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HybridZ
5 Star Rising

240z misses and spudders same issue thought I fixed it..

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It looks like the prior owner had a wideband installed but he took it out. The probe and wire is still mounted to the header. The needles that are in the carbs are large (fat) compared to a cheap set I ordered and returned a few months back, didn't have any other needles to compare them to, but they were bigger than the needles that came in the cheap carb rebuild kit and the needle valves were bigger in my carbs than the ones that came in the cheap kit too.

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 Feels like I get to work on my Datsun every other month. Here we go again still dealing with the same issue. Not knowing the history of this car kind of sucks but the latest thing that I found may be the cause is that the prior owner possibly installed race needles in the SUs.. I cannot get rid of the miss, or tune these carburetors properly . how can I tell if I have race needles? And if I do, where can I get a set of factory needles that don't cost an arm and a leg?

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race needles...You have to take the needles out and measure the diameter at various intervals. If you give me your data, I can tell you if you have modified needles. Note that needles are only part of the equation. The quality of the seat needs to be considered. Not something that is usually  replaced in a rebuild. Since you mention that they look fatter leads me to believe that they are not race needles.

In any case, the video you posted looks very much like a lean condition at quick throttle apply. I assume when you mention happening at mid RPM, you are punching the throttle and get the hesitation/backfire. "race" needles should help with this vs the other way around. The second reason I think it lean is again your comment on the needles begin fat, which supports you having a lean condition at quick throttle. My question is what oil are you using? I would expect that the piston is moving up too fast and causing the lean condition. In essence, you have "disabled" accelerator pump design of these SUs (the "accelerator pump" is built into the design of the movement of needle and piston) That relationship is critical for good response. On another note, these SUs are happier if you run slightly rich, so tune them that way.

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One of the Pistons moves up slower than the other like it has more resistance but when you let off the piston they both fall about the same speed. I have a constant Miss or flutter throughout the RPM range I also adjusted the carbs to Max rich settings but still get the backfire.  I am using ATF fluid for the oil.

 

Last weekend I got some needles and the complete bottom seat assemly off a 72 240 that was completely rusted in rotted out including the carbs or at least they looked pretty beat. I have yet to take my carbs out and swap the needles and seats but at least now I have something to try. And now you got me thinking about the Pistons.

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