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Quick and dirty SU Tuning

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It wouldn't run with the modded SM's so I put some modified N27's in today. Still wouldn't start. Since it ran with them before figured it must something else. Had a look at the points, they were rather played. The points cam was worn down, not enough gap. Bumped it around until it was on the cam and opened up the points a bit. It started right up and ran great, except for overrun when I shut it down after a drive.

Tomorrow will install a new set of points and set the dwell with the meter AFTER I get some points cam grease. Maybe the modded SM's were OK. I'll try putting  them back if it goes lean.

Idle was set high couple days ago so it would run. I forgot to readjust it, so today it was way too high once the engine warmed up. Maybe the cause of the overrun.

Edited by Stanley

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hey guys,

So, I spend the whole day on the car. The car starts right up with the fast idle screw and balance screw completely open and carbs are synced. I adjusted the float again but this time to the 70/71 specs. I ran the car until warmed and no overflowing. Now what I need is a tach so I can finalize the tuning.

I also received today a new coil, hoping that will fix my RPM guage but Iam having issues installing it. I posted in the electrical section.

Thank guys for your input.

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Can anyone comment on how strong the requirement is to keep mixture screws with the same setting for each side?  Certainly it's good to keep them as similar as possible, but even with the most perfect float level setting there may be slight differences for each.  I would assume slight differences are acceptable to get the mixture correct on each side?

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I disable one and get it to idle as smooth as possible with the mixture wheel. Let it clear up and then do the other one. Like Blue said "equally shitty".

When I'm happy with them the wheels are within 1/4 to 1/2 inch apart. I have one of those protruding knubs painted black on both carbs.

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You do have to compromise however.  One tuning item to note w/o air fuel meter::

70-73 domed SU's tuning:

1. have an air balance for idle (throttle valve stops)

2.  have air balance for off-idle (back carb rod screw)

3.  only have one fuel adjustment (jet height)

When tuning SU's, you are forced to adjust fuel height for either idle or off-idle.  Since you drive the car more than you idle, it is best to tune for off-idle.

The "equally shitty" stumbling (at idle when running on one carb) is good for initial tuning but it would be better to match the rpm drop (off idle... say 3000 rpm when running on one carb).

For this adjustment, use the high-speed screw to get the rpms up to ~ 3000 then balance air. Once balanced, disable the front carb and note lower rpm.  Repeat by disabling the rear carb and note rpm.  Compare rpms. The carb that was  carrying the load with the lower rpm needs its fuel level adjusted.  Repeat the test by balancing air then measuring rpm drop, then adjusting fuel until both are synced.  Revert back to idle to see how far off they are.  If the carbs are in good form and the needles are well profiled for the engine then you should be good to go.

Run the car for a while on the roads and note plug colour and exhaust soot.  Equally adjust each jet up or down to enrich or lean mixture to taste.

 

 

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There's nothing in the FSM that says both mixture nuts should be set the same. It has a graph that gives preliminary settings dependent on altitude and temperature. 2.5 turns down is about right for 70 F near sea level. Hotter ambient temperature and higher altitude require a leaner initial setting, richer for cold weather, but that's just the starting point. If the engine or carb needles aren't stock then the initial setting should probably be different. Then the mix is set per FSM or other more advanced or simplified methods. It's important to use syncrometer or similar and adjust balance screw to get them balanced. Since it's difficult to get the floats set exactly right, and since the carb pistons move differently depending on the oil level in each one, and maybe other differences, it's unlikely that the best mix would be where both mix nuts are set exactly the same. But if they're way off when it's running the best something's wrong, probably the float levels, and needs to be corrected.

Correct me if I'm wrong, but I think once the carbs are balanced and a test drive shows it's a little lean or rich, the carbs can be adjusted by moving the mix nuts of each the same amount, say 1/6 turn before re-testing, without rebalancing them.  Since you can't adjust them with the air cleaner(s) on, and air cleaners make it run richer according to British SU documentation, I set them a little lean and richen them up according to test drive, plug color and paper towel test. Air/fuel gauge would be better.

Edited by Stanley
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On 22/6/2017 at 0:32 PM, Mark Maras said:

 What color are the plugs at 4 turns down?

Ngk Bp6es Plugs at 3 turns clockwise both su carbs after some hard running and couple of traffic lights.  Should i richen or lean them?

20170730_114752.jpg

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 Sorry, I can't really tell judging by the pic. I'm assuming that's an old plug. The black portion indicates a rich mixture. The white portion indicates a lean mixture. You'll have to start with new plugs that have all white porcelain to get a proper reading. That said, I'd say that the black portion was from running the mixture too rich when the plugs were new. Then it looks like the plug was cleaned, the mixture and or floats were re-set to a lean mixture. Neither color is what you're looking for. You're shooting for a tan color. Normal driving.

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Is the tan color still attainable with unleaded fuel? I run 93 octane and its clear when the mixture is too rich as they are dark, but the lean condition seems like it might be harder to diagnose with the plug color using unleaded fuel?

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 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.

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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..............................

image.png  

image.png

 

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.....  

'73 240z Engine Update 04-2019 (1).jpg

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 by moritz55
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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.

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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.

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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 by moritz55

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 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'.

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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. LOL

 

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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 by zKars

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 @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.

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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.  

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