Guest bastaad525 Posted August 2, 2005 Share Posted August 2, 2005 Hey all. Well it's been a long time since I've worked on SU's A fellow Z owner I met recently has a 260z with L28 motor and 3 screw round top SU's. He is having some problems with them, and I offered to help him out. His main issue is that the motor will bog or choke once he gets up to speed at WOT. He can take off from a stop, and 1st and 2nd gear go by just fine, but then in third gear, once he gets up to 4000-5000rpm, the motor falls flat pretty badly. I'm thinking fuel starvation. He's unsure what needles are in the carbs, and I took a look at them and I couldn't tell either just by looking. So I'm thinking either 1) he's running the stock 240z needles, and they can't provide enough fuel for the L28 (I recommended SM needles to him), 2) he's got a clog in his fuel lines somewhere (the car sat for a couple years undriven before he bought it, maybe old fuel sitting in the system has gummed up?) or 3) his fuel pump might be on it's way out... it's an electric fuel pump, not the mechanical one mounted on the side of the head. What do you guys think? Am I on the right track? Are there other things that might be causing it to bog like that at high rpm in 3rd gear? Also, I did a tune up on the carbs, took them partially apart, cleaned them up, and adjusted the fuel needles in the pistons as per the Ztherapy method. Basically just leave the needle loose in the piston, then press the piston down and the nozzle up to make sure they seat against each other when the fuel mixture nuts are adjusted fully closed. I also balanced the airflow with a Unisyn. however, even after I did this, I found that with the motor running, if I closed the mixture nuts all the way, the car still idled fine. And also, as I opened them up/richened them, the idle did NOT change at all until after I had opened the mixture nuts up over 3 full turns!!! I find this odd... when I had my SU's if I closed the mixture nuts fully the car would stall (no fuel), and the idle would change noticeably from even small adjustments of the mixture nuts... like 1/4 turn. At any rate, I left the mixture nuts adjusted to 2 full turns open... with such odd results from adjusting them I really wasn't sure where to put them. I'm thinking this means the nozzles are worn? The needles looked fine, they weren't scuffed up or worn looking at all. Any oppinions on what else this could mean? Am I on the right track? Anyways... was kinda fun working on SU's again... it's been a couple years... god they are the simplest carbs on earth. I'm hoping I can help this guy get his car running right. Quote Link to comment Share on other sites More sharing options...
Thumper Posted August 2, 2005 Share Posted August 2, 2005 I had the same problem on my su's and it turned out to be the floats. Get a clear tube and set the floats. This is more than likely it. If it was needles it would happen all the time no matter what gear. Since 3rd, 4th, and 5th last longer thus in the higher rpms longer the float runs out of gas. Only other problems would be bad float check valve or fuel pump. When you adjust the mixture nuts make sure to push the nozzle up because sometimes the choke lever will not fully seat them without alittle nudge. Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted August 2, 2005 Share Posted August 2, 2005 cool thanks for the tip. Yeah I wanted to check out his floats too, make sure they were properly adjusted and the needle valves weren't sticking or anything, but unfortunately we didnt' have replacement gaskets handy, and I have no idea how old the gaskets are that are on there now, so I didn't want to damage them and then he's got a fuel leak. I did tell him if he could get ahold of those gaskets though to bring the car back by and we could check out the floats too. Quote Link to comment Share on other sites More sharing options...
Guest rspiecha Posted August 2, 2005 Share Posted August 2, 2005 I had the same issue, ended up being the fuel tank vacuum locking. Run the car hard, pull over, turn engine off, and remove gas cap. If you hear a gush of air, you probably have the vacuum lock problem. I drilled my gas cap above the one way valve to fix the problem. Rob Quote Link to comment Share on other sites More sharing options...
Silent Posted August 3, 2005 Share Posted August 3, 2005 check the floats, i bet one of them is way off. also check the timing. that will also do the same thing as of running out of fuel. he doesn't need sm needles for a stockish l28 Quote Link to comment Share on other sites More sharing options...
Guest tony78_280z Posted August 3, 2005 Share Posted August 3, 2005 *begins to stir the pot* Why not just get a ArizonaZ intake and switch to a holley carb? That is what I would do. (Ok, that was uncalled for) Quote Link to comment Share on other sites More sharing options...
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