Strange Engine Problems, This is a good one Options

[cdmcse]

OK I have an issue with my engine and I'm not sure where to go next so I defer to you, the experts. (IMG:style_emoticons/default/biggrin.gif) It is a 1.8L (probably a bus engine) with Dellorto 40s. It has a very recent unkown rebuild by the PO.

Symptoms:
-Won't rev above 5K rpms
-Feels underpowered at all rpms
-Below around 1700 rpms both cylinders on the left side do nothing (I can remove both spark plug wires at idle with no effect) After 1700 rpms all four cylinders come to life.
-At idle I can hear a slight knock sound coming from the front center of the engine in an irregular pattern. Seems to go away under load.

What I've done:
-Swapped carbs from side to side, problem stays on the left bank.
-Replaced the Dizzy twice, it is now a stock 1.8 dizzy with a pertronix, new cap, wires, rotor, and plugs. all stock.
-I have fantastic spark to all four cylinders (fat blue spark).
-I pulled the lifters/pushrods and everything looks to be in good shape with no abnormal wear.
-Valves have been adjusted 3 times and adjustments seem normal valves seem to operate correctly when turning by hand
-All intake and exhaust gaskets have been replaced.
-Compression is 105#-120# (The low # IS on the left bank) The car doesn't burn or use any noticable oil.
- Oil pressure is 15psi at idle hot and 50psi at speed
-Oil temp stays ~180 degrees.
-Timing is set to about 34 degrees and seems to be spot on.

(IMG:style_emoticons/default/headbang.gif)

I'm guessing at this point it is some type of valve train issue, but I thought I'd see if you guys had any guesses. I planning on tearing the engine down this winter.

Thanks for any input you have!