5 Star Rising Posted September 23, 2017 Author Share Posted September 23, 2017 Checking back in. For the last few months I have been building a engine on my 71 Chevelle, It is finally done and runs amazing.. I am ready to start trying to figure out this issue with the 240z again running like crap and sometimes backfiring. Technically I never rebuilt the Su's because the kit they sent me was wrong and it was of cheap quality. I cleaned the carbs, put back all the same parts and set the float levels. I'm wondering if the guy before me put race needles in these carbs or something to make them not tune correctly. Been wanting to slap another set of Su's on the car and see if the carb's are my main issue but cant seem to find a good set local and they are a grip $$$ online just for testing purposes. I will see about getting a video up. Quote Link to comment Share on other sites More sharing options...
5 Star Rising Posted September 25, 2017 Author Share Posted September 25, 2017 240z Quote Link to comment Share on other sites More sharing options...
5 Star Rising Posted September 25, 2017 Author Share Posted September 25, 2017 Just now, 5 Star Rising said: 240z Apparently that video upload failed. 40 mins so far trying to upload Quote Link to comment Share on other sites More sharing options...
5 Star Rising Posted September 25, 2017 Author Share Posted September 25, 2017 So here is a super short clip of my car running. Sounds like it's running lean with those backfires. Its not smooth and responsive when driving. I get a miss in the mid rpms also. Does this sound carb related? 240z backfire_001.mp4 Quote Link to comment Share on other sites More sharing options...
HuD 91gt Posted September 25, 2017 Share Posted September 25, 2017 (edited) My car would do the same when cold and trying to rev. Aka Lean condition. With proper float levels, and a good idle mixture it is possible racier needles have been installed in the past (Do you have a wideband o2?). SM needles or the like, like to idle very very rich to get a proper mixture in the power band. If your setting your mixtures at idle for stoich, then they will be running lean everywhere else. Richen up the mixture, and maybe order some new factory needles from Z therapy. Edited September 25, 2017 by HuD 91gt Quote Link to comment Share on other sites More sharing options...
5 Star Rising Posted September 25, 2017 Author Share Posted September 25, 2017 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. Quote Link to comment Share on other sites More sharing options...
5 Star Rising Posted December 14, 2017 Author Share Posted December 14, 2017 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? Quote Link to comment Share on other sites More sharing options...
jsteg Posted January 12, 2018 Share Posted January 12, 2018 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. Quote Link to comment Share on other sites More sharing options...
5 Star Rising Posted January 13, 2018 Author Share Posted January 13, 2018 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. Quote Link to comment Share on other sites More sharing options...
5 Star Rising Posted January 21, 2018 Author Share Posted January 21, 2018 Still haven't had time to pull my carbs, but do you think that because one of the pistons has more resistance going up than the other carb that it may be the issue? Quote Link to comment Share on other sites More sharing options...
Miles Posted January 21, 2018 Share Posted January 21, 2018 (edited) 2 hours ago, 5 Star Rising said: Still haven't had time to pull my carbs, but do you think that because one of the pistons has more resistance going up than the other carb that it may be the issue? 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. Google search results: https://www.google.com/search?biw=1536&bih=741&ei=GB5lWof6K8LYjwPRqZmACg&q=240z+su+carb+lean+backfire&oq=240z+su+carb+lean+backfire&gs_l=psy-ab.12...48161.51822.0.54621.8.8.0.0.0.0.107.703.7j1.8.0....0...1c.1.64.psy-ab..0.6.545...33i21k1j33i160k1.0.TrwnWHR2y0U Edited January 21, 2018 by Miles Quote Link to comment Share on other sites More sharing options...
5 Star Rising Posted January 21, 2018 Author Share Posted January 21, 2018 Good read. Looks like he had similar issues. Yeah when you get like 30 minutes a week to work on your car you want to dive into it with a plan in hopes that you can solve the issue. Hoping to nail the issue before summer. Hardest part is trying to figure out what the PO has done to this car. I see evidence of mods. Had a wideband, oil cooler, wide open exhaust, the car has a little lope, could have a different cam in it, oversized pistons, racing needles. Trying to set everything back to factory as much as possible if possible but depending on what he did I may not be able to. Quote Link to comment Share on other sites More sharing options...
jsteg Posted January 22, 2018 Share Posted January 22, 2018 23 hours ago, 5 Star Rising said: Still haven't had time to pull my carbs, but do you think that because one of the pistons has more resistance going up than the other carb that it may be the issue? yes and although both are acting different, both could still be moving improperly. As mentioned, would be good to see what needles you have and watch the pistons when you apply throttle. The interesting part is that it backfires out of both carbs. Not a lean condition in just one set of cylinders. (1,2,3 vs 4,5,6) couple other potential culprits. 1) Make sure you have no vaccum leaks and that both Vac and Mech advance is working. I'm sure you have checked this but never hurts to check 3 times. A vac leak is of course going to cause lean cond and not allow and changes you make in the carb tuning to take effect. Don't only check the hoses. There are so many intefaces to check compared to other engines (car to insulator blocks, blocks to intake runners, intake runners to manifold, etc) 2) If you think that you have a non-stock cam, do you know if it is degreed right (Cam timing)? Did the car ever run right? Wonder what kind of overlap you have. Quote Link to comment Share on other sites More sharing options...
5 Star Rising Posted February 8, 2018 Author Share Posted February 8, 2018 The car never ran right, i used cans of carb cleaner to find small vacuum leaks never found any the car ran the same. Tonight I started the car after months, it idled good and reved up good and smooth with the choke pulled 3/4 of the way back, as soon as the car fully warmed up I open up the choke all the way and reved the car. I got the back fire out of the carbs and that miss. Im thinking maybe the carbs are just super lean even though I have the adjustments on the bottom of each carb set to the richest possible setting maxed out . Maybe it's still not enough fuel. Is there a way to make these carbs even more rich? Like maybe adjusting the float levels more? Quote Link to comment Share on other sites More sharing options...
morbias Posted February 8, 2018 Share Posted February 8, 2018 Without reading the whole thread again, I would suggest checking the ignition system including strength of the spark, whether you have the correct weights and springs in the distributor, whether the variable timing plate in the top of the distributor is moving freely, if the vacuum advance is functioning properly. It also might be worth checking your valve clearances if you haven't already. Quote Link to comment Share on other sites More sharing options...
HuD 91gt Posted February 9, 2018 Share Posted February 9, 2018 If you don't know if your running lean, pull the plugs. The wideband 02 dinosaur way Plugs don't lie. Quote Link to comment Share on other sites More sharing options...
5 Star Rising Posted February 9, 2018 Author Share Posted February 9, 2018 (edited) 5 hours ago, morbias said: Without reading the whole thread again, I would suggest checking the ignition system including strength of the spark, whether you have the correct weights and springs in the distributor, whether the variable timing plate in the top of the distributor is moving freely, if the vacuum advance is functioning properly. It also might be worth checking your valve clearances if you haven't already. I went through the ignition system already, timing, new distributor, new coil, got rid of the points, went electronic ignition, same issue. How can I make the carbs run rich with the choke open all the way? Seems like it is not getting enough fuel. Runs better when I choke it and cut the air consumtion in half thus balancing the air fuel mixture better. As soon as I open the choke all the way it seems like the carbs are getting way more air than fuel. Would raising the float levels so that the fuel almost pours out the top of the nozzles help richen the mixture? It backfires out of both carbs when I Rev it and the choke is fully open. Doesn't really back fire when I pull the choke back a bit. I have both carbs set to maximum rich settings and it doesnt seem like it is enough fuel. Edited February 9, 2018 by 5 Star Rising Quote Link to comment Share on other sites More sharing options...
Miles Posted February 9, 2018 Share Posted February 9, 2018 2 hours ago, 5 Star Rising said: I went through the ignition system already, timing, new distributor, new coil, got rid of the points, went electronic ignition, same issue. How can I make the carbs run rich with the choke open all the way? Seems like it is not getting enough fuel. Follow the directions given in the ZTherapy video. What you are doing over and over and over again is not going to get the result that you want. Diagnose and fix the carbs. Runs better when I choke it and cut the air consumtion in half thus balancing the air fuel mixture better. As soon as I open the choke all the way it seems like the carbs are getting way more air than fuel. Would raising the float levels so that the fuel almost pours out the top of the nozzles help richen the mixture? It backfires out of both carbs when I Rev it and the choke is fully open. Doesn't really back fire when I pull the choke back a bit. I have both carbs set to maximum rich settings and it doesnt seem like it is enough fuel. Quote Link to comment Share on other sites More sharing options...
morbias Posted February 9, 2018 Share Posted February 9, 2018 A bit further up the page you mentioned you don't know what engine setup you have or what sort of needles you have installed; that's pretty critical with regard to your issue tbh. You really need to hook up a wideband O2 sensor and AFR gauge to figure out what's going on, or even just take the car to a shop and see if they'll hook up the gear they use for the emissions test while you rev the engine. That would at least give you some rudimentary idea of if the carbs are leaning out further up the rev range - if they are then no amount of fiddling with the carbs is going to fix it and you probably need different needles, assuming the carbs are in good working condition and don't have hidden issues like worn throttle shafts. Quote Link to comment Share on other sites More sharing options...
5 Star Rising Posted February 10, 2018 Author Share Posted February 10, 2018 10 hours ago, morbias said: A bit further up the page you mentioned you don't know what engine setup you have or what sort of needles you have installed; that's pretty critical with regard to your issue tbh. You really need to hook up a wideband O2 sensor and AFR gauge to figure out what's going on, or even just take the car to a shop and see if they'll hook up the gear they use for the emissions test while you rev the engine. That would at least give you some rudimentary idea of if the carbs are leaning out further up the rev range - if they are then no amount of fiddling with the carbs is going to fix it and you probably need different needles, assuming the carbs are in good working condition and don't have hidden issues like worn throttle shafts. I have a old school air/fuel mix machine with a stinger that I got at a garage sale a few years ago. I hooked it up last year and it said the carbs were a bit rich at idle, I'm not sure how accurate that machine is. I watched that z therapy video 3 or 4 times already and followed it. But still getting the same result. This isn't my first Z, and I never had a issue like this before. Going to check the needles this weekend. Wondering why I can clean up the miss when I pull the choke back. Also going to watch that video again..... this is getting old. Quote Link to comment Share on other sites More sharing options...
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