If you have drag pipes, not two into ones, the vibration will be much more noticeable. If you watch the exhaust values coming from the front pipe on a Road Star you will see the front cylinder leaning out pretty drastically below the torque peak. The rear cylinder does not do this at all. So there is a difference in power between the two cylinders in that rpm range. It gets worse as you approach the torque peak. After that it levels out and smooths out.

By changing up your mix, you probably found a better compromise between the two cylinders that is letting them be closer in power output through that range. You can tell this on any Road Star, but more so on a bike with drags. Just rev up slowly in neutral. As the rpms start to climb, the vibration will get progressively worse (thats the front cylinder leaning out), then will all of a sudden smooth out and start racing up. When the vibration stops (it happens suddenly) the front cylinder is back to normal mix values, the two cylinder are pulling more equally and the motor will want to start racing up.

Environmental changes (air density) can more drastically affect a bike that is set to the lean side on mix values. It pushes that front cylinder even leaner and makes the vibration stronger.

Really interesting analysis, Gram. Wonder why the front cylinder runs leaner, though (?) Seems like it should be the same since it's fed by the same carb. Is it because there's less of a turn in the header? Seems like that would only affect really high RMP operations.

In any case, I'm liking this bike more and more with every ride. Rode it all over today without any pronounced knock or vibration, so I'm pretty close to perrrrrfect, I believe. Just about time for a BAK so i can start all over again

MidStar07 wrote:
Really interesting analysis, Gram. Wonder why the front cylinder runs leaner, though (?) Seems like it should be the same since it's fed by the same carb. Is it because there's less of a turn in the header? Seems like that would only affect really high RMP operations.

In any case, I'm liking this bike more and more with every ride. Rode it all over today without any pronounced knock or vibration, so I'm pretty close to perrrrrfect, I believe. Just about time for a BAK so i can start all over again

We have discussed it a lot over the years here. We really don't have a good explanation for it yet. But I observed it on every single bike I jetted that ran drags. No exceptions. Some pipes were better than others, but if they were drags it showed up. It probably does it on two into ones as well, but I only have one tailpipe to measure there so it tends to be the average of the two cylinders as opposed to one by itself.

Two into ones have a lot better scavenging that would probably help this problem somewhat. I know they tend to run smoother than drags, so assuming that might be the reason why.