Chatting to a member privately on arb site about tuning and thought I’d share a message I sent.
For those who want more in depth info on the topic, why I recommended what I did in the video, this may offer more clarity and context. Many may not be interested, but for the few that are:
There are many different types of carburetors, and some dependent butterfly carbs aren't as sensitive as others. However, some can be quite problematic, with the high-speed screw needing to be turned out 4.5-5 turns just to start and it’s impossible to lean the engine out enough on the high circuit, for good full throttle of operation without the low side being far too lean. Homelite, Mac and Poulan homeowner saws circa 40cc from the early 2000’s (all look identical but rebadged) with walbro dependant carbs are notorious for this.
Since the majority of the dependant butterfly carburetors I've come across can be tuned similarly to independent systems, I tend to recommend tuning butterfly carbs that way, as shown in my video, even though fuel has to flow past the high-speed screw first. The amount of fuel flowing through the high-speed drilling is crucial to how it affects the sensitivity of the low-speed circuit and altitude, diaphragm age, condition of fuel lines, filter, carb kits etc all affect how sensitive the screws are.
This could explain why manuals sometimes don't distinguish between tuning methods for different carburetor types. Though this is me speculating, as mentioned before, I give very little weight to the carburetor manufacturers tuning recommendation for the reasons and examples I pointed to earlier. (These were how walbro, for example, recommend setting the high speed screw at its fastest rpm without offering any context to what engine should be tuned that way. Doing so with a saw will lead to setting it far too lean and possible seizure)
Ideally, every dependent carburetor should be tuned starting with the high-speed screw, as the fuel supply to the low-speed circuit depends on the high-speed screw's position. If the high-speed screw allows sufficient fuel flow to the low-speed circuit within its typical operating range, the engine will be less sensitive to adjustments from the high-speed screw at idle and progressive off idle / acceleration.
Rotary carbs seem to be much more sensitive and should be tuned as dependent systems (high-speed first, then low-speed), which is why I recommended that method in my video.
In a perfect world, I’d provide specific tuning instructions for every carburetor, including the best methods and starting positions, which screw can be adjusted first, which should be and which carbs it doesn’t affect even if they are dependant, but that's not realistic to fit in to a 12 minute video. I can’t fit years of obsessive in-depth testing, tuning and research into a 12 minute video. There is always a compromise with the intention of offering the most value for the timeframe I have available. Not to mention my audience comprises of a wide range of experience. Some haven’t turned a single screw on a carburetor, others have 40+ years of experience - far more than me with more knowledge than I’ll ever have. It’s a hard balance to fit in enough detail that it offers useful value, not too much that people don’t get overwhelmed and a balance between what is necessary, what isn’t and finding where to draw that line to suite the majority. Concise enough that people don’t just click off and on that same vein, not too in-depth that people start to get overwhelmed with information.
No matter how good a mechanic, teaching on YouTube is always going to be a compromise. It’s impossible to cover all intricacies, not to mention I’m human and I forget details that I later wish I added into the video. I work off a script I have pre written, but even than isn’t perfect and leaves me wanting to add more - hindsight is 2020 as they say.
I have a video lined up on how to determine whether a butterfly carb is dependent or independent, and I'll direct viewers to the flowchart from there. As with anything, one video can't cover every scenario, machine, or setting in every possible sequence, but it's a solid starting point for those facing challenges.
Lastly, every rotary carb I've encountered on small engines is, by design, dependent. They don’t have separate idle or progressive idle drillings.
