Alfa 33 16V runs rough at 2000+ rpm
I have an issue with my 16V Alfa 33. A bit of background, the car is a series 1 (1985) with the engine from a 1990 series 3.
The engine was originally a Cat-converted version with throttle position switch. I rebuilt the engine completely including some minor mods like skimming the heads, anti-reversion groove on inlet valves, polished exhaust ports, conrods lightened, internals professionally balanced. I also gave the fuel system a pod filter and thermal wrap on the exhaust manifolds. Finally, I converted the engine from cat to non-cat by removing the cat-converter, fitting appropriate fuel quality relay, swapping the ECU for a non-cat ECU and disconnecting the lambda sensor.
Recently I've noticed that when its sitting still in neutral, when I rev it up slowly, at around 2000rpm the exhaust note sounds a bit 'confused'. It doesn't sound out-of-time like my old mechanical distributor used to sound, but it doesn't sound like its combusting very efficiently or smoothly. In addition, the car and particularly the steering wheel will shake slightly (the car isn't rolling). When I'm driving I can't notice the steering wheel shaking (because of the natural steering) but the engine sounds a lot rougher than my old carburetor'd 1.5L engine used to sound - particularly in the midrange. When I'm cruising, it sometimes has trouble keeping steady revs.
My guess is it's either a) bad fuel mixture, b) bad ignition timing, or c) something internal isn't right.
Is this usual for these cars? Is it perhaps because its a throttle switch rather than a sensor, and therefore not a perfect fuel mixture? Or is is ignition? I've always wondered how close to perfect the motronic ignition is. I can buy a DIY stand-alone ignition system pretty cheaply, would this offer any advantage? Or does the stock system provide accurate timing under all conditions?
Thanks for your help.