Thanks Mark. I'm pretty sure I changed these out when I took the engine apart last year. I did all the o-rings and seals. But I will check. Thankey
By the way, wouldn't a leaky intake make for a lean condition?
Not when the engine is run with a pulse-feed type carb, at least below speeds that supply a more-or-less constant vacuum.
Here's how these things work: the idle circuit actually mixes at approximately 8:1 ratio (air:fuel), but in a real short pulse during the intake stroke. This pulse is about 63% of the total intake stroke at low engine speeds, due to having to get the air moving, then slamming the valve shut in its face shortly afterward. This rich mix then works out to (8 / .63 ) = 12.7 air/fuel mix rate at best. Usually, true-mix charts on dyno pulls show it to be closer to 12:1.
As the engine speeds up, the pulses become closer together, and by about 2500 RPM they start to act more like a constant-vacuum carb. It is at this point that the idle jet loses its ability to send gas up its tiny pipe, because the slide (or butterfly, on some carbs) is open so far there isn't much airflow over that jet tip. Instead, it is coming from the mainjet far more. The mainjet is set closer to 14:1 air/fuel ratio. This leans the mix out for a more "normal" type burn, as is found in most IC engines.
So...if the airflow in, say, carbs 1, 3, and 4 is normal, but #2 has a vacuum leak at the head: the #2 carb "sees" less vacuum from the pulse of that one's intake stroke. This makes that carb mix at the lower airspeed (i.e., closer to idle speed while the others are near 1/4 throttle, for example), which makes it run richer on that one cylinder. This is why we ALWAYS see the cylinder with the darkest plug being the one with the biggest vacuum leak, in normal commuter riding.
For those who work on cars with carbs, this seems backward until they realize that there is only one vacuum chamber on all of those cylinders. This creates a much more constant vacuum, much like these bikes see above 4000 RPM. It is true that if the bike is hiway-ridden only, the vacuum-leaking cylinder will (finally) show up leaner, but that's only because once it passes the 4000 RPM range, all 4 carbs are running above this rich idle circuit. But, it takes about 50 miles of hiway riding to make this show up - or, several laps at the racetrack, where it never goes below 6000 RPM anyway.
Hence, the arguments continue here, while most don't seem to realize that they are not riding the bikes at the speeds needed for car-like carb-and-vacuum readings. This may also be why so many don't believe that these idle air screws run LEANER when turned inward, and RICHER when turned outward. They just don't seem to understand Bernouli's principles that make these little mixers work?
