It sounds to me like the hoses between the carbs and head are leaky...probably hard as a rock?
Nope carb boots are about a year old, I should start having a list as well of all the other work I've done on the bike.
Wouldn't leaky carb boots lean out the mixture? When my initial problem was too rich.
As I've posted many times (and is outlined in my book), a leaking hose makes that cylinder run richer below 3500 RPM. This is due to the fact that the pistons are all pulled along by one crankshaft: if one of the intakes has a vacuum leak, then the vacuum at that one will be less. These carbs mix much richer, by design, as the vacuum pulse depth becomes less (i.e. less vacuum), because it presumes that cylinder is running slower. When this is happening, the vacuum draw will not be long and consistent, like a faster-running cylinder's intake will be: faster-running intakes have a longer draw-time on the jet's venturi. At about 800 RPM the roundtop carbs mix at 8:1, by 2500 they are closer to 12:1, and by 4000 they are about 14:1 air-to-fuel mix. The PD carbs act the same way, but mix more accurately due to them being the solution for tighter emission controls in 1976 and later.
The pipes: these bikes are too short for the pipes to have any effect on mixture (the lone exception was the 750's Dunstall 4-2 setup of the 1980s, which ended almost 6" past the rear tire). The typical 4-1 and 4-2 pipes are even shorter than the 4-4 pipes were, which makes them even less effective at modifying the mixture needs. In short, the carb mixture can stay as OEM (except the 1978 PD42/b carbs, which were set very lean and can stand a 10% increase) with any and all engine mods save a longer and earlier intake cam duration: then they must go leaner below 2500 RPM or the plugs will be black in short order. The roundtops typically go from #40 to #37 (now #37.5) pilot jets for the Megacycle 125-00 cam, for example, while the mainjet for that cam should be about #108. This assumes a standard airbox.
On that latter topic: the airbox designs on the SOHC4 are to die for. There is nothing aside from cutting the frame uptubes under the seat to allow longer snorkels on the carb intakes (8-9" long on 750, 4-7" on the 500/550) and installing long, carb-bell-sized intake tubes, that can improve on the airbox performance....DESPITE what the Internet has to say about this... the Internet has not gone to schools nor thru extensive aerospace training for decades to actually learn how much Bernoulli taught the world, and his laws reach far higher and further than the Internet's rumor mills on the topic. The afore-mentioned long tubes were developed by endurance racers in the late 1980s and proved to work well, but require relocation of all that stuff under the seats in install them. This obviates nice things like electric starters, as the resulting batteries are 1/3 as large as ours.
