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Why is my 323i refusing to rev over 4000 rpm?

I took the car for a road test this afternoon after swapping the K-jet system out of a known good runner, the car pulls fairly well upto 4000 rpm exactly. It then starts misfiring and sounding awful. After a 15 minute drive, the car's temperature was 3/4, this tells me I may have a timing problem on top of the break down at high rpm.
Car had been sitting for about 5 years before I got it running around 2 months ago.

In the last few days I've replaced the entire fuel distributor system, this has eliminated my cold start problem as well as reducing backfiring on overrun.
After replacing the fuel system, I lost spark, playing with the dizzy, leads and coil ended up solving this.
Swapped the rotor, coil and dizzy cap with an E30 323i which has no rev limiter.

The car has aftermarket headers+full exhaust, and my dad thinks maybe a cam, which could effect the standard ignition timing. But there could be more engine mods, I just cant tell at this point.

The car now starts and runs pretty much as i'd expect under 4000rpm although lacking some power, considering it should feel about the same as my stock E30 323i
Could it be something to do with the dizzy vacuum advance at 4k?


Any help or advice will be taken!

Like at the turn of a switch it goed bad at some point? Ulikely to be the vacuum advance then, perhaps coil or rotor? Did you check ignition timing? I'd check that and exchange some parts to see if that changes anything.

My first suspect should be the rotor, but as you replaced it already that is rather strange. Check the cam for where, I can remember a E23 728i which I couldn't rev over 4000 because it was worn out completely.