Density Altitude

For years, I’ve been making a rookie’s mistake that I should’ve learned to fix long ago: I’ve been tuning my carburetors based on MSL altitude. Basically, I would just check a map or ask at the airport for the elevation, and I thought I would be good to go. This method, however, does not take into account barometric conditions. Instead, the carb should be tuned to "Density Altitude." There is an explanation for this posted here. The abbreviated version is below.

In order to calculate density altitude for the location where your JEEP sits, get a little scratch pad and pencil handy, then go to:

http://www.weather.gov/

After you type in your ZIP, gather up the following:

1) You need your current elevation in feet. Usually located in the upper left corner of the NWS page in parenthesis. If it doesn’t show up, just Google it.
2) You need your air temperature in degrees Fahrenheit.
3) You need the barometric reading in inches of Mercury(Hg).
4) You need the dew point in degrees Fahrenheit.

After you have jotted this information down on your scratch paper, go here:

http://wahiduddin.net/calc/calc_hp_dp.htm

Enter the data into the appropriate blocks. Note that “altimeter” is your barometer reading. Don’t forget your decimal point and two numbers to the right of the decimal or else it will give you erroneous data. Click on “calculate.” There will be several numbers spit out. You can do what you want with them, some are just FYI, but the most important number is “density altitude.” With this number, you can now refer back to your carb literature to make educated changes to your jet sizing.

 

Clint,

Interesting stuff. I gave the calculator a shot. It really helped that I found a weather station that posts historical data on the net, and the station is located about 1/8 mile from me. This allows me to plug in the data for any day in the past year.

No wonder I'm getting lousy mileage, I'm running rich almost all year, except in the winter (when I don't drive the thing much). I live at about 3500ft, in the mountains, and was amazed to discover my density altitude runs anywhere from 2788ft to 7200ft. Without leaving my driveway. DA really goes up when its hot! I can imagine the DA 8500ft on a hot day.

This info gets the German riled up, and it must be fixed. I've got a carter YF carb, and even though I've read the FSM, I'm still really unclear of how to change the main jet sizing, or even where different size jets are available. NAPA have such a thing? What about the rod? From what I can figure the shape of the rod has to be changed when you change the jet? I'd take it to a mechanic, but so far, I haven't found one I'd trust to rebuild a carb. Seems like most of them are parts replacers. So I'm gonna have to figure this out myself. Can anybody point me in the right directon?

 

The only thing you need to change is the Metering Rod itself, You do not need to change the Main Jet and the Idle/Low Speed jet does not usually need to be changed.
Its true that from summer to winter even at the same elevations air/fuel ratio can vary up to half a step on the Rods.........Problem is there is only three factory Rods listed for the 938SD ........standard, one step lean and two steps lean. These three cover a wide range of acceptable use from a factory perspective but may not be ideal for someone that is willing to tune for all the variables.
PM me, I think I can point you in the right direction.

 

interesting, very interesting, kinda starting to like fi now

 

Is there some reason you can't provide the possible right-direction to the whole group?

 

You dang pilots....think you can apply aviation matters to your Jeeps...R)R)R)

 

I went through a similar period of confusion, then denial, then astonishment. I was standing at the machine shop one day when I first heard about this. It was pretty much the summer conditions I described in the link above. I was complaining to the machinist how I had followed Holley's directions, and yet my JEEP was still running pig rich. He was pretty short with me and brutally honest about letting me know what a moron I was inbetween expletives. Muttered something about the elevation being at 4000' that day. (Lawrence sits at 810' MSL) He was an older guy, so it was just one of those situations at the time where I was like "Yeah. Sure. Riiiiiiight. I'll just take my Jeep now and be going." Boy was I ashamed later on when the carbed dialed right in for a 4000' density altitude.

 

If she had a set a wings man I know she could fly...

 

OMG What was I thinking There is no reason at all. I have had allot on my mind this last week, My Apologies to all my new Friends.
I would be happy to , It may take me an evening or two to think about how to word a few things and it will be a long post but I will get some information up that I think will help.
Thanks for the wake-up call......Stay tuned

 

I don't mean to speak for anyone other than myself but, from my searches of the archives and other sites, the lean rods are hard to come by (?) and, any help w/ this (rich) prob would be greatly appreciated

 

Somewhere on the net I ran across a company (in the southwest, IIRC) that will custom build you a YF for your altitude. Would custom machine a metering rod for you. Trouble is, I've searched so much that the memory might be from a page 10 years old, and the company no longer exists. I had no idea I would need such a thing at the time or I would have bookmarked it. Anybody have any memory of such a company?

 

Don't worry about it too much, but the beauty of the forum is that somebody a year from now can search and find the answer to their problem. I'll be interested in seeing your write up on it.

 

Never accured to me to do this. Mainly I suppose because of an aversion to pulling the top off the carb everyday.:?

On of the drag racing sites about 10 years back, or may have been one of their magazines, ran an article about this kinda thing. The guy would set up a small portable weather station and then calculate this stuff out for each run. Seemed to make a big difference in his times.

 

The right direction I am referring to is learning to make our own Metering Rods to get the best possible performance from our rigs at the elevation and seasonal conditions where we live ( Density Altitude ).
Even if your choice of the three original Carter Rods were still readily available it would still be a performance compromise for many of us.
I will try to explain and will use the YF938SD on my F-Head 134 as an example, The Carter Bulletin I have lists the Main Jet as .0935, The Idle/Low Speed Jet at .028 and the three Factory Rods sized as follows-
Standard ( 75-1500 ) .07325 .0655*.057*.04725
One Step Lean ( 75-1570 ) .07525 .070*.063*.056
Two Steps Lean ( 75-1571 ) .07525 .0735*.067*.059
If you take the area of the Main Jet and subtract the area of the Rod Step you will get the resulting fuel flow area. When you compare the resulting areas on a percentage basis you will find that the Factory offered One Step Lean Rod restricts fuel flow area throughout the range an additional 15%-16% over the Standard Rod and the Two Step Lean Rod will restrict fuel flow area another 7%-8% over the One Step Rod.
It is my personal opinion that the manufacturers further tried to shave inventory and parts costs by quietly reducing the number of Rods even further over time, you will find allot of so called " Standard Rods " that are One Step Lean or more on the first and sometimes Second Step and Standard on the High Speed Step, A good example of this is the Standard Rod that came out of mine. The Carter Part Number is 75-1500 and is so stamped on the Rod but..........It measures .070*.062*.053*.046.... seeming to conflict with the Service Bulletin, I think the early series rods were sized to run a little richer than the later series rods even though the part numbers are the same. If you compare with the above sizes you can see why some F and L heads may idle around the woods ok but drink fuel getting there and back at 45mph, Especially at any altitude over 4000'.
Installing a smaller Main Jet by itself would not solve this problem, It would probably have a lean stumble at low speed and still drink fuel at 45mph.
I feel that many of us are running a Metering Rod that is only half right at best.
If one is willing to make their own rods it will open up a whole new world in performance and help fuel economy.
One will find that depending on where you live a Rod that is 1/2 step lean or 1 1/2 step lean ( or any other percentage you need ) may be perfect for 90% of the driving you do.
If you take the three old factory Rod Options and calculate the area percentages for 1/2 steps we get,
Standard .07325* .0655*.057*.04725
1/2 Step Lean .074* .06755*.060*.052
One Step Lean .07525*.070*.063*.056
1 1/2 Step Lean .07525*.07155*.0655*.0575
Two Steps Lean .07525* .0735*.067*.059
You could do 1/4 steps, 1/3 steps or any other % increment you wanted
Once you catch on to calculating the percent of area for each rod step you can make the entire rod rich or lean throughout the range or just on a specific step.
All kidding aside all one needs to make their own rods is a 1/2" variable speed drill clamped to the edge of a workbench, A 6" fine single cut mill file ground safe on one edge, Micrometer or Caliper that you can read or guess down to half a thousandth , Some 400&600 wet and dry ........and patience, We will all ruin at least two rods before we get the hang of it
I do understand that not everyone will care to do this but I hope this information will help over time, Maybe those of us that can make them could help out some of our jeep brothers now and then, I just wish I had time for all of you.

Here is how I make them.
1. Cut the brass rod stock to a rough length of 3", I like stock in the .078 to .080 range.
2. Form the eye end of the rod first, Clamp two .125 drill bits in a vise just far enough apart to get the rod stock between them. Place one bit .125 above the vice jaw and the other about .250 above the vice jaw. Insert just enough brass rod stock between the bits to hold it in place against the short one while wrapping it around the tall one to form the eye. The eye does not have to be perfectly round, It can look a little like a Q-Tip with a slight gap where the eye would normally close against itself - This gap is needed to get the rod on the adjusting screw type of rod arm found on the later model carbs, If you have the older " bend to adjust " type make the eye as round as possible with little to no gap.
3. Find some heavy wall tubing that will just slide over the rod stock, Cut 3 short pieces with square ends the following lengths. Large piece cut to 1.200" , Medium piece cut to 0.990", Small piece cut to 0.800". These will serve as guides for the safe edge of the file and determine step length & location.
4. Run the smooth end of a small drill bit through the eye of both your new rod blank and the original, With the bit held level and the rods freely hanging trim the new rod blank to the same length as the original.
5. Insert eye end of rod fully into drill chuck, The 1/2" chuck should have enough room in it to accommodate the eye and still allow the jaws to hold the rod firmly. Tighten chuck to just allow rod to be moved back and forth by hand, Pull rod out gently until it stops, Place the large piece of tubing over the rod and push rod back into chuck until it is flush with end of tube, Tighten chuck lightly and double check for flush, Remove tube.
6. Adjust drill speed to a slow setting, Just above stall speed for lack of a better analogy. You can adjust speed to suit after you get the hang of it. It does not have to be very fast.
7. Place medium tube over rod, Place safe edge of file against tube while using a finger from your free hand to hold tube in place against chuck and counter the force of the file to prevent bending the rod. Move file smoothly against rotation slowly working down to within .001 of the desired diameter. When getting close measure often to ensure high speed step is not tapered, If it is color it with black marker so you can see where you are removing material and straighten it out. TIP; When getting close its easier to hold the file under the rod and draw it towards you, That way you can see the marker and what you are doing alot better. Use a piece of the wet and dry paper folded on a popsicle stick or similar item and remove most of the file marks and bring to finished dimension. A little water or oil will help the paper work better.
8. Remove medium tube and place small tube on rod, Begin turning down the midrange step in the same fashion, When straight and polished down to the desired diameter remove small tube so you have room to work and turn a taper on the midrange step from a point one third down its length to the high speed step. This taper should not blend into the high speed step but have the edge lightly chamfered, take care not to cut into the high speed step, A perceptible ridge that you can just feel with a fingernail is acceptable. Polish taper. TIP: Taper is easiest to form pulling file from underneath the rod, Use black marker frequently to keep track of where you are removing material. A small triangular or pattern file works well.
9. Using the safe edge of the file against the drill chuck turn down and finish the low speed step ( sometimes referred to as the economy step ). Carefully turn a faint 45 degree chamfer on the bottom of the low speed step, Do not run a taper up the rod, You are only trying to get rid of the sharp square corner. Again a perceptible ridge you can just feel with a fingernail is better than cutting into the midrange step.
10. The short pieces of tubing will wear so keep an eye on their length. You can cut them to match any original rod step length, Remember that they are only a spacer to guide the file thats minus the rod step in length from large to medium to small.
11. Be patient, You will be amazed at the quality of work You can produce with simple tools....Enjoy.

 

Excellent commentary and info! (your solution was too easy Thank you!

Now, a different angle: Are the Solex or Weber carb folks experiencing similar probs? The reason I ask is, I have my YF doing all it can (without rod mod) and it works great, month after month but, I still think it's a bit rich (economy) ... does a Solex or Weber eliminate all probs at all elevations, ala FI? Or, do you have to re-jet them for different elevation ranges ... say 300' - 6000'?

 

If we're patient, I think this thread could be of immense value to many of us.
Please don't hijack.

 

These forum things are hard to gauge sometimes, without a direct quote, so, please bear with me. Is the quote I posted here referring to me and my comments?

 

Nope. Carb postings in general get wacky, it seems. Too many times, I've seen the one poster on a thread with valuable info, stop posting because things got off track.
My post was probably a plea more than anything else, I really need Brian's info, and I'll bet a few others do too. No 'fence meant.

 

Some one asked a valid (to me, at least) question.
Will this stuff that BrianP outlines work for the Solex single barrel and the Weber Redlines and others, too?:?

 

I have no personal experience with Solex or Weber Carbs but the stuff I outlined will work and does apply to any unit that has Metering Rods provided they are not part of a solinoid activated-computer/sensor controlled emission control system.

What makes the YF easy to make Rods for is that in this particular carb they are a physically large part compared to their function, Making Rods for the primary system of a Q-Jet would not be impossible but would be difficult with common tools because of their small size.

Other Carbs like the M2100/2150 require that the actual jets be changed to alter Resulting Fuel Flow Area because they are designed with Manifold Pressure activated enrichment valves ( these valves actually have jets in them too but they are a fixed drilled orfice ) .The same problem exists with these units to a point as jets and enrichment valves with different orfice sizes are not readily available anymore.

Hope that helps