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[Fot] Crank Harmonics and Carburetion

To: Friends Of Triumph <>
Subject: [Fot] Crank Harmonics and Carburetion
From: Phil Gott via Fot <>
Date: Tue, 25 Aug 2020 07:50:12 -0400
References: <>
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 Bernouliâ??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. Iâ??m stumped.
The car runs very strong nonetheless. You see the results in Daveâ??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

Sent from my iPhone


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