Video of Jet Drop When "Choke" Applied

[240260280]

[Mark Maras]

 Too bad he didn't take a measurement of the drop. I've been puzzling about the distance the nozzle drops below the fuel level when the choke is applied. At the tail end of Jalex's fix, the Cap'n brought this subject up when I was trying to get Jalex to set his fuel level just below the nozzles at 2 1/2 turns down. I believe that Jalex's fuel level is lower than the nozzle tops and it seems to run fine.  I'mm left with the opinion that there is a grey area in the fuel level that works just fine for the street. Opinions?

[siteunseen]

Get him to pull a steep grade, mountain. Do they have mountains in Panama?  That's how I discovered my low levels. Ran fine on the streets around here but Alabama's highest point, Mt Cheaha, stalled it out only running off the back carb.

[Captain Obvious]

7 hours ago, Mark Maras said:

I'm left with the opinion that there is a grey area in the fuel level that works just fine for the street.

I'm no carb expert, but that is my contention. I do not believe the absolute fuel level is as critical as many people believe it to be

It is my contention that the majority of the pressure drop occurs at the needle restriction, and not much caused by the different height of the fuel. What is the psi reading at the bottom of a 2 mm tall column of fuel?

[Mark Maras]

11 hours ago, Captain Obvious said:

 

It is my contention that the majority of the pressure drop occurs at the needle restriction, and not much caused by the different height of the fuel. What is the psi reading at the bottom of a 2 mm tall column of fuel?

 I got lost when you explained this. Can you expand on this theory? I've always believed that a precise fuel level was critical in all carbs. Now I'm wondering how large the fuel level "grey area" is. Perhaps @Bruce Palmer can help us out. I believe he said the optimum fuel level should be just below the nozzle tops at 2 1/2 turns down.

[240260280]

I used to think it made little difference but it sure does on triple Webers.

Maybe the fact our SU's are flowing for 3 cylinders (3X weber flow), makes the fuel height differential less significant.

 

Edited June 25, 2018 by 240260280

[Captain Obvious]

The Webers aren't constant velocity.. The venturi vacuum is all over the place with those depending on throttle position and air flow.

With the SU carbs, you have constant-ish, predictable venturi vacuum, even at idle.

[240260280]

I'll flow test the differences of  fuel flow vs. jet height and fuel bowl height when I get a chance.

I just checked some modelling figures against measurements when tuning an L28 yesterday:

• SK Synchrometer shows 10kg/hr of air flow at 800rpm  for 3 cylinders (4.8cfm)
• An old  model gives ~10cfm at 800rpm for 6 cylinders (~5.0cfm for 3 cylinders)
• Flow per cylinder at idle would be ~1.6cfm
• Can't find any formulas for a rectangular venturi (like the bridge/Piston space on an SU).... maybe the flow has to be sliced into sections along the width and only the slice of flow above the exposed annular ring pulls the fuel.

 

 

[240260280]

Thoughts on measuring:

 

Step 1:

(Benchmark using Synch Tool)

 

Step 2:

(Calibrate Benchmark)

 

Step 3:

(Conduct measurements)

Edited June 25, 2018 by 240260280

[Captain Obvious]

Actual testing would be awesome. I'm just operating off anecdotal and uncontrolled data.

When I get a chance, I'll try to whip up a sketch or two showing why I contend the CV carbs would be less finicky than other designs.