Loss of power above 4,000 RPM

[Itsa Scorpion]

It was great to get to drive the Scorpion for the first time this weekend but I am puzzled by a noticible drop in power above 3500-4000 RPM.

I do not know the specifics, but the car has 40 DCOE Webers, MSD ignition, new fuel pump, aftermarket cams and an Ansa exhaust. It idles and accelerates great until about 3500 when it feels just like I backed off the throttle about 30%! It never misses a beat, no "pops" or " backfires", runs very smooth all the way to redline, just drops off a ton on power.

I tried advancing the timing and it made no difference.

The timing belt is new and all the marks line up where they were albeit what looks like almost one tooth off measured between the crank pulley mark and the marks on the back of both cam pulleys. Not knowing the history of the cams I replicated what the PO had when in stalling the belt.

We found that the numbers on the block indicate it's a 1.8 Abarth block but it took a late model 2.0 cam belt?

Also, the car used to have nitrous, so who knows what the PO could have done.

Where would you guys look first?

[1,6 HF]

I'd check the MSD module, coil, distributor, and the rest of the ignition system.

3500 - 4000 rpm is also the point at which the Webers are transitioning from the slow speed circuit (aka idle jets) to the main circuit. If it runs smoothly to the redline, then it's probably not a synchronization problem (which usually shows itself in some roughness), but that would be worth checking anyway. It's possible that there's some blockage in the main circuit of one or more of the barrels.

But I'd check the ignition system first.