I already watched the Just SU DVD and still have some queries on how to tune both Hitachi SU Carbs on my 240z. The procedure i am following is:

 

1. Remove air cleaner box

2. Verify each piston moves freely

3. rotate both idle mixture knob until it seat counterclockwise

4. rotate both idle mixture knob 2 full turns clockwise.

5. Crank the engine with the choke on for 3 minutes until it warms up

6. fully unscrew the fast idle screw

7. fully unscrew the balance mixture screw which is besides the rear carburetor

8. set the rear and front carburetor at 800 RPM using the idle screw on each carburetor and measure with the unysis that the ball is raising to the middle in both carbs.

9. How do i dial in the balance mixture screw and when?

 

Is there is something i am missing here? The car seems to be stumbling a Little bit and if i go over 7000 RPM the front carburetor is farting.  

The first thing to do before adjusting the SU's is to make sure all other systems are properly functioning: Emissions, Valves/Engine, Timing .... and then carbs.  Also want to make sure you have any and all vacuum leaks addressed first.  I have the Just SU's video as well and its great, but I also found this series on YouTube that is very useful:

 

Background on SUs:

   

 

   

 

Tuning:

   

After you have the idle speed and balance set with the fast idle screw backed off, screw the fast idle screw in to raise the engine speed and use the balance screw to balance the rear to the front. You will probably have to move back and forth between the balance and fast idle screws to maintain a good engine speed for the off-idle balance.

After they are balanced, back off the fast idle screw. It is just used for tuning.

Make sure your vacuum advance is working properly. If it's leaking or not hooked up you will never get your SU set-up right. 

 

TIP: Take of the dizzy cover and suck on the end of the tube to see if the mechanism moves. If it does then suck and hold suction for at least 10 seconds. If it doesn't more back until you let suction off then your diaphragm is fine. If the mech begins to creep back to resting point then you have a vac leak. Also, check that the vacuum port on the SU isn't obstructed. You can blow into the tube and should hear the air entering. I always perform this test using brand new vacuum hose. (You won't get a mouth full of gas fumes that way!)

 

Dave's MG SU Video's are great!!

 

>>3. rotate both idle mixture knob until it seat counterclockwise

You want to screw the Jet adjust wheel #1 up toward the carb body #4 on diagram.  When they hit bottom of the carb body #4 rotate back down toward #7. Make two full turns 360deg x 2.

 

>>5. Crank the engine with the choke on for 3 minutes until it warms up

Nope, you want to drive around for about 15min prior to adjusting anything to make sure the carbs and engine are warm.

 

>>8. set the rear and front carburetor at 800 RPM using the idle screw on each carburetor and measure with the unysis that the ball is raising to the middle in both carbs.

Ditch the unisys ball thing and get an "STE Synchrometer" flow meter gauge. You can leave it in the venturi without blocking air flow. Mine reads about 9.5 on front and back carbs. Use an external tach meter so you can watch the RPMs from your engine bay. 

 

Turn the Jet adjust wheel #1 down in 1/4 turn increments till you get the front and back balanced.

 

>>9. How do i dial in the balance mixture screw and when?

This is very last. You just want the screw touching the plate so that both throttle plates open in unison. 

I think you have the clockwise and counter clockwise backwards.  "Righty tighty" all the way up then "lefty loosey" 2.5 turns down.  Simplier, turn them towards the cylinder head until they stop, then turn them towards the fender 2.5 turns.

 

http://www.classiczcars.com/topic/39221-quick-and-dirty-su-tuning/

 

 

>>3. rotate both idle mixture knob until it seat counterclockwise

You want to screw the Jet adjust wheel #1 up toward the carb body #4 on diagram.  When they hit bottom of the carb body #4 rotate back down toward #7. Make two full turns 360deg x 2.

 

>>5. Crank the engine with the choke on for 3 minutes until it warms up

Nope, you want to drive around for about 15min prior to adjusting anything to make sure the carbs and engine are warm.

 

>>8. set the rear and front carburetor at 800 RPM using the idle screw on each carburetor and measure with the unysis that the ball is raising to the middle in both carbs.

Ditch the unisys ball thing and get an "STE Synchrometer" flow meter gauge. You can leave it in the venturi without blocking air flow. Mine reads about 9.5 on front and back carbs. Use an external tach meter so you can watch the RPMs from your engine bay. 

 

Turn the Jet adjust wheel #1 down in 1/4 turn increments till you get the front and back balanced.

 

>>9. How do i dial in the balance mixture screw and when?

This is very last. You just want the screw touching the plate so that both throttle plates open in unison. 

 

Can you explain the balance mixture screw dialing process a Little bit more? I have read that before you start tuning the carbs you should unscrew it all the way out? Is this true? if i start screwing it in the rear carburetor rpm increases.

You must tune the carbs in two stages.
 
1. Idle
2. Off idle

 

 

**** DISCONNECT CHOKE CABLES ****  Pull them out of the levers on the carb!  Make sure the jets move up and down freely and do not bind.
 
To tune the idle, you must warm up the car and, as mentioned above, ensure that timing and valve lash are set correctly with no vacuum leaks. The pistons in the carbs should fall freely and both at the same rate when dropped by your fingers. Do this with the oil dampers removed. You should also visually ensure they fall completely to the bottom of their travel.
 
The needles should be straight and placed in the piston so that the shoulder of the needle is aligned with the flat of the piston bottom, not with the machined trough. (refer to the next two pictures).
 

 
 

 

 

 
Before warming up the car, turn the knurled knobs under each jet all the way up then back down 2.5 turns. The next picture shows how to turn the knob.  It is like a regular screw and turning the jet up would be turning CW if you were looking at it from below.
 
 


 
The first step in tuning the idle is to ensure the linkage from the accelerator pedal to the carbs is disconnected.  Simply pull the nylon socket off the Aux. Throttle shaft (8) in the figure below.
 
The next step is to balance the air flow into each carb at idle.  You do this by turning the "throttle ajdusting screws" (10) in the figure below.

 

There is one for the front carb and one for the back.

 

The goal is to get equal air flow into each carb AND get the idle where you want it.
 

 

 

It is good to use a syncronizing device to measure air flow.

 

Once you are happy with the air flow balance, the next step is to adjust the fuel balance between the carbs.

 

A quick check of fuel balance is to lift each carb piston up a little (one at a time with a 10 sec break between lifting) and note how the engine stumbles.  It should behave the same for each piston being lifted. Next lift a piston all the way up and see if the car will run on just one piston. Repeat with the other piston.  If the car stalls out, restart (you may have to blip  the throttles by hand to get it going as the rod coupling the throttle linkage should still be disconnected). 

 

When you raised a piston, you disabled THAT carb so the car was running on the other.  If one carb runs worse than the other, give it more fuel (turn its jet down 1/2 a turn) and re-balance the air then try disabling both carbs and compare.  Both should run equally crappy. Repeat until happy then have a look at the plugs #2 and #5 so verify that the front carb #2 and back carb #5 are similar.

 

 

I'll add off idle tuning later.

In the diagram Blue posted you are adjusting the #14 balance screw correct?

 

The screw's only function is to make the front and rear carbs open at the same time. It is an adjustable link between the front and rear throttle plates (internal butterfly valves) so they open in unison (at the same time).

 

You back this screw off a few turns to remove any possible preloading and because after you re-tune the carbs they might not sync in the same position as before. When you screw in this screw too far you are preloading (opening) one throttle plate way before the other. This is why the RPMs went up on that carb. 

 

The screw is only supposed to touch the linkage balance plate with the lightest of pressure, or till there isn't any slop in the throttle shaft linkage. You can test for slop when you push down on the throttle shaft #11 in the diagram. Check to see that the screw and plate move at the same time. Remember the end of the screw only needs to be touching the linkage plate.

 

(screw> []=== * | <plate) *with gap (slop),  (screw> []===| <plate) no gap.

 

Hope this makes sense.

I already watched the Just SU DVD and still have some queries on how to tune both Hitachi SU Carbs on my 240z. The procedure i am following is:

 

1. Remove air cleaner box

2. Verify each piston moves freely

3. rotate both idle mixture knob until it seat counterclockwise

4. rotate both idle mixture knob 2 full turns clockwise.

5. Crank the engine with the choke on for 3 minutes until it warms up

6. fully unscrew the fast idle screw

7. fully unscrew the balance mixture screw which is besides the rear carburetor

8. set the rear and front carburetor at 800 RPM using the idle screw on each carburetor and measure with the unysis that the ball is raising to the middle in both carbs.

9. How do i dial in the balance mixture screw and when?

 

Is there is something i am missing here? The car seems to be stumbling a Little bit and if i go over 7000 RPM the front carburetor is farting.  

 

7k rpm ? Way to much for a stock engine.

That's some nice shop clothes there, jacket and cuff links. 

 

I always tune my carbs in a suit with French cuffs and cuff links, Doesn'

t everybody?