E30 Zone

Hi all, I'm new poster in need of some understanding. My mind seems to get stuck at some point getting to some logic .
In the last 10 years, I have collected and rebuilt a few E30s in various models. I keep my flagship, a 91 318i with M40 motor as original as I can. Anyway, at some point I had a 325i that was downgraded to 316i with M40 (Downgrade?? Yes, It happens a lot in my part of the world). Well it had a very nice intake pipe and unmolested afm. I decided to swap these before selling that car. To cut a long story short, I am now facing challenges on that decision. What are the symptoms of crossing afms between 316 and 318? I have richness/ over-fueling and at times wet plugs. Like a case of some slightly off timing At some point after a rebuild, I did have a relatively good run for about 270km and my fuel consumption was slowly coming down. I have already ordered the "correct" afm for 318 and should receive it in the next few days. My question: is there no way to get the 316 afm to work on 318? Anyone have a clear explanation why it can't work? (something about the stroke causing a timing mismatch on the two engines??). Thank you in advance for any (fair) inputs.

AFM works by a spring loaded door in the airflow sweeping a movable contact across a resistive carbon track. A fixed voltage is applied across the resistive track, so a voltage appears on the movable contact which increases as the airflow increases.
The carbon track is not just a simple linear resistor though - it is customised to suit the requirements of the engine it is produced for, and the track customised for a 1.6 engine differs from that required by a 1.8.
The differences aren't great, o the wrong combination will work, but it won't produce either the power, economy or emissions that it should.
A custom made 'chip' for the ECU could probably correct things, but it is much easier just to use the correct AFM.

Thanks for the timely and very informative response Brianmoooore. So in essence the board is different (in the carbon track). Got it! Thanks. Based on that, do you think my over fueling issues could come as a symptom of those differences? I did run sweetly for a cumulative 270km. I was down to 15km/ltr.... on old filters.... I figured some remapping was taking place in the background???
I have ordered the correct one (used) but I've also seen somewhere I can actually get the afm board on its own and fit it to the housing??? I'm thinking the correct AFM connects to a distinctly larger intake hose, so I will have that pickle in a bit.... Some temporal air "plumbing" can do in the interim. I realise there is an air restriction there but going by that 270km run I mention, I can live with what I was getting in the power and economy. I am an average driver.