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Massimo

Trouble tuning SU carbs

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I am having a heck of a time getting the correct air/fuel mixture set on my carbs. I have a 260z and the engine was just rebuilt with .020" over pistons, stage 2 cam from MSA and 3-2 MSA header. I am running the stock 260 intake/balance tube with emissions removed and 4 screw round top SU's. I also just rebuilt the carbs with the ztherapy kit. The problem I am having is at idle the wideband is reading an air/fuel ratio around 13 and at part throttle, it really leans out at around 14.5 - 15.5. I am pretty sure the needles are n27's and was wondering if I need to switch to SM needles? I have followed pretty much every guide to tuning that I could find on here but I can still not get it right. 

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I would try the SM's. They're reported to run richer from 2500 rpm all the way up and are possibly better suited to larger bore engines (2.8) and free breathing engines. They don't seem to work well on stock 2.4's. I'd also give Ztherapy a call and ask for their opinion.

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My rebuild kit came with SMs from Ztherapy.  Runs good on my slightly modified free flowing 2.4.  msa 6 to 1 header and 2.5 premium exhaust.

Their dvd has old info on setting the floats though. You can't measure .5" with the lids upside down. Needle valves are too sensitive.

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3 hours ago, Massimo said:

I am having a heck of a time getting the correct air/fuel mixture set on my carbs. I have a 260z and the engine was just rebuilt with .020" over pistons, stage 2 cam from MSA and 3-2 MSA header. I am running the stock 260 intake/balance tube with emissions removed and 4 screw round top SU's. I also just rebuilt the carbs with the ztherapy kit. The problem I am having is at idle the wideband is reading an air/fuel ratio around 13 and at part throttle, it really leans out at around 14.5 - 15.5. I am pretty sure the needles are n27's and was wondering if I need to switch to SM needles? I have followed pretty much every guide to tuning that I could find on here but I can still not get it right. 

I can't seem to find a problem in your description.

 

What is the mixture at WOT?

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at WOT it is still around 13.5. It is my understanding of afr is that at idle it would be 14 - 14.5 and at part throttle, it would richen up to around 13 -13.5 and at WOT it would be around 12.5. I did check the needles and they are in fact the SM needles. 

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I took it out for a drive this evening to reconfirm the afr and it wants to idle around 12. Cruising in 4th at 3000 the afr is reading at 15 - 15.5 which seems really lean. I noticed going uphill it goes really lean getting around 16.5. Accelerating at WOT it is exactly where it needs to be at 12.7ish. Could this be a problem relating to the float levels? When I rebuilt them I followed the ztherapy video setting them at .5" as siteunseen said could be a problem.

Sent from my SM-N915V using Classic Zcar Club mobile

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Such is the problem with tuning the SUs, they only have one circuit. You're either going to have to live with creating lots of NOx due to the lean mixture, compromise by setting the mixture nut richer, or get new needles.

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What weight oil are you using in the Su's? I believe the factory recommended 20 w. I had good luck with ATF.

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I was running atf and then switched to 20 wt fork oil. The 20 wt seems to be better to keep the mixture from going extremely lean on quick accelerations.

Sent from my SM-N915V using Classic Zcar Club mobile

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Your fuel/air mixture rations seem ok to me. It is suppose to lean out a bit at cruise. If it running ok leave it.

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