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Black puffy exhaust is a rich condition either due to over fueling or not breathing.
I think you need to look into the carb set up, a stuck choke or a valve adjustment.
If not all of them
I know this. What part of carb setup? This does not have a choke. It has enrichment needle valve. How should it be adjusted? Valve adjustment has been checked and is oK.
The fuel mixture & float adjustment. That has an auto choke on it, & they get stuck.
I am sure choke is not stuck. Scooter starts immediately and idles well even after warm. This suggests the start enrichment system is closing properly. The only fuel mixture is for idle and is a "D" screw near the front of the carb which I cannot turn. Would this adjustment have any effect on high speed operation? Float bowl is not overflowing and seemed to close properly when I reassembled the carb. Is float level critical?
Yes, if the float level is too high you have rich condition.
There is no indication thus far that the choke is not stuck and I'm not sure why you are trying to convince me otherwise when your symptoms contradict your thinking.
Idle adjustment is not fuel mixture & all carbs since the 80's have had to have mixture screws on them & in fact your carb can not be cleaned without removing it.
Yes, fuel mixture adjustment can effect all or certain rpm ranges depending on your fuel mixture needs.
Unfortunately because I don't have the carb in front of me I can only suggest to you what I suspect as an issue based on your symptoms
Since you know the valves are good I think the issue is the mixture or float level, with the "by-starter" deal being last on the list.. definitely over fueling
Are you familiar with this exact carb? It does not have the traditional low speed, high speed mixture screws common on other carbs. There is one screw with a "D" head which is not meant to be adjusted according to what I read and it is for idle, not high speed. You need a special screwdriver for it which I don't have. It does not have a "choke"; instead it has a needle valve which is open cold and electrically closes as the engine warms up. The only thing that makes sense to me is the float level. Any other ideas? Could the throttle position indicator be involved?
There are no carbs that I'm aware of with two mixture screws. Most have 2 jets but all only have 1 mixture screw.
An idle adjustment only adjusts mixture on a 2 stroke carb, you do not have that. & yes, you set the idle & only adjust as necessary.
It does have a choke, it an auto choke called a "by-starter valve" by honda, otherwise know as an electric or auto choke, if you need to refer to it as a needle valve that is fine, but be clear, it is an auto choke.
You have to understand that fuel mixture adjustments & idle screws are not the same & you are dealing with 4 stroke carb not 2 stroke small engine carb. They are very different & are tuned differently.
Do you know where I can get a driver for this "D" shaped mixture screw? Or is there some clever way to use existing tools to turn it?
You have a 2 jet carb with a fuel mixture screw right on the side of the carb. As per epa regs the mixture screws are capped from the factory, but I would think on an 04 the carb would have been serviced several times & the cap removed.
That is just a thumb screw as far I know, maybe I'm not clear on what you are referring to & since that is an idle adjustment only, is not your issue.