SU carbs WILL work perfectly on my modified L28 and give DCOEs a run for their money!!! | Page 2 | The Z Club of Great Britain
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SU carbs WILL work perfectly on my modified L28 and give DCOEs a run for their money!!!

Discussion in '240Z / 260Z / 280Z' started by AliK, Jun 11, 2019.

  1. RawlinSTR

    Z Club Member

    Enjoying this thread so far, thanks for taking the time to document it.
    AliK likes this.
  2. 240L31

    240L31 Forum User

    Ali, I just created an account for the sake of replying to your topic! :)

    I love your scientific approach. I did a lot of tinkering with my SUs, too. I am certain that they can perform really well if tuned properly. My engine is a L31 stroker, running fully rebuild SUs with great success. I started with SM needles but the engine was running lean. Following, I switched the needles for RH needles which work great so far:

    AFR 12.5 to 13.5 at idle/low cruise,
    AFR 14 at high cruise (120-150 Km/h 3000-4000rpm),
    AFR 12.5-13 at really high cruise (180-220 Km/h)
    AFR 12.2-12.8 at hard acceleration.

    The only thing I would like to change are higher AFR numbers during low cruise.
    I didn't start changing the piston springs yet. Another issue to be analyzed: How important is the actual fuel bowl level? I'm starting to think that it doesn't matter too much (played around with no real effect).
    Last edited: Jun 17, 2019
    AliK likes this.
  3. AliK

    Z Club Member
    Staff Member Moderator

    Glad the musings of a madman have some value! Kind of you to say so.

    Welcome to the forum!!

    Well sir I feel humbled by your kind words, thank you. I’m a mere hack having a lot of fun with this. I find it enjoyable in equal measure to blasting around the countryside in the Z!

    BTW do you get any torque flat spots / dips on your lovely torquey stroker?

    Your AFRs IMHO look fab! The literature / research papers I’ve read all concur that ideal WOT is around the 12.8 mark with modern fuels. I would love to sit between 12-12.8 all the way around on WOT! As you say, the idle / cruise is a little rich. Have you considered getting a second set of needles and “polishing” the top end richer so that you can set them a little leaner at idle? I will have a look on my spreadsheet to see if I can see an RH “like” needle that runs a leaner idle.

    On float bowls - I am the furthest thing from an expert, but again, all that I’ve read says it’s a vital part of SU carb adjustment/tuning.

    Now here’s an admission of guilt: in theory it’s the first thing you should check before adjusting SUs but I stuck my head in the sand hoping z Therapy set them up perfectly. Then I read that they get “re-adjusted” by the bouncing in transit. So I got curious and since you did ask ...

    I tried a new approach by using a Sharpie and colouring the needle of each carb.


    Then I lowered it into the jet and pulled her out quickly.


    Here is the theoretical ideal ...


    So my rear carb “appears” to be way low and the front practically wiped the whole needle clean! Several times!!

    Not trusting my Sharpie-test I tried the “10 turns down” method (each turn being 1mm) and discovered that I still can’t get a ‘well’ of fuel. I even cranked the engine for a while to no avail. So I suspect that the rear is very low. Interestingly it is the one on the video!! I haven’t yet had time to check the front one properly but as the float-synch tool is no longer available for sale, I bought these and will use some 5mm clear tuning to make a level checker ....


    The beauty of it is that you can check the levels with the engine running / blipping the throttle / fast idle to see what is happening beyond static conditions. I wouldn’t feel happy driving with it though!

    I will post my finding again when I next get to play.

    On springs, I was going to experiment with the HS6 red ones (4oz) but decided that I should change one thing at a time to know the effects of that thing in isolation.

    The next video will hopefully have a vacuum gauge in the field of view.

    On that note a very wise man told me that I am not consistently comparing things: I.e. the reflected pulses off the inner wing and lack of the trumpets on the airbox, means that what you see in my video is not what is happening with the filter housing screwed on.
  4. 240L31

    240L31 Forum User

    I don't get any flat spot so far - the engine is pretty torquey as you said and really picks up from 3000-5500 rpm. CR 10.7:1, 235 deg@0.05" cam, Zstory Sport/Race header. I just came back from a 2600km road trip with no issues :) I did check my float levels using the banjo bolt/clear tube method. It is still pretty difficult to measure the right fuel level height (not even to speak of what is the correct height). But I am fairly certain that both levels are within 2mm of the target height.

    The dampers are filled with ATF fluid, seems to work good.

  5. Farmer42

    Z Club Member

    The level of the float makes a big difference to the mixture levels. It can either flood the piston chamber & jet making it too rich on idle & cruising or not allow enough fuel through to get acceleration & top end.

    You need to set the correct height of the float before you mess about with jets, needles etc. An easy way is to take the float lid off and turn it upside down and measure the distance between the bottom of the float and the lid. I used a correct size drill bit for mine, bending the tang to the right distance but mine are crude British SUs not the refined Datsun Hitachis. You might need a more refined measuring method for those. Unfortunately I can't remember the measurement and I can't get to my info right now but I am sure it is in the Haynes manual or in one of the Burlen booklets.
    AliK likes this.
  6. AliK

    Z Club Member
    Staff Member Moderator

    @240L31 - that sounds like a very sweet engine and it’s great that you aren’t experiencing any flat spots or dips.

    @Farmer Paul - you make a very good point there. One to be prioritised for my next tinker. It may even smooth out my idle a little. Interesting method you used. Having taken out the float once before and having seen how delicate the whole thing is, it’s one of those jobs that scares me!

    Shame about the banjo method not working as well as I had imagined it should. I’m thinking that the only way to accurately know the heights is the “10 turns” method.
  7. RawlinSTR

    Z Club Member

    Here's a good video showing the float height check on SU's Paul was talking about:

    AliK likes this.
  8. AliK

    Z Club Member
    Staff Member Moderator

    Right, so the weekend was a little too hectic for words and progress slow.

    One interesting thing that occurred to me was that (and pretty obvious to anyone else who understands SUs) the notch at the bottom of the piston, just below my 11 mark is c.1.1mm. With the jets being set at 1.9mm down the cranking position on the needle is position 1. Watching the webcam at start up and idle (1000rpm) station 3 on the needle is being used.


    Got the KD needles in and played with the mix - I think I ended up at half to 3/4 of a turn lean doing it on the side of the road. I need to take off the domes and measure to be sure.

    I think we are getting pretty close now. The AFRs are very close to target and the 3-3.5 krpm torque hole is more like a flat spot now, as opposed to a dip in power. The sacrifice has been a leaner idle than I am looking for, but knowing exactly where that is on the needle, with visual evidence then I can easily sort that at a later stage when I’m completely satisfied with the rest of it. So now we have AFRs of:

    Idle: 15.5-16.2 ish

    Cruise at 40-50: 14.7-15.2

    Cruise at 70 and high rpm in lower gears: 14.7-15.8

    WOT in any gear from 3300 to 6500rpm 11.5-12.8.

    Holds rock solid at 12.8 at the very top - from 5000 to 6500 rev limit. I tested this several times late at night on a steep uphill dual carriageway in 3rd so as to stay at safe public road speeds but give the piston time to fully raise under full load.

    For now I will live with the idle and observe if it really is a problem or not, before even thinking about polishing the needles.

    I also did the 10 turns down test on both carbs and while the photo doesn’t really show it, the fuel was about 0.5mm below that point. Just touching the black flexible pipe from the float bowls made t come above the jets. Both carbs were virtually identical in this sense.


    I suspect my actual fuel float bowl levels are close to 11mm down.


    Unfortunately I can’t get VLC to record sound off the web cam but I will upload some footage with improved lighting at normal speed. Also need to find a marker pen that is resistant to ethanol / petrol and do a proper video at some point (with sound).

    Next up: taking the domes off and measuring the jet heights to note what I have set them at in millimetres and to ensure they are both set identically. Then I plan to run the same tests again with the airbox and filter fitted to fine tune the AFR which I believe gets richer once everything is back on. Also the trumpets on the inside of the airbox making the inlet tract longer will have an effect on the mid range and low end torque.

    I cannot for love nor money get the joints at the back of the exhaust system to fully seal. So I will resort to some exhaust assembly paste one evening this week and see if that makes any difference to my power dip /flatspot.

    This worked very well on the primary to collector joints - But horrid stuff to work with:


    After that and the Silverstone 50th (oh and sorting out slippery tyres) I will RR power run, then swap exhaust system to do a back to back comparison with a further power run.
    Last edited: Jun 24, 2019
    atomman and Huw like this.
  9. toopy

    Z Club Member

    Have you bumped off the wife and kids? I'm jealous of all this Z time you seem to be getting!
    Paul_S and AliK like this.
  10. AliK

    Z Club Member
    Staff Member Moderator

    Ha! Saturday morning is her turn with piano lessons and my 2-3 hours of peace. Then after the little monsters go to bed and I’ve done my domestic goddess duties, I maybe can squeeze out an hour ;)

    All in all you get very efficient in the available time :p
    toopy and Paul_S like this.
  11. 240L31

    240L31 Forum User

    Hey Ali, any progress with your fine tuning? I will look again at alternative needle profiles during the winter season (preparing spring/summer 2020 tests).
  12. SeanDezart

    Z Club Member
    Official Trader

    A similar manifold on an L24 using 1 3/4'' round-tops with a DJR race head and cam.

    Attached Files:

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