djhedges Posted March 7, 2012 Share Posted March 7, 2012 Has anyone had any experience or tips for tuning these carbs for specifically for racing? I recently bought a ITS prepped 260z with the flat tops. All the emission stuff is plugged and the choke removed. The car has EGTs which show about 1100F at WOT on the straights. Last weekend we had a test and tune day at Oregon Raceway Park. The track is at about 2300ft elevation and the temperature was in the low 40 to mid 40's that day. We pulled the carbs and measured the needle at 45mm. We tried moving the needle to 45.5mm, 46.5mm and backwards to 44mm. EGTs still read 1100F except for the 44mm which was at 900F. We readjusted the valves and checked the compression. The plugs were gapped at 1.2mm instead 0.8mm. After regapping the plugs it idled much better but I won't have it on the track again until the 17th. Got any other suggestions? Quote Link to comment Share on other sites More sharing options...
Perfect240z Posted March 8, 2012 Share Posted March 8, 2012 Ive heard the flat top carbs are better used as a boat anchor lol. Get some refurbed roud tops from z therapy and they can install some better needles for you I think. Way worth it. They're better then new when you receive them. Quote Link to comment Share on other sites More sharing options...
rossman Posted March 8, 2012 Share Posted March 8, 2012 (edited) nevermind Edited March 8, 2012 by rossman Quote Link to comment Share on other sites More sharing options...
djhedges Posted March 8, 2012 Author Share Posted March 8, 2012 I can't run the round tops in ITS and I've heard of others getting the flat tops to work really well. The guy I bought the car from won plenty of races with the flat tops so they are doable. Quote Link to comment Share on other sites More sharing options...
cockerstar Posted March 8, 2012 Share Posted March 8, 2012 For an all out race effort, the flat top carbs have been modified to be more effective than round top carbs from what I've heard. I'm sure TonyD will chime in on the subject! Quote Link to comment Share on other sites More sharing options...
Tony D Posted March 9, 2012 Share Posted March 9, 2012 (edited) All Archived. And yes, class rules dictate the carbs. It was the intelligent ones who figured out there was no advantage to Round-Tops, but rather the Flat Tops were preferable because they have a separate idle circuit. Meaning, unlike the Round Tops, you CAN have a nice, properly adjusted idle mixture, and be fat rich for peak torque and power off-idle. It won't load up idling at the gate like a rich round top setup will. The key is the transition plumbing. But I'm having an especially trying day and don't have the energy to elaborate. Just about anything you do with round-tops is applicable to flat-tops, other than the idle circuitry, and the power valve. My preference is 73's since they have a larger float bowl than the 74's but look almost identical. You can make the needles pig rich and still have idle correct on them, you don't have to compromise between 'right at the top end, or at the bottom end'... Get a real EGR and stop looking at EGT other than for plugged jet diagnosis. The dyno tells all. Good Luck. This comment, though: "Ive heard the flat top carbs are better used as a boat anchor lol. Get some refurbed roud tops from z therapy and they can install some better needles for you I think. Way worth it. They're better then new when you receive them." Just makes me weary! Edited March 9, 2012 by Tony D Quote Link to comment Share on other sites More sharing options...
Joeoski Posted March 11, 2012 Share Posted March 11, 2012 I seen a PDF once to make the flat tops performance carbs but forgot where i seen it at it's on one of these Z forums some where for download start hunting around i know i will be looking for it again . Quote Link to comment Share on other sites More sharing options...
Perfect240z Posted March 12, 2012 Share Posted March 12, 2012 All Archived. And yes, class rules dictate the carbs. It was the intelligent ones who figured out there was no advantage to Round-Tops, but rather the Flat Tops were preferable because they have a separate idle circuit. Meaning, unlike the Round Tops, you CAN have a nice, properly adjusted idle mixture, and be fat rich for peak torque and power off-idle. It won't load up idling at the gate like a rich round top setup will. The key is the transition plumbing. But I'm having an especially trying day and don't have the energy to elaborate. Just about anything you do with round-tops is applicable to flat-tops, other than the idle circuitry, and the power valve. My preference is 73's since they have a larger float bowl than the 74's but look almost identical. You can make the needles pig rich and still have idle correct on them, you don't have to compromise between 'right at the top end, or at the bottom end'... Get a real EGR and stop looking at EGT other than for plugged jet diagnosis. The dyno tells all. Good Luck. This comment, though: "Ive heard the flat top carbs are better used as a boat anchor lol. Get some refurbed roud tops from z therapy and they can install some better needles for you I think. Way worth it. They're better then new when you receive them." Just makes me weary! I heard it from classic Z members sorry : / Never looked into flat tops because of those comments when I got my first Z. Quote Link to comment Share on other sites More sharing options...
macambra Posted March 12, 2012 Share Posted March 12, 2012 (edited) This guys flat tops look nice!!! http://www.youtube.com/watch?v=VgbQ33VJhIo http://www.youtube.com/watch?v=TR_5h-unBbE&feature=related Edited March 12, 2012 by macambra Quote Link to comment Share on other sites More sharing options...
randy 77zt Posted March 14, 2012 Share Posted March 14, 2012 Somewhere I have heard that these carbs will flow more cfm than round tops.As far as mixture goes I would get a 2 channel wide band o2 sensor gauge to monitor the carbs with.This would take out the guess work and give reliable data to work with.I have heard of people chucking the needles in a lathe and machining them with emory cloth.complying with class rules makes you use your brain instead of your wallet Quote Link to comment Share on other sites More sharing options...
Tony D Posted March 14, 2012 Share Posted March 14, 2012 "Complying with class rules makes you use your brain instead of your wallet." Not entirely true, those with big wallets and small brains simply buy guys with bigger brains to help them comply...er or cheat and not get caught. But rules do make you think, either of how to do it or how to cheat and not get caught. So to that use of the brain: AMEN BRUTHA! Quote Link to comment Share on other sites More sharing options...
BlueStag Posted March 14, 2012 Share Posted March 14, 2012 "Complying with class rules makes you use your brain instead of your wallet." Not entirely true, those with big wallets and small brains simply buy guys with bigger brains to help them comply...er or cheat and not get caught. But rules do make you think, either of how to do it or how to cheat and not get caught. So to that use of the brain: AMEN BRUTHA! Tony, You need to clear out your PM folder. Quote Link to comment Share on other sites More sharing options...
Tony D Posted March 15, 2012 Share Posted March 15, 2012 No, I just need to make my donation and get the old capacity back! I'm going to China for three days on the way back from Thailand. So much for getting caught up with Paperwork. Quote Link to comment Share on other sites More sharing options...
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