Your
carb or carbs are running very lean. Cold means the chokes are working enriching the mixture.
First, figure out which one or both- Here's how: Air cleaner off. Find the brass air jets. Next to them is a small hole in the casting. it's about the size of the end of a normal paperclip. With the engine running, close that hole. That hole chokes off air to the idle circuit. If your lean, the engine will speed up or smooth out. ( less air- more rich) If rich, the idle will stumble and not like it.
You can do the following:
1. Slightly close each butterfly, reducing the air, thereby increasing tthe richness.
2. Turn both enrichment screws ( idle screws) outward a bit, thereby opening more idle fuel.
Repeat. When both carbs do the same- slight stumble- that's it.
You can read your plugs too. Heat has much to do with lean and crappy alcohol blended fuel. Check your distributer advance and that's why you use ported vacuum.
Advance trick- guess where heads blow? Between number 3 and 4 cylinder. that also corresponds to where heat and lean conditions cause problems on M30s. Tune for those cylinders and chances are the others will be a bit rich.( Read the plugs.) Stay cool ya'll.
