So the remaining big thing I haven't talked about yet is the pulsating effect from the firing of cylinders 5 and 7.
As a simple diagram a standard 2-carb setup across 2 rotations of the engine looks like this:
Note that LH and RH on this diagram refer to the ports being supplied by the carbs, not their physical location. And of course I've ignored the effect of exact firing point for simplicity.
Everything is happy, nice and regular.
The Boxer 4 carb looks like this:
And there's the 5 and 7 issue - everybody else has a more regular pattern.
Just for interest, and never mind about how it might be engineered, here's what it looks like if we were to have 4 carbs connected transversely:
Now we have 3 carbs behaving like 5 and 7 were in the last case, and only 7 and 8 behaving what we'd call more reasonably.
Back to the Boxer. So this is why we need a sizeable balance pipe, and I would have thought connected to all 4 carbs to maintain the suction on 5 and 7, and a heavier oil in the dashpots to stop the 5 and 7 carbs jittering about.
If I have this right, because the plunger/needle with heavier oil has a greater lag response following an increase in the throttle disc, the transient airflow across the bridge is increased relative to what it would have been with a lighter oil. This works like a pump jet in a fixed jet carb and improves the pick up of the engine. If you drive with a heavy right foot you'll see the effect in terms of increased fuel consumption.
The thought also occurs that we generally think of the dashpot oil from the point of view of it's primary job in smoothing the movement of the plunger/needle in response to throttle change. But it must have a secondary job. In real-world terms the springs are fairly weak. The movement of the plunger is a bit like having a weight on the end of a length of knicker elastic. So if you're driving at a constant speed across rough ground the plunger is going to want to bounce around and will do weird things to the engine response. The dashpot oil minimises this. I would guess this effect will be well known to the off-road guys?
Measure CO at the exhaust-manifolds individually
