Hi Stan - Thanks for your considerate response.
I too have been building these carbs as well as gas and alcohol FI systems for many years - you're right on in many aspects although your post seems to be slanted to carbs that have a secondary idle circuit.
As I alluded to earlier in my response about the 1:1 throttle actuation - We have found that it has a tendancy to even out fuel distribution (good) under steady throttle settings, but introduces too many other variables that are exceptionally difficult to manage efficiently or cost effectively - a couple are;
1) Driveability - 1:1 actuation is much faster given the same amount of pedal travel and the flow of air is too great at too low of an RPM for most roadrace applications. This makes the engine less controllable off the corners in my opinion. Some people call them "Light-Switch" carbs.
2) Accelerator pump circuitry in a 1:1 has always been a royal pain in regard to getting them to be responsive - but not overly - in all the ranges. Unless you are willing to change final drive ratios to optimize your setup for a specific RPM band at each track, you will find yourself re-working the carb tune-up for each track. While that's fine for you and I - your average Joe-Track-Day guy is going to be lost.
In regard to fuel being introduced by a secondary idle circuit which the carb is at part throttle (on it's mains) and the secondaries are closed - I have not seen this to be the case. When considering that the vacuum signal is relatively low at part throttle (depending upon the size of the carb vs. displacement), the balance of pressure between the plenum and inlet side of the carb will not typically allow the flow of fuel through the secondary idle transfer slots - or at least not an appreciable flow to upset the fuel mixture towards/away from Stoich. I would find it a stretch to think that the minimal amount of fuel that may be drawn from this secondary idle circuit, during part throttle operation, would be substantial enough to cause cylinder washdown or elevated exhaust temps.
In conclusion:
In the grand scheme of things, the absolute best carb is one that has a true single variable venturi system that works correctly (Kendig / Predator were close). The second best (in my opinion) is one that actuates the secondaries via vacuum control where the engine will not see the additional air until it can use it.
That being said - the Double-Pumper carbs were originally built because people could not "FEEL" the secondaries coming in or were having problems with bad diaphragms / actuation so they would put "The Screw" in the secondary safety link to FORCE the secondaries open (we know what that does.. I will always maintain that if you can FEEL the secondaries coming in, they are setup wrong...)... Still I, and a number of other builder / tuners rely on the mechanical secondaries as the mainstay of competition carburetors because they are typically easier to tune and the removal of the secondary diaphragm eliminates another point of potential failure.
Again - Thanks for the conversation, it's good to get the neurons working!
Hybrid Mode
