Although it idles at approx 400 from dead cold it idles really smooth and doesn't stall. I may have wound in the PDU knob fully when I had it in bits! I set it up spot on before and it was working fully and the measurments were correct! It's probably that. Will retest. Won't the expansion element cam hold it off using 3PU for a while till warm though?
If it idles at about the right speed when fully warmed up, the 3PU 13mm nut must be about right. Anyway, that adjustment won't affect the fully extended position, which is the one that's relevant to the first few seconds from dead cold. As you correctly say, once the 3PU pulls back from fully extended to the middle position, the waxstat is in control until that cam rotates out of the way, so the exact position of the 3PU plunger has no effect during that period. I think the choke flap may be your problem, if that doesn't open far enough, fast enough, the engine just can't get enough air to rev fast enough to run on its own from dead cold. Check vac hoses going to both ports on the pulldown, the little one-way valve, the green ball hose, and the big vac hose that comes off the inlet mani, these sometimes split on the underside where you can't see.
Still kind of have this low rev start-up prob for a minute or so, Engine runs absolutely spot on with plenty of uhhum power and idles okay 900 ish. Just had another good look over the threads and I've checked the carb choke flap. The pull down unit opens it as it should 1st and 2nd stage but... From dead cold engine (all be it on a 14'c day) the choke flap has nothing pushing it closed, (iirc it should be the bi-metallic strip) and it is about 45' open or so and moveable through 0' to 90'. Shouldn't there be resistance from dead cold even on a warm day?
I had cause to look at this the other day, helping out someone with similar probs. It seems that the choke flap only closes fully if it's really quite cool. 4 or 5 hours after I last drove it I looked through the little rubber bung-hole in the airbox above the carb with a torch, and the choke flap was sitting at about 60 to horizontal. Engine/outside temp was 25C. The bimetallic coil spring is pretty weak, so you won't necessarily feel much resistance unless there's a big mismatch between where the ambient temperature 'wants' it to be and where you're trying to push it to. Just had a look at mine again now, outside temp 17, choke flap at about 40 or 45. I also cut up and examined my last waxstat, and was surprised to find the wax was liquid at room temperature (again 25-ish, maybe a tad more from my gentle sawing). I'm sure most of the expansion occurs at the solid-liquid transition, so this suggests that the waxstat only covers a lowish range of temps. So it seems like a lot of the cold-start stuff only acts partially when the ambient temps are summery, kinda like you'd expect bearing in mind that the operating temperature range probably goes down to minus-quite-a-lot. Start it and drive straight off, and you won't notice any cold-idle 'wobbles'. Unless there are other probs making the engine harder to turn that it should be.
