It pains me to see someone buy a Motorcraft kit for $400 and then report 11 mpg in their Eagle and terrible idle issues. For about $600 I'll have that same Eagle running perfectly, starting immediately, and averaging 25 mpg with TBI. An automatic Eagle is capable of 28 mpg on the highway and should never get less than 22 mpg, even driven aggressively in traffic.
I'm doing the majority of my Eagles with TBI. I have already installed and removed the Howell system from my green 1980 Sport wagon. It was TBI for about a year and I removed it at the end of this summer to CNC copy the adaptor before this next summer. The positive difference is huge and it is very simple and cheap. The only reason to go MPI is if you are pushing for power over economy or if you already have a whole donor XJ to work with. Economical daily drivers are better off TBI than MPI usually.
My CJ8 was owned by the JP magazine guys and in 2004 they purchased two new kits, one a Howell MPI kit based on the 1992 High Output 4.0 system and the other the Howell TBI system based on a GM TBI. The Howell kit cost $3200 and the Howell kit cost $1300. They put both systems on my Scrambler and ran them 100 miles to record the difference. It was a huge benefit over a carb but there was nearly no difference in performance between the two. When I bought my CJ8 it still had the MPI installed and the Howell kit was still boxed up neatly and basically brand new.
When I installed it on my Green wagon I did find that the Howell kit is flawed. The included fuel pump is very noisy and unreliable when installed by Howell instructions. If you run out of gas you need to loosen the input clamp and get the air out, or you won't be able to start and the pump will burn up and die. Been there, done that. Howell wants $140 for another one. They give you flex line to get from the pump up to the TBI, which a stone punctured and I got a nasty fuel leak one day from it. If you buy a stock Chevy unit for like $30 off rock auto it goes right into the AMC fuel tank where it belongs. It will autoprime, it will stay cool and last a long time, and it will be totally quiet.
Also they rely on the TBI pressure valve to knock down the fuel pressure. It returns through the original AMC vent line. Since the vent line is a small diameter and prone to rusty out from moist vapor, that can be a big problem. My Eagle hissed badly and ran terribly until I replaced the vent line. Ideally you should have the fuel pump in the tank then a pressure control outside it. The pressure should get to the TBI closer to the right value so that you're not pushing a huge volume of gas through the tiny vent line.
I also found that the Howell spacer is prone to air leaks because they made their bolts recessed with huge holes that provide a hole from outside inside. The instructions simply state you're supposed to RTV the crap out of it. I got another company's adaptor that is no longer sold but has precise flush mounted flat heads that easily seal with a very small amount of RTV on the gasket. I'm making my own adaptor using the proper hole design and the nicer shape of the Howell one and I intend to sell them here on this site.
The Howell kit also eliminates the spark control and has a special program on it to make it run properly without that. I'm going to follow the Binder Planet plans and simply use a GM factory program and incorporate my Skip White HEI distributor into the spark control. I happen to also own a Scout and I bought a full TBi system for it from another guy on the Binder planet. In a couple years I'll be able to post pictures and details about an IH 345, AMC 304, AMC 360, 3.0L Iron Duke, and a couple AMC I6s all happily running with TBI. It's a good way to go.