I have struck a few with dirty injectors; they seem to get a build up of carbon under the caps and this restricts the flow. It can give uneven running more so than the actual lumpiness.
However, when I did my last cambelt change, I tried to refit all 3 pins before I started and discovered that the cam sprockets were about a half a hole out. The car was lumpy as all hell and I'd put that down to the combination of cam sprockets I was running.
According to Peter Taylor over here who runs a couple of 205/Mi16s at sprints and does Mi16 rebuilds and mods, there are 3 different cam sprockets numbered 2 3 & 4. It seems that most cars have the 2 / 2 combination whilst some have the 2 / 3 and apparently there's a slight difference in valve timing with the perfectionists preferring the 2 / 3 I understand.
I noted that the belt seemed too loose between the camsprockets, so as I had just completed a cambelt change on a VTS Xsara and had been forced to make up a cam sprocket lock, I decided to use this on the 16V also.
With the belt off, I set the sprockets with the stepped pins and fitted the new belt. When I got to the cam sprockets, I slackened the centre bolts on each one about a turn and as I did, I noticed the camshafts move minutely. When the belt was fitted, I then retightened the bolts, set the tensioners and removed the cam lock. I then refitted the pins (all 3) and found the pins fitted into the holes so easily, you'd almost believe the holes were oversized; no wriggling, no playing with tensions, just popped straight in.
I then turned the engine using a socket and again tried the pins; same result. I then started the engine and let it run for a few minutes and again the pins refitted perfectly.
Since then, I have never had my car hunt once.
It starts and runs as smoothly as an old low compression V8 but still goes like the clappers.
It's only a theory, but I reckon the cams occasionally require realgning with their keyways to correct the results of high revs over prolonged periods at varying temperatures and from other cars I've managed to feel the cambelts on but not been involved in any fitting, that a lot of cars are fractionally out on valve timing through this.
I since did an Mi16 and used the same principles with identical results, so it's not a one off.
It also removed that "mystery" tapping sound that some 16Vs get when first started, so my opinion is that both of these symptoms are caused by the same fault in belt sprocket setting.
Alan S