[Fot] Crank Harmonics and Carburetion

[Phil Gott]

Can anyone help with this weird phenomenon? The discussion of the second harmonic being at 5200 rpm on the TR4 crank got me thinking:
On a chassis dyno we could watch the SU (HS 6)  pistons during the dyno Pulls. We started each pull at 3000 rpm. The pistons went all the way to the top almost immediately after going to full throttle. The front carb piston stayed all the way up for the entire run. The piston in the rear carb DROPPED DOWN to about 3/4 open at around 5300 rpm and stayed there for the rest of the pull. This repeated consistently.
As these depend on air flow via Bernouli’s law, I examined the valve train thoroughly, fearing a stuck valve, problems with the valve train, a bad lifter etc. but everything looked fine. No noises from errant valve operation either. 
The crank harmonic discussion got me thinking. Could it be something to do with vibration that would upset the air flow or cause the piston to drop? Note that the piston was steady when it dropped. No apparent fluttering. I’m stumped.
The car runs very strong nonetheless. You see the results in Dave’s latest video at Thompson. He and I are thinking about putting a go pro camera and a light under the hood to watch that carb in real life track situations to see what happens.
Any thoughts as to why the Piston in the rear carb only should drop at about 5300 and above?
Phil Gott


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