless of intake or exhaust restrictions, altitude etc. The sensors are supposed to monitor the various parameters and adjust fuel flow accordingly. If the system is working properly when you put on a less restrictive exhaust it should send the same fuel ratio that it is presently set up for, which is too rich according to your account. I once had a BMW that had one potentiometer adjustment for fuel enrichment but it was supposed to be adjusted using an exhaust gas analyzer and it was looking at the carbon monoxide in the exhaust, the higher the number the richer the mixture. Does the enfield have such a fine tuning adjustment on the ECM? Let us know how you make out.