Quick and dirty SU Tuning

[Mark Maras]

 Others have reported that the old standard tan color is not attainable with the fuel formulas currently in use. I can only assume that a much lighter tan color is what we're shooting for.

[moritz55]

My intent is not to hijack this thread but thought adding to it may help others on some data I compiled as I fine tuned my New SU's (from ZTherapy).  I can delete this if recommended... 

My '73 240z engine:  L24 rebuilt with Stock Cam, Mallory Breakerless Disty with Mallory Promaster Coil, MSD 6A Box, New Rockers/Lash Pads adjusted valves to spec.

                     NOTE Spark Plugs:    NGK BP6ES Plugs gapped to 0.062" (due to Mallory Promaster Coil and MSD 6A Spark) ... I run 87 Octane Unleaded.

Tuned SU's using ZTherapy DVD:  18degrees BTDC, Idles great at 840rpm (when my electric fans kick on RPM drops to ~800rpm).  Both SU's dialed currently at 2-1/4 Turns Out (Front & Back are the same).   I DO NOT have O2 Sensors so am not able to measure precisely during driving.... 

Background:  I tuned the SUs using SM Needles, 20wt damper oil (ATF) as described in the DVD and it ran extremely good.  Balanced them real well and rock solid idling, starting well and no sputtering during acceleration and top cruising speed.   However after about 250miles on them, I noticed it ran a little rich during accelerating and cruising based on the plugs so I tweaked Mixture Screws Leaner (Turned In) ... after repeating this for about 1month putting 300miles on it, I could not get the idle as stable as I liked... so I began to research on what Needle profile may work better for my engine config during accelerating and cruising. 

I compiled some data to share (all from this site) on Needle comparisons for anyone's interest to use.   There are 2 Needles that come close to the N-27 profile during working ranges and I went with one that is a slightly richer at Stations #3 through #6, but leaner than "SM" ...   I am currently testing the "SR" Needle that is close to the N-27 (but slightly richer through all stations when compared to Idle Diameter), have set the Mixture Screws on the SUs back to 2-1/4 Turns Out and it's idling rock sold again... put about 100miles on these needles and the car is running real well and will stick with them for now. 

The closest Needle profile to the N-27 is actually the "SL" , and may work better for those with a more Stock Like engine configuration.  

I used pages 10 & 11 for Needle Profile testing during driving from attachment:   SU Needles - Tuning_SU_Carburetors.pdf

I purchase my 0.100" Needles from SU Midel:   http://www.sumidel.com/shop/category/-0100-fixed

Data table in the last attachment..............................

  

 

Maybe this information is helpful to others ....   if not I can delete this ....   again I did not want to confuse or hijack this thread.  

Many thanks.....  

SU Needle charts pp 47 to 49.pdf SU Needles - Tuning_SU_Carburetors.pdf SU Needle profiles vs N27 Stock.pdf

SU Needle Stations.bmp

Edited May 10, 2019 by moritz55

[Mark Maras]

Excellent info. Thanks for sharing.

[AK260]

Utterly brilliant!!! Thank you for sharing. I'm trying to do the opposite I.e. Modify SM needles to keep SUs on modified L28. So this is a great resource. [emoji106]

[w3wilkes]

I wonder how much effect altitude has on all this? Those of us in the Rocky Mountain region of the USA are typically at 4,400 elevation or higher.

[moritz55]

Hey w3wilkes... good question. I am probably stating the obvious but assuming the tuning/idle is stable and setup properly at 4400 feet above sea level the same procedure should apply.  Therefore if following page 10 "Needle Testing" method of the SU Needles - Tuning_SU_Carburetors.pdf ; based on how the car behaves during accelerating in 4th gear (Top Gear) and how well it runs steady at Top Speed at the 4400ft above sea level environment, it would indicate which Needle Stations should be richer or leaner.  So when determining the optimum Needle profile of one Needle to another, the diameter reduction (or change) comparison must always reference off of Station #2 Idling position.    

Maybe a "SA" Needle profile works better at 4400 ft altitude versus an SM, SR or SL in order to be at 2.5turns out?  

Edited May 13, 2019 by moritz55

[Mark Maras]

 My 4 screw SUs were set to 2 1/2 turns down at sea level. The only time I had to lean them out was around 5000' heading up to Mt. Hood Timberline Lodge. (6000') At that time (hot day) the engine developed a miss. I adjusted them to 2 turns down, which cleared up the miss, and then readjusted them back to 2 1/2 turns when I got down to around 3000'.

[w3wilkes]

Thanks to both for the replies re: high altitude!

[siteunseen]

1 hour ago, Mark Maras said:

 My 4 screw SUs were set to 2 1/2 turns down at sea level. The only time I had to lean them out was around 5000' heading up to Mt. Hood Timberline Lodge. (6000') At that time (hot day) the engine developed a miss. I adjusted them to 2 turns down, which cleared up the miss, and then readjusted them back to 2 1/2 turns when I got down to around 3000'.

That's adjusting on the "fly" right there. I love these old cars.

 

[zKars]

This is very helpful information. Thanks for posting it.

I was reading the above post about the case where  Mark had to add 3 turns to compensate for an elevation change to get back into drivable state.  I had to richen it up about one turn to go from Whistler BC down to sea level in Squamish on one trip. Geez 3 turns seems like a lot I said....

So just as a point of reference in this subject matter, let's connect the number of turns required to move one station on a needle. The threads on the nozzle height adjuster are 1.0mm pitch I believe, so moving 1/8 inch, the distance between needle stations requires just a bit more than 3 complete turns. Makes me wonder how I get significant changes to AFR when tweeking the knob just 1/2 a turn or so during garage tuning sessions .... Crossing a threshold from one to the next? Maybe this stuff isn't so simple..

Edited May 13, 2019 by zKars

[Mark Maras]

 @zKars Perhaps my description was lacking. I didn't add 3 turns to the mixture screws. I adjusted them UP 1/2 turn. I ran 2 turns down at higher elevation. Back to 2 1/2 turns down at lower elevations. The only other adjustment I ever made was one frigid winter I adjusted the mixture screws down 1/2 turn more than usual because the engine ran better with a bit of choke. During that time the screws were at 3 turns down. Hope this clears up any misunderstanding.

[zKars]

Ah, that makes more sense. A total of 3. 

Still I marvel at how moving the nozzle less than a full station makes much of an effect, yet it clearly does.  Seems to me the flowrate of fuel from the nozzle may be a more complex function of the total needle profile that is immersed in the nozzle than simply the difference between the needle OD/nozzle ID area at the top of the nozzle.