[Fot] Crank Harmonics and Carburetion

Tue Aug 25 07:32:07 MDT 2020

Check your cranking pressure for each cylinder first.
I suspect you either have either a bad hole or a spring issue, both are easy to check. The piston is drawn up by the velocity / CFM being pulled into the engine.
Rick

 "Doing the right thing is always the right thing, regardless of consequences"

    On Tuesday, August 25, 2020, 09:00:49 AM EDT, Paul Ricco via Fot <fot at autox.team.net> wrote:  

 Phil, 

Wr went through this with SU’s about five years ago. We had the rear SU piston “winking” at us on the dyno on high rev pulls.  We resolved it. 

1. When is the last time the carbs were totally rebuilt?  Any chance you have any air leaks at shafts or anywhere else?

2.  What are you using for a manifold gasket?  Any chance of a vacuum leak?  

3.  Same for the carb/isolator block end of the manifold. If you added a heat shield that is two more faces that need to seal well with gaskets. It starts to get thick and the studs are not very long. 

4. What are you running for dashpot oil?

5. Which weight springs do you run in the carbs?

6. Any other vacuum leaks anywhere?

All can be factors. However I would be looking for an air leak. Including at the carb shafts depending on the age. 

Just my two sense from having been through this and resolving it successfully. 

Paul Ricco

Sent from my iPhone

> On Aug 25, 2020, at 7:34 AM, Chris Marx via Fot <fot at autox.team.net> wrote:
> 
> Hi Phil,
> 
> interesting you are having visible vibrations. My engine doesn't show things like this.
> I guess the rear carb drops from a lack of flow. Maybe fuel supply is not working properly, valve springs too soft with valve bounce.
> 
> Something like that.
> 
> To help the crank....making everything as light as possible:
> Piston with rod - 1000gr
> Flywheel with ring gear - 3500gr
> Clutch with plate - 1000gr
> Pulley - 400 gr
> 
> Seems to work fine. I let balance the crank all 2-3 years.
> 
> Cheers
> Chris
> 
> 
> 
> -----Ursprüngliche Nachricht-----
> Von: Fot <fot-bounces at autox.team.net> Im Auftrag von Phil Gott via Fot
> Gesendet: Dienstag, 25. August 2020 13:50
> An: Friends Of Triumph <fot at autox.team.net>
> Betreff: [Fot] Crank Harmonics and Carburetion
> 
> 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
> 
> 
> Sent from my iPhone
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