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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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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 by Miles

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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.

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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.

 

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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?

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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. 

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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 by 5 Star Rising

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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.

 

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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.

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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.

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What distributor are you using (get the letters and numbers off of the casting)?  You need this in order to know what initial timing should be as different distributors require different settings.  Previous owner(s) could have changed it.

 

You said you've set timing many times.  What did you set it at - initial setting in BTDC (Before Top Dead Center) degrees?

 

Have you watched the timing change as you rev it up?  Are you able to see the timing advance working?  There is supposed to be both a mechanical advance (accomplished with fly weight) and a vacuum advance.  Confirm that when you rev the engine up from idle that the ignition timing changes... (it should increase).  What is the total degrees it advances to by, let's say 3500 RPM?  It should be at least in the 30 to 36 degree territory.  I understand that there may not be marks going that high.  However, you can estimate by adding some paint marks on your pulley based on the distance between the existing ones.  Paint additional marks as necessary so you can see the total advance that is occurring.

 

Have you checked that the cam timing is set properly?  Take valve cover off  (and spark plugs out) and rotate engine by hand/socket until a bright (or dark) link matches the mark on cam gear and then align cam gear to mark on front cam plate.  Be aware that there are two links on the chain that are notably different from the rest, so you may have to do this for both of them.  After many revolutions, you will see a marked link will line up with the mark on the cam gear.  When you further align the cam gear with the mark on the cam plate, you should be able to look at the timing pointer and its relation to the crank pulley markings to verify cam timing is set right.  If the pointer is not pointing at the TDC mark, then keep rotating until the next mark aligns with the cam gear mark - align cam gear with plate and check pointer at crank gear.  If neither of these line everything up, your cam timing is off.

 

Also, you should check that the distributor drive spindle is correctly installed - you can remove the distributor and view it compared to reference pics of where it is supposed to be at top dead center (look on this forum or google images).  You need to know that all of the mechanical alignments are correct before you can conclude that your ignition timing setting efforts are being accurately performed.   

 

Until you knock these things out as possibilities for the problem(s), you have too many variables to troubleshoot systematically.

Edited by inline6

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Thanks Inline. That is some good info. I actually replaced the distributor and have the same issue. The shaft on the old distributor had a ton of play and the vacuum advance diaphragm was shot.   So I got a rebuilt unit from Autozone, the new shaft was tight and the diaphragm in the vacuum advance acually held vacuum pressure with a vacuum pump how ever the advance mechanism did not move very much even applying a bunch of pressure.

 

I cant see the engine producing as much vacuum as I pumped in that thing, so the advance probably does not work too well with the reman unit. However with the new distributor and electronic ignition excluding the points and condenser, I still have the issue. I tried the timing in mutiple positions btdc and extremly advanced and had the same issue. I wish I had a spare set of good running carbs to bolt on and see if it ran different. 

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Here is a picture of one of the needles that was in my car. It has a "Z" on the side. The other needle on the left in the picture is out of a 72 240z that I got from a friend it was not in my carbs but just used as a reference. Does the one on the right look like a racing needle? I have no idea what to look for in comparison.

20180210_203201-1.jpg

20180210_201609.jpg

20180210_201602-1.jpg

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14 hours ago, 5 Star Rising said:

Thanks Inline. That is some good info. I actually replaced the distributor and have the same issue. The shaft on the old distributor had a ton of play and the vacuum advance diaphragm was shot.   So I got a rebuilt unit from Autozone, the new shaft was tight and the diaphragm in the vacuum advance acually held vacuum pressure with a vacuum pump how ever the advance mechanism did not move very much even applying a bunch of pressure.

 

I cant see the engine producing as much vacuum as I pumped in that thing, so the advance probably does not work too well with the reman unit. However with the new distributor and electronic ignition excluding the points and condenser, I still have the issue. I tried the timing in mutiple positions btdc and extremly advanced and had the same issue. I wish I had a spare set of good running carbs to bolt on and see if it ran different. 

 

Honestly, until you confirm the cam timing, and the distributor drive are installed correctly, doing other things that rely on those to be right are half efforts.  Even with the remanufactured distributor, you should get the markings on the casting so it can be properly identified.  Assumptions, such as that they sold you the right reman distributor for example, don't help solve your problem.  Facts are better.

 

You need to do what I outlined in my last post.

 

I became an expert with my 4 bolt SU carbs - I ran them for something like 15 years.  There is a list of things that need to be done right for those to be right.  However, quite a few things can be wrong on them, and they still work reasonably well.  

 

Just looking at the needles visually isn't going to help you identify them.  New old stock ones I bought for my car came with black "ink" N27 printed on the fat part of the shaft that goes inside the piston.  Easy to rub off... and once it is gone, no way to identify them by looking at them - you have to measure their diameter at specified intervals along the needles length and compare to notes others (search the internet) have made in that same regard.  You can find that info online.  The "Z" is probably just what a previous owner wrote on them with marker - no bearing on whether those are actually z car needles, or which z car needles, obviously.

 

A lot of people don't know how to set the needles in the pistons right.  And getting the needles in the correct position is a crucial thing about the Z's SU carburetors.  To start, you have to wind the nozzles all the way in, have the domes off.  Grab a piston and have the needle set screw loose enough to slide the needle in and out of the hole easily.  Then put the needles in the pistons so the fat base portion sticks out easily an 1/8th of an inch.  Next, set the set screw so it is just tight enough to keep so that the needles do not move on their own, but can move easily if pushed upon.  Test this.  The needles should move easily if you push or pull them in and out, but they should not move if you hold the piston vertically and shake it back and forth.  Set the needle again so the base is proud of the piston bottom (both the channel and the bottom of the piston). Then set the pistons in the carb bodies making sure not to bump the needles as you align them to go into the nozzles.  When you set the piston down all of the way, the ridge on the fat base of the needle will contact the nozzle - and the needle will be pushed up into the piston.  This sets a "touching" or no gap fit between the shank of the needle and the nozzle top surface.  Remove the piston carefully and tighten the needle set screw. 

 

When unscrew the nozzles to their initial settings per the manual, you will have the correct gap between the top of the nozzle and the base of the needle.   The needles are now at the correct height.  

 

Other things:

Don't bother with ATF in the piston tubes.  Just use standard 10W30 motor oil.  I tried ATF and it didn't work as well.  There are low and high markings on the screw in "dip" sticks.  Set them both to full when the sticks are threaded all the way in - don't go over.  I used a small bottle (like air tool oil) which allowed me to add single drops of oil at at time - as necessary to get the level set precisely.

 

If you don't have a carburetor synchronizer tool, buy one.  You need to get the air coming into each carb to be the same amount.  This is done at two places: at idle, and "off idle".  Until you set this correctly - the front three and back three pistons will try to operate like two different engines.  Obviously, not a good idea - not smooth.

 

With these things and those of my prior post - if you do them and can supply specifics back, maybe the solution to your problem(s) won't be far off.

 

 

 

 

 

Edited by inline6

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Thanks Inline. I actually watched 2.5hrs of the Ztherapy SU video again last night as I had not watched it for about a year. I had written down most of the info you gave me above about the carbs and I also remember following the video and setting the needles correctly last year, what I did pick up in the video this time around was my float levels, I remember trying to set them like the video but I did not have gross jets, as I tried to set the float levels the way it shows on the video the weight of my float kept calapsing the needle valve so I was unsure if it was set correctly, I am getting a lean condition and back firing out of both carbs when the choke is off, I also have the carbs set to the richest position.

 

Before I pull my valve cover I'm going to raise my float levels a bit and see if that gets rid of the lean condition and backfire. I had only messed with the float level one time and I felt unsure at the time if they were acurate. I will try this today and see if something simple like the float level is causing this. This could actually be a separate issue that I created from the very beginning, as in earlier posts I had mentioned that I had found 2 spark plug wires swapped from the PO. It obviously made a huge difference, but I am stuck with the backfiring lean condition that I may have caused by trying to fix the first issue lol. I know.. sounds stupid. It is a possibility. I will definitely start all over if this float adjustment does not work today.

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I just looked again at your pics.  That is not a Z on the base of the needle.  It is an N.  That was followed by the number - most likely 27.  Those are the right ones.  You really don't want to have scarring at that point of the needle.  You may want to polish them up with some 2000 grit sandpaper, or think about replacing them down the road.  Also, when assembled can you lift the pistons to the roof with your fingers, hold them tight against the roof and let go of them both really quick... and see if the pistons drop both at the same rate, and if they clunk on the carb floors.  They should drop at the same rate. 

 

And they both should clunk.  If they don't you've likely got a bent needle which can keep the pistons from seating.  That will cause a lean condition as well.  

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I cant seem to set these floats right. Do I have the right float and hardware in here? Seems like the fuel bowls are not filling up enough, when I pull the floats out on both carbs the bowls are only like a 1/4 of the way full. Cant even see the fuel in the nozzle when looking straight down into the nozzle with a flashlight.

 

I cant seem to bend the tabs enough so the bowls will fill more with out having the float touching the lid at a steep angle, but then I run the risk of the bottom of the float hitting the side of the wall first before closing the needle valve. Any suggestions on this?

20180211_164522.jpg

Edited by 5 Star Rising

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