I have the 225 in mine, and I will say it's the best engine I could personally ask for in a CJ. It's a close fight when you compare it to the
AMC 258 i6 / 4.2l , but if you're working with a short-nose CJ you won't be fitting one of those in. At least not without way more work than it's worth.
I would stay away from the 2.5. Everyone I know who had one (keyword "had") one either got rid of the Jeep for one with a straight-six/V8 or swapped one of those motors in.
It's the only gas engine of it's physical size that puts out the kind of power it does. Some people don't like the odd-fire ones because they claim they vibrate more at idle than a normal even-fire engine. On a theoretical level, yeah it probably does, but my personal motor is smooth as anything else I drive. Besides, if your engine vibrating at idle is the worst of your problems on an old, presumably lifted Jeep, you're doing pretty well.
I personally love it. Tons of torque, decent gas mileage for the power, and it's put together in a way that makes it easy to work on. Once you get it tuned up right and squared away though, it doesn't need much maintenance. The Rochester 2G carb that typically comes on them is a piece of cake to rebuild/tune, and they can be easily converted to HEI.
Here are the only drawbacks as I see them: in stock condition, they don't have any valve adjustment so they can be hard on cams. There's probably someone out there who makes adjustable rockers for them. On the odd-fire 225 and 231 engines, it's possible to get the distributor installed "out of phase" so that when you time it, the passenger-side bank is firing 30* late, which will make it run like
, and before too long, blow your exhaust valves up in that side. Don't ask me how I know that, I don't wanna talk about it. This happened to me because the haynes/chilton cap diagram is just plain wrong.
This isn't a concern in the even-fire 231s.
If you want to know more about odd- vs even-fire, Google is your friend. Basically, the firing interval on the distributor isn't evenly separated. On even-fire 6 cylinders, the distributor rotation between sparks is 60-60-60-60-60-60. On odd-fire 6 cylinders, the rotation between sparks is 45-75-45-75-45-75. They did this because it makes for a stronger crankshaft.
That all said, I wouldn't trade my 225 for anything. IMO, it's the best possible choice for a 71' or earlier CJ.