Most members have found 3 to 3 1/2 turns out on the idle air mixture screws seems to be the "happy place".
The idle port sits in front of the transition ports and it wouldn't surprise me that the idle transition supply passage to those ports is plugged . Scotch's cleaning tool provided a way to reverse flow cleaning fluid backwards through that passage and clear out the blockage. the idle port and transition ports are about .020" in diameter and block easily.
Although the main jet passage feeds the idle transition passage, the fact that the main jets fell out and laid in the bottom of the float bowl would have little effect on the idle circuit.
Most people who rebuild their own carbs will dunk the carbs in carb cleaner, then when they pull them out of the cleaner, will blow out all the open ended passages that they see with little regard to the way the fuel flows in those passages. For the most part this works BUT when dealing with the Idle transition passage, the idle port and the transition ports in the venturi at the engine side of the venturi are so small that if you blow air down the passage in the direction that the fuel and air flow, you will blow all the blockage to the smallest part of the passage which is the port itself which as previously mentioned is around .020". ONE MUST BLOW THIS PARTICULAR PASSAGE OUT IN REVERSE TO THE FLOW OF GAS AND AIR MEANING < BLOW THE PASSAGE OUT FROM THE IDLE PORT (1), AND THE 3 TRANSITION PORTS. It's an absolute must !!!! And when doing this, the idle jets must be removed before blowing out the passage and also the enrichment valve.
I was at one point in my life a licenced automotive technician and probably rebuilt more than 300 carbs. I had on several occasions run into difficulties with idle circuits and ended up replacing carbs because of it. It was only after Scotch's carb rebuild write up that I came to realize how critical it is to clear out the idle passage in reverse flow to the gas /air mixture and I owe a great thanks to Scotch for the enlightenment.
Hope this helps,
Had them off and given them a good clean, a couple times now, reverse then forward cleaned those idle jets and they blow air through.
But still not firing on 3, issues with 4 seem to have gone, but not firing at all on 3 at low rpms, also messed up balance a little so 5/6 are iffy, waiting on the correct threaded adapters to put the balancer on properly, but now sure why it’s now only firing 4 and not 3 on that carb at low rpm and when it does at higher is running lean still
If after that I’m still having issues tempted to send them off to get fully reconditioned, unless it is something else somehow. Am I correct that all scotch’s cleaning tools are gone?
Edit: also, good lord do I hate putting the carbs and air box back on, what a faff.
A few thoughts here
1) Ultrasonic Cleaners- marvelous at cleaning external surfaces but probably has little effect on cleaning internal passages. The microwave frequency would have to stimulate the body of the carb and transfer that energy through the metal to stimulate the fluid inside of the passages to surface erode the crud in those passages. Stick your finger in an ultrasonic cleaner while running and watch it clean the crud off your finger but you won't "feel" any sensation in the nerves of your finger. That should say it all.
2) Many members have found that they needed to clear out those Idle Supply Passages several times before they finally got things running right . I know it's painful. My thinking here is that the crud buildup in those passages is a combination of Varnish from stale gas being left in the carbs and breaking down becoming varnish and then there's the Ethanol effect. Ethanol loves to absorb moisture until it's a fully saturated solution of Ethanol and water. Then it sits in the carb and the water corrodes the aluminum creating aluminum oxide. Once you start cleaning the passages, you break up the corrosion and dissolve the varnish one layer at a time so to speak, and then you have to encourage that "stuff" out of the passages. Some of the particles maybe to big to flow out of the idle port or the transition ports, so they block up at the ports. Until those passages are clean and not releasing particles to the flow of gas/air, you'll be fighting idle circuit issues.
3) Not enough attention is being paid to gas filters. Most fuel tanks are full of surface corrosion and when it breaks off, flows with the fuel and it should get trapped in the fuel filter. Automotive fuel filters for carbureted cars may not be fine enough for the carbs on our bikes. Think of it. A 2or 4 barrel carb has 2 idle circuits to feed engines that are 5 to 10 times larger in displacement then our bikes so the fuel flow through those 2 passages will be 5 to 10 times greater. That would infer that the 2 idle ports are much larger and able to pass larger particles.
In my opinion, I believe we should be running 15 micron filters to ensure that whatever fuel we put through our system, it won't have particles in it that could block those idle ports.
Looking forward to hearing what the intake readings are once you get the vacuum gauges on. They may tell the tale here. if 3/4 intake vacuum is low, they may not be drawing enough to flow the idle mixture and that could happen if the butterflies for 3/4 are completely closed.
Another trick I like to use when diagnosing issues like this is a spray bottle with gasoline in it. If I suspect a lean issue or misfire on certain cylinders, I will spray fuel down those intake throats and listen for rpm changes. If the rpm goes up, then it's likely a fuel problem. If it doesn't it could be an ignition problem. I've also used a propane cylinder with the burner nozzle removed and a piece of surgical hose on the end and then direct the hose to the suspect intakes . I've also used this set up to find intake leaks by directing the hose around carb flanges and listening for rpm changes.
good luck and please let us know what you find.