Density Altitude

I'd wheel it.

 

A big thanks for the well written info Brian. Very valuable info for those of us who don't live in the flatlands. Should probably be in the tech section, IMHO.

When searching to try to solve my problem, I have run across a post here or there that state "you have to make your own rod", this is the first time I have ever seen instructions on HOW to do it.

Kudos!

 

My thoughts as requested :tea:

To the best of my knowledge and observations the location of the step ( or start of a taper ) is directly related to throttle shaft position and resulting airflow. I believe the step or taper is calculated to begin at a point when the throttle is opened far enough from idle where the resulting airflow in cfm would begin to proportionately need more fuel to maintain the required air/fuel ratio than atmospheric pressure in the float bowl could push past the fixed restriction in the jet ( low speed step ), at this point a step, series of steps or a taper would have to begin. The steps or taper would have to be the correct decreasing diameter/configuration as the restriction is lifted out of the jet to maintain the proper air/fuel ratio as the throttle is opened ( transition or midrange step ), The design of the linkage controlling the lift rate of the restriction as the throttle is opened would determine step/taper location, length of step/taper, configuration and when transition is no longer required and we again have a fixed restriction ( high speed step )

I have experimented with step location a little, You can place the Low Speed Step a little farther down the Rod and it will effectively force a leaner mixture longer but what I have found is that this actually decreases any potential for economy, A perceptible lean surge between Low and Mid Range Steps is generally the result, This is followed by a drop in manifold pressure as the driver attempts to compensate with the throttle, the rod is lifted out of the jet delivering a richer mixture than needed. From what experimenting I have done I dont think step/taper location is worth fooling with much for average driving, That seems to be figured out pretty good.

I have salvaged some straight taper rods out of YF carbs ( of unknown origin ) so I know they exist ( they could be early series rods ), They must work ok for some applications. To be honest I have never tried to run a straight taper rod in my 938SD on the F134, Now my inquiring mind wants to know. I will have to make a One Step Lean Rod this weekend with a straight taper and let you know how it works.

 

I think I get what you are asking, I am not as up on the complex calculations as I would like to be either but I do think if you plotted air flow on a graph it would be on a slightly parabolic curve and would vary considerably depending on the volumetric efficiency of the engine ( uneducated guess ).

I think the steps are there because the technology and manufacturing costs of the day did not make it practical to do it any other way, Maximum economy was not the concern then that it is now and it was probably easier to determine what size restriction would give acceptable performance over a wide range and be compatible with the four basic fuel metering stages of the era carbs. ( the idle circuit + three on the rod )The transition points only represent where a change in fixed restriction has to occur to maintain an acceptable air/fuel ratio. If an acceptable air/fuel ratio is provided it should not affect any power or airflow curve. I would think that would be more dependent on engine rpm and load.

I dont think the step as we know it on the early rods actually has to be there, It ( a straight taper ) may however have to start slightly higher on the rod than the step does. I have some rods out of later units that have straight tapers and as we get into more emission controls we see a series of tapers and sometimes a small step in a specific location that to an old school carb man would not make any sense at all, Anyone attempting to make a rod for a later model should duplicate the existing rod configuration.

The basic fuel metering stages on a carburetor essentially represent a series of compromises in maintaining an air/fuel ratio that will deliver acceptable performance.

And now for the most important question, How high is up?..........Well I stepped outside with my coffee just before sunup and called a conference, Between myself, a starving coyote, three rabbits and a stray cat we decided to establish up as anything over 5280'

 

This is a really good thread. Very informative.

 

OBD for ECJ5s! Tuning your carb in style: Just plug in and cruise around to study your own driving habits and the engine's response to them. Thanks for the tip, Warloch! Might just find someplace to make it permanent.

 

The weather cooperated today and I was able to conduct road tests with a straight taper rod, I used the same One Step Lean Rod dimensions of .070 * .063 * .056 but substituted a Straight Taper for the Mid-Range Step as you suggested may be worth trying.
The resulting Rod Dimensions are .070 - Taper - .056 NOTE; Although it sounds simpler the Straight Taper requires more patience to make, The only way to get a good long straight taper is to grind a small triangular file safe on two edges to the same width as the taper is long.

The Straight Taper sure seems to work well, I tried my best to conduct an objective test by trying every loading condition in every gear I could think of. I did several runs of slow progressive throttle application at speed in high gear up a mild grade and did not feel the classic " lean hesitation " at any time but I do think we are getting close to that point. Power seemed smooth and predictable, The longer transition time ( distance down the rod to obtain/equal the .063 of the midrange step) and slightly leaner mixture resulting from the Taper did not seem to adversely affect performance of the upper half of the Mid Range, So far so good.

I plan on making another Straight Taper Rod that is 1 1/2 Steps Lean just to see what happens, Untill then I plan to run this Straight Taper Rod for awhile to see how it does when the weather ( Density Altitude ) changes and if there is any notable difference in economy.

Test was conducted with a clean properly serviced stock oil bath air cleaner, Less restrictive air filters may produce slightly leaner mixtures requiring a proportionately richer rod. I will also admit my engine is due for an overhaul, Its well worn and getting tired.

I would also like to thank Warloch and Jayhawkclint for adding the reminder of how valuable a tuning tool the Vacuum Gauge is to this thread.
Vacuum Gauge update, Bench testing done on 07/05/08 with a modified carb body revealed that it takes 10"hg of vacuum to keep the metering rod bottomed in the jet well. Below 10 inches of vacuum the pump diaphragm will begin lifting the metering rod and at 4 inches of vacuum only the high speed step remains in the jet orfice. UPDATE; Due to a vacuum bleed port between manifold and pump diaphragm there is always enough vacuum to keep the metering rod in contact with the rod arm so rod position is really regulated by throttle position, this was also verified by tapping into the vacuum passage and attaching a gauge just under the diaphragm and comparing this with throttle position via a throttle position gauge under actual driving conditions.

Field testing results for straight taper rods as of 05/02/08. After running the 1 step and 1 1/2 step straight taper rods my opinion is as follows, I could not sense or feel while test driving any excessively lean condition that hampered performance or that would lead me to think a straight taper rod would not work well or not be worth pursuing if one chose to do so. I do feel / sense that with this configuration you are getting all thats easily achievable economy and performance wise out of the upper half of the midrange step area but I do suspicion the lower part of the taper just before the .056 high speed step may be to rich. If one choses to make a straight taper rod they should pay close attention to the engines reaction to throttle application around the 1/4 to 1/3 throttle position area. If one notices during slow smooth throttle application that there seems to be a place that does not produce acceleration but can be slowly throttled through without a stumble this would indicate that a faint amount would need to be removed where the midrange step would have started on a stepped rod, I would suggest removing .002 overall on diameter then test run.
My F-Head is being overhauled at the moment so when I can test the straight taper rod on a fresh engine I will do so and update this post with any useful information.

 

As to the thoughts of the tapered rod:
First, my thanks to Brian for the GREAT input!!!
I had a moment of paying attention to my vacuum gauge, when I usually pay the least attention to it ... just crusing my hilly rural neighborhood, slowly in 2nd gear ... great economy: I'm running ~15"hg ... then a bit of hill ... I apply a bit of pedal, then a bit more ... instantly, I go from 10-15"hg to 3" ... ahhh, I hit the "step" on the rod, and it's "on or off".
For pure performance, who cares ... but for economy, the "steps" seem to be counter productive and, the tapered rod possible help

 

When you posted the above manifold pressure numbers it got me to thinking..........Just exactly what kind of numbers should we be looking for when trying to get the most efficiency out of the YF on the F or L Head.

I had an extra carb body, pump diaphragm and a few springs so I did a few modifications that would allow me to conduct vacuum tests and note how it affected metering rod position. The following was noted, On average it took a minimum of 10"hg to keep the metering rod bottomed in the main jet well, Keeping the rod on the low speed step. Below 10"hg the pump diaphragm spring rapidly began lifting the rod out of the main jet. At 4"hg the midrange step area has cleared the jet orfice and only the high speed step remains in the jet.

Based on this limited bench testing it looks like we need to keep manifold vacuum at or above 10"hg for maximum economy.

UPDATE; The bench testing turned out to be a little deceptive as there is a bleed hole between the manifold and pump diaphragm that I bypassed during the test, this bleed hole reduces the amount of vacuum required to keep the metering rod in contact with the rod arm, the pump spring is calibrated to this lower vacuum, more road testing with a vacuum gauge connected directly to the diaphragm chamber and a throttle position gauge installed showed that under all conditions there is enough vacuum present to keep the rod in contact with the rod arm meaning that the rod position is really controlled by throttle position. Based on this attempting to drive a YF equipped engine with a vacuum gauge wont net us anything in the economy department.

 

this is such an informative thread.
any updates?

pretty please?

 

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