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round top swapped 260z carb issues


Alekb

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Hi guys, about a year ago i bought a 1974 260z that had an N54 l28 swapped in, it also was converted to 4 screw round top su carbs. I rebuilt the head and the carbs. I got it running last spring and it idled great but whenever i tried to accellerate hard it would hit around 3500 rpms and then start to stutter, cough and die, almost like it was hitting a limiter. I did some trouble shooting and ended up trying a couple pulls without air filters on, and the z ran great, I could pull way past 4k and more without any stuttering or coughing. I thought it was the stock air box restricting air for some reason, so i bought itg air cleaners off of MSA. Even with the performance cleaners the issue persists. I've checked ignition timing and it is around 10 degrees advanced, I'm really at a loss and have no clue what could be the issue any help would be greatly appreciated

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 It would seem that the air filters are at fault. If that's true they have to be affecting the carbs in some way. Engine off, lift both carb pistons to the top. There should be considerable resistance but it should feel even. No lumps or bumps. Now let them drop at the same time. They should both hit the bottom with an audible clunk at close to the same time. If the drop rate is substantially different, swap the pistons and test again. The other drop test, tests the rising rate. Remove the domes and pistons. Tape (masking) all the holes in the bottom of the pistons. Place the domes upside down resting in the tops of something like coffee cups. Place both pistons in the domes at the top (equal heights) and drop them at the same time. If the drop rates are substantially different, swap the pistons and test them again.

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I did both tests and for the first drop test the front piston dropped slower, however the gap between speeds was  smallest with the original piston orientation at about half a second between them. I did the rise test and they dropped at nearly the same rate. 

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The drop tests don't show any serious anomalies, IMO. A few more questions. Did it run well prior to the carb rebuild? Have you balanced the carbs? A better method for reading the plugs is to drive it until it starts to cut out, kick in the clutch, shut it off and then pull a couple of plugs before it idles. My gut feeling is the poor performance under load is caused by lack of fuel but judging by the air filter info it would seem to be the opposite, lack of air or too much fuel. You may want to run this problem by the folks at classiczcars.com. I'm Mark Maras over there.

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