[quote author=HondaMan link=topic=194710.msg2279978#msg2279978 date=1711859913
Your #1 is running a little lean, which will make #3 dark and wet, and the #4 is running perfectly, so the #1 is being dragged along lean.
Mark can you explain how this works? I am experiencing similar things but had no idea one cylinder can affect another. However so far the weather has been completely lousy so no chance so far to experiment.
[/quote]
Sure:
since the crankshaft slaves all these carbs together, so to speak, the intake suction at each cylinder needs to be similar (so-called 'carb balancing') in order for the airspeed to be similar thru each carb, thus making the carb mix in the same air-fuel range. As the air speed increases the cylinder runs leaner, and vice-versa: at idle the carbs mix about 8:1 air-fuel and around 2500-3000 (depending on which 750 cam yours has, F0 and later being 3000 RPM) and by about 3800+ RPM it drops (gets leaner) to less than 12:1, becoming around 14:1 when the slides are open past the 'wasp-waist' in the middle of the throat.
So, when the first cylinder fires with its inhaled mixture and the crank accelerates to that speed, the next cylinder 'down the line' in the firing order receives that crank speed from that previous cylinder, and the airspeed thru the venturi of that carb causes mixing accordingly. If this next slide is open exactly the same as in the previous cylinder's carb, but the first cylinder was a tad lean, the crank will run momentarily slower at the intake stroke of this next carb. Since the mix is richer when the air is moving slower for a given slide opening, the mixture falls toward the slower mix rate with the same-size slide opening. This difference isn't a lot: if I assigned a (arbitrary) value of 1 CFM airflow for the #1 carb and the engine stumbled for leanness in #1, then the #2 carb's slide will be open too far for the crank speed that #1 caused, making its air intake only 0.999 CFM (again, arbitrary value, for example here), and since it gets richer as it slows down, the next cylinder #2 ends up with a wetter mix. Thus the darker plug.
This issue is critical in multi-carb'd airplane engines and is a point of scrutiny big-time when the engines are serviced, and especially when rebuilt. When I mentioned this to a fellow engineer (and SOHC4'er, CB400F) one day in the lunchroom at an aircraft manufacturer I worked for in 2006-2008 and the 'big' boss was behind me listening to it (I didn't know that...), the boss later created a meeting of the Power Group of the airplane's engineering team and told me to explain that to them all, because he knew it to be true but had never been able to explain it to them - so, they were also arguing about it. It became a part of the company's video-teaching series. I didn't even know they were recording it, standing there in my bluejeans, talking to a team of guys in suits. There I was, the Senior Electrical Systems engineer, not even IN the Power Group...
