Here is the situation, if anyone can help. When I bought the bike, the carbs were leaking and the jets looked well used so I bought a refurb kit (jets, needles, floats, gaskets) from 4 into 1. I replaced the jets (they were main 80, pilot 38) with the refurb kit (they are main 75 pilot 40). Now, after the new parts were fitted, the bike starts well but doesn't hold idle and has a heavy rev-hang. I am not able to adjust the idle screw to get it to stabilize. I have set up the slides to have a 1/8th inch (3 mm) gap when they are fully closed. The plugs are sooty black so I guess it is running rich. The previous owner had fitted pod filters and I want to replace those with the original airbox/filter. I am about to take the carbs apart again and refit the original parts. However, if I am correct the new jets are the standard size and should be the right ones for use with the original airbox in place. Any advice? Thanks!
The old jets were the standard size: your new #40 jets are a whopping 5% fuel increase below 3500 RPM, so yep, it's running rich. Keep in mind the idle circuit in the 400F runs up to 3200 RPM, so it's dumping lots of extra fuel into the mix. A 2% increase is usually enough to push things in the low-RPM range over the cliff, so to speak. For the record: the 400F came with #78 mainjets. The 1975 version had #37 pilot jets, which changed to #40 sometime around 1976. With #37 pilots the airscrews were set at 2.0 turns out, with the #40 pilots this was more like 1.5-ish turns out. Both setups were slightly cold-blooded, so the fancy choke cable setup was supplied so as to create a high-idle-while-cold capability.
Use of the #75 mainjet is a good place to start while you try to adjust the 400F to the 21st-century fuels: they all run rich compared to the 1975 fuels, so lower octane will help keep things in trim once you have the right jets aboard again. The octane is only a hint (today) to how fast or slow the fuel burns: in the 1970s the higher octane simply burned slower than the lower octane. Today it's not quite that simple: all (USA) fuels burn slower to help ignite catalytic convertors faster by burning into the exhaust system on purpose (especially when cold), so there will be some tweaking with the smaller Fours to make things tip-top.
You can also benefit from using a bit of oil in the gas: about an ounce or two per tank is good (I've done this since the 1990s to mitigate the damage from outright acidic fuel blends, and ethanols). This helps because modern fuel is very 'dry' compared to when these bikes were designed: the oil will smooth out the low-speed engine response (up to 4k RPM in particular) and make the final jetting less critical and more flexible. The 1990s, when acidic MTBE fuels were forced down our bike's throats, the brass parts (jets, float valve seats) took a beating unless oil was added to the tank: yours probably didn't get that, and the jets, in particular, were damaged. This eroded the wetted surfaces of the jets (and the old float valve's seat) and caused the problems you started with: uneven mixing and too-rich mixes. I see it a LOT when folks send me their carbs for rebuilds. It made the inner metering holes non-smooth and slightly larger, and damaged their outer surfaces sometimes so much the numbers disappeared(!) from sight.
If your 4into1 kit came with emulsifier tubes (these hold the mainjets), check the sizes (and number) of the aeration holes in them: the upper ones (4 holes total) will be smaller than the lower ones. In many aftermarket carb kits these holes are both the wrong size and often also the wrong count. If these holes were the same sizes as the OEM emulsifiers, they need to be opened up slightly for today's ethanol-laced fuels: about 5% to 10% increase is about right (Example: the 750K1-K6 use 0.033" to 0.035" holes in the lower section OEM, but 0.0375" to 0.039" works much better now, reducing plug fouling and 'flat' throttle response at in-town engine speeds). This is needed because our fuels now need more air to burn fully (which is then consumed in the car's catalytic convertor to make it hot, and work right) without increasing carbon buildup on the sparkplugs.
