Quick and dirty SU Tuning

[240260280z]

For balancing the carbs here are a few quick and dirty methods I use when I don't have a flow meter or WB A/F meter:


Setting Idle
0 Assume good plugs, valve adjustment, no vacuum leaks, good fuel pressure and float levels set correctly.
1. Remove air box
2. Warm up car and remove all linkage connections so that the carb's throttle plates are resting on their set screws.
3. Adjust each set screw so that the suction felt when you put your knuckles against the throat of each carb and feel the air flowing between your fingers is the same on both carbs.
4. Stick you head down between both carbs and listen to the intensity of air whistling sound from each carb. It should be the same amplitude
5. Repeat 3&4 until happy and your rpms are 600-800rpm (exact rpm should be suited to your taste and performance when moving from stop or what the FSM states... your call baby)

Setting Off Idle
6. Reconnect linkage
7. Make sure the screw from the fast idle servo is not pushing on the throttle rods (that screw is the one next to the front carb).
8. Turn the fast idle screw at the center of the linkage (way up high) so that the car is idling ~ 3000 rpm
9. Adjust the balance screw on the linkage (next to the back carb) so that the air flow is the same in both carbs (as you did in 3&4 above)
You can also observe the point at which both throttles open when you push the main linkage. The balance screw should be set so that both throttle plates open at the same time.


Setting Mixture
0. Warm up car
1. Screw both jets all the way up to the top
2. Screw the jets down 2.5 turns (default starting point)
3. Take the car for a run and listen closely for popping in the exhaust (rich), popping in the intake (lean) and note power
4. Screw both jets up or down 1/2 turn and take another run to see impact.
5. Repeat 3&4 and do 1/4 adjustments until you are happy
6. Take the car for a spirited 30min drive with plug wrench then while driving fast on a long stretch, hit clutch, kill motor then pull over to side of road, look at all plugs and compare colour. If there is a difference between the front 3 and rear 3 then the fuel jets need tweaking. Adjust the darker 3's carb by turning up the jet to lean the mixture a 1/4 turn
If all plugs are too dark then turn both jets up 1/2 a turn.
7. Repeat 6 until you are happy. All plugs should be the same colour and very light brown. Listen for popping out the front to tell if you go too lean. Don't worry, you can take your time and do this leisurely over a couple of days to fine tune and colour match your plugs.


Power and Timing
1. Fill up with premium gasoline
2. Advance timing to 10° BTDC
3. Grab a 10mm wrench
4. Take car for a run and note power
5. Pull over and use 10mm wrench to advance timing a bit (turn distributor clock wise) Don't worry about a timing light
6. Take it for a run and note power increase and listen for pinging under load (try acceleration on a hill to force a load)
7. Repeat 5&6 until you are happy with balance of power vs. pinging
8. Check plugs again. You may need to add more fuel to get the last bit of power
9. Make note of timing at idle using a timing light.

Follow Up
1. Turn each jet up and count turns. Record these numbers for future reference then turn the jets back down.
2. Measure timing with a timing light at idle with no vacuum advance and at 3000 rpm Record these numbers. Aim for 36 to 38 maximum degrees at 3000rpm.
3. Measure engine vacuum and record number.
4. Measure air flow (with proper device) and record numbers
5. Measure O2 with WB at idle and at various RPM's underload and record numbers.
6. Re-balance carbs at idle and 3000 rpm's with airflow meter.
7. Adjust jet height using WB O2 sensor.
8. Close mouth of each carb (one at a time) set rpm to 3000 and run rough on one carb and measure with WB. Compare both carbs and strike balance.  (AF ratios should be the same and rpm's the same when running on one carb)
9. Do the "lift each piston 1/16" and compare how engine behaves as well as "Norm's lift each piston fully to disable" trick to also compare each side to the other's behaviour.
9. Measure 02 with WB at various rpms' under load and select or augment needle to taste. Repeat 9 until happy with WB and butt dyno performance.
10. If you have the luxury of a dyno then use it.
11. Once jets and needles are set Blueprint (measure evertyhing) including depth of jet from bridge of carb with vernier caliper's depth gauge.

Edited November 30, 2014 by Blue

[Powderkeg]

Philip,

Thanks for the tip.

I look forward to your posts and glad to see here!

Dave Ruiz

[Dobber]

Thanks for taking the time to post all the great tips.

[JLPurcell]

Philip

Thanks, I will keep this for future reference.

JLP

[240260280z]

Some drawings.

Edited May 7, 2011 by Blue

[240260280z]

more pictures and diagrams

[siteunseen]

I'm reviving this thread, it's easy to understand and very useful to new SU owners.

Thanks @Blue, I use this all the time (well not really, I've done them this way so many times now )  and recommend it to others as well.

[madkaw]

Good timing Cliff. I just went thru these SU's on the 72 and was about to look for a good thread for tune. I need to rig some hoses for float level and I will be set to tune 

[madkaw]

Need some tuning advice so I'm tagging on to this thread . SU's idle good and drive decent , but getting some lean spitting out the carbs. Pulling on the choke eliminates and adds gobs of power . Already 2.5 + turns on the little L24(stock), but it wants more. SM needles installed , timing checked, idles too good for vacuum leaks. Plugs showing rich idle but lean white burn on tip. 

Big question for me is where the shoulder of the needle should be relative to the piston. Flush with the groove or flush with the bottom of the piston. 

Float levels have been checked numerous times. Just tried richening them up and overflowed one. 

Help me

[Mark Maras]

 Needle shoulder flush with the outside of the groove. Enriching the mixture won't cause the float chamber to overflow. That's a problem with the needle and seat. IMO, IF, (BIG IF) the floats are adjusted properly the problem is the SM needles. Many in this club have an opinion on them. Good and bad. I've always liked the stock needles. But your original description confirms that the engine is running too lean. Are you double dog sure the float level is correct? That's the most likely cause of a lean mixture at 2 1/2 turns down.

[240260280z]

I just tried tuning Vin 249 as is.  Even with the jets screwed all the way up and needle shoulders down to meet, it was 13.7"1 A/F at idle.  The needles were stock N27.  The nozzles looked bigger than norm so I swapped out and it ran better. Still rich at idle and lean at higher rpms.  I think fine tuning needles is needed in some cases. 

 

[Mark Maras]

 Agree with the needle tuning especially if you have an A/F meter. Much more accurate than old school "reading the plugs". One of the things I love about SUs is, if you suspect the mixture is lean a gradual pull on the choke will not only tell you if it's lean but also give you an idea of how lean it is.. 

 Madkaw  The leaking (overflow) needle and seat in the offending float bowl will still need to be addressed. IMO, There's still something amiss in there. Unless the overflow occurred after a major float adjustment and the overflow stopped after another major adjustment, theoretically, I guess it's possible. but if the needle, seat and float tang are operating properly, logic tells me that there will be no overflow. no matter where the float is adjusted. If the floats were saturated the bowl would overflow but then you'd be flooding the carbs with gas all the time. Did you ever turn the mixture screws down to see where it ran good without the chokes?