911 carrera rough cold start

[harryC3targa]

HELP! I'm at my wits end! Already spent $2K and still no closer to figuring this out. It's a '76 3.0 Carrera Targa, twin plug with Max Moritz (9.8:1) 3.2 kit. Engine has rough cold start, but slightly smooths out when warm. Seems to idle OK, but at 1600 RPM it gets rough all the way to higher RPM's. It sputters and pops when dead cold, when warm not so much. Tested injectors-OK; replaced all 12 wires-OK; plugs-OK; leak down-compression-OK; new warm-up regulator, new fuel distributor, new fuel pump, new filter and 3 year old accumulator. Plugs have light gray color on electrodes, not too rich. On warm start engine cranks excessively, catches on one cylinder, then two, then three...., finally coughs and pops, then runs rough. When driving home from PCA event the engine felt like it lost a cylinder, and has been running rough ever since. This problem started long before I rebuilt the fuel system. Anybody with ideas of what to do next, other than sell the car?

[T77911S]

I did not see anything about a cold start valve. that's what really gets the car started when cold.

it also sounds like you could be lean.
check for air leaks.
cracked air box
rubber sleeves on intake runners
intake to head bolts
not sure if the 76 has this but if you have the metal pipes behind the CIS for the AAR and AAV, one of them can rub on the air scoop for the oil cooler. it can get a hole in it.

check pressures
check mixture. you really need to know what the mixture is doing.

not sure about that ignition system. never worked on a dual plug.
what plug gap are you running?
if wide you close it up
timing at idle?

[Discseven]

Harry... What was done to the engine just prior to the rough running?

[pmax]

Does the idle change when the oil cap is removed ?

If it doesn't, in all likelihood the airbox is leaking. Here are other recent threads on the same topic.

pop off valve
1974 CIS Airbox replacement questions
911SC shut off and won't turn back on.