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balancing SU's


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Just got my Synchrometer/SK and took it out to see what the balance was like on my still shiny ZTherapy SU's, Well the lowest I can get the idle down to without stumbling/stalling is about 900rpm.

At that idle the SK reading is up in the 18-19 kg/h scale, so no accuracy and almost impossible to see.

Should I have bought the BK model with the air bypass? What am I doing wrong?



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  • Turn each jet all the way up then turn each down 2.5 turns.
  • Warm the car up.
  • Disconnect carb linkage between carbs from the long rod that is parallel and next to the balance tube (just pull an end off the ball and socket rod connecting the two between the carbs.
  • Make sure choke is off and jets are all the way up
  • Use STE synch on carb one and set air using idle stop screw adjusting screw
  • Use STE SK sync on carb two and set air using idle stop screw adjusting screw to match the flow on carb 1
  • Repeat until the same and you are happy with the idle.
  • Reach in and lift the piston on carb one all the way up. This disables carb 1 and makes the car run on the back carb only.
  • If car can not run turn the back carb jet down a 1/4 turn and try step 8 again. Repeat 9 until the car can run . You may have to turn the jet up.
  • Set carb 1 to the same number of turns down that carb 2 ended up at.
  • Repeat but run the engine on carb 1 by lifting the piston in carb 2 all the way up. Adjust the jet on carb 1 by no more than 1/4 turn up or down to run.
  • Rebalance the air flow as done in steps 5 & 6.
  • Reconnect the throttle linkage and turn the screw near the middle of the bar near the balance tube. This will increase the RPMS (off idle). Use the STE Sync tool so that carb 1 reads 30 or 35.
  • Adjust the screw on the linkage to the left of carb 2 and measure the air flow. Adjust so that carb two is also 30 or 35 (depending what your target was on carb one).
  • Unscrew the screw near the middle of the bar near the balance tube so that the carbs go back to idle

I can ring and walk you through it if you want.

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One more step to add...

16. Get out Colortune (or similar device) and match the flow based on combustion.

Like many on these forums, I've been using a synchrometer for 30 years. And I've always got good results. But a few years back I decided to spend $25 and try a colortune. Now I get great results. From what I understand there are better devices then a colortune as well.

The synchrometer gets me 90% there, but matching the combustion puts me right on the money.

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Yup. a/f is important. I use an O2 in the tail to get the A/F balanced when running on a single carb. In steps 8 to 11 I set the A/f to ~ 27 to 30 for each carb by adjusting the jet on the working carb. I then go for 14-15 at idle. Anything else requires piston weight and spring changes...and/or needle re-profiling.

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Also the initial turns down on the nozzles depends on temperature and altitude per the graph in the FSM. Using an IR thermometer helped me get the about the same A/F on each cylinder, but you need to take a lot of readings and average them - that doesn't take too long though. Reading the plugs gives a ballpark A/F. In my planes (in the game FSX) you set A/F using the EGT gauge. Problem with the colortunes (want to get one, though) is you can't read it under load at WOT. Maybe all you really need is 6 into 2 headers with a wideband for each secondary pipe, and an EGT on each primary close to the exhaust port. Would that be too many gauges? Colortune, IR, and reading the plugs, and butt dyno is way cheaper though. Oh yeah, and a stopwatch or dragstrip.

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Hey grannyknot

I take a spin down to see Laverne in Kitchener everyother spring.

He does use a colour tune.

He has my Z running great.

I get wanting to do it yourself but I can't argue with success:)


Edited by Casey_z
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Reading plugs is always a good learning experience. You can do it at idle... just try idling for 5 min at 1.5, then 2.5, then 3.5 the 4.5 turns for each setting. Read the middle plug for each carb (plugs 2 & 5) and watch the colour change. Both plugs should get darker and should be identical if you have the jet heights and needle to piston fit correct.

btw I set the air fuel on my oil fired furnace eyeballing the flame colour. A year later i had it tested and was within 2% so the principal of colour-tune is sound...however it only sets idle. You really need an O2 sniffer for checking idle, cruise and WOT (if you want to try to adjust these for performance or fuel efficiency or for the best compromise). The SU's are designed for set and forget so colour tune should be fine for daily driving.

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Blue and all, thanks. Lots to digest. Our local Zfest is next weekend so I don't want to screw around with it too much as the car is running fairly well at the moment.

After Zfest I'm going to switch back to the SK triples that came with the engine to see if there is a power gain to be had.

Blue, I'll take you up on your offer, I'm pretty sure I have the triples set up correctly but will probably need help.

If the triples prove to be too finicky for me I'll be heading back to the SU's.


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