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SU carbs WILL work perfectly on my modified L28 and give DCOEs a run for their money!!!
- Thread starter AliK
- Start date
240L31
Club Member
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).
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).
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Glad the musings of a madman have some value! Kind of you to say so.
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 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).
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! [emoji33] 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 ....
(M8x1.0)
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.
240L31
Club Member
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.
Farmer42
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.
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.
@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.
@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.
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.
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.
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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
All in all you get very efficient in the available time
SeanDezart
Well-Known Forum User
A similar manifold on an L24 using 1 3/4'' round-tops with a DJR race head and cam.
Attachments
Apologies for the lack of updates guys, been very off the Z world for a few months and just catching up.
Once I sort out the new ignition issues (plasma ball 123 ignition cap under bonnet), weather permitting, I plan to get back into this.
Franky, I’m not ruling anything out but equally the same Sean system is producing a very lovely and progressive power curve on Woody’s engine; with a slightly wilder cam but less compression. So I can’t point to the header. I suspect it’s more than one thing combining to produce that effect - for example I don’t know my cam overlap - but am able to wind the engine up a thousand or more RPMs than Woody can.
The tuner was saying that it’s a cam / header interplay but as you say, a back to back test is required to know for sure what is causing it. Even before that, I’m going to play with the needle profiles first. I’m pretty convinced that leaning out the low end should minimise the effect and give me a smoother BHP line. The fact is, I absolutely love the way Sean’s header has opened up the top end and want to stick with it.
Sean, thank you for sharing that curve. Interestingly, I’ve noticed that most modified engines with wilder than stock cams / shaved heads have the two torque dips at pretty much the same revs and the same rev separation of about 1k rpm. Mine has the same bumps but way deeper dips and much higher peaks, which I’m hoping the next set of leaner low / mid range needles will straighten out a bit. Quite a few dyno charts to look at below for comparison. Obviously every engine / carb / exhaust combo is different but you’ll see what I mean about how common those dips are ...
https://forums.hybridz.org/topic/123487-compare-your-dyno-sheets-here/
The thing that is missing from most dyno charts is the CO2 plot which would show how rich or lean the mix gets throughout the rev range. Mine shows it going super rich at the torque dips (where on the web cam we saw the reversion effect). Hence why I think leaning up the earlier part of the rev range and sacrificing a little low end power may be a good solution. Although I’m not kidding myself that it will ever be fully cured. Most performance engines / setups have some compromises somewhere.
Once I sort out the new ignition issues (plasma ball 123 ignition cap under bonnet), weather permitting, I plan to get back into this.
Franky, I’m not ruling anything out but equally the same Sean system is producing a very lovely and progressive power curve on Woody’s engine; with a slightly wilder cam but less compression. So I can’t point to the header. I suspect it’s more than one thing combining to produce that effect - for example I don’t know my cam overlap - but am able to wind the engine up a thousand or more RPMs than Woody can.
The tuner was saying that it’s a cam / header interplay but as you say, a back to back test is required to know for sure what is causing it. Even before that, I’m going to play with the needle profiles first. I’m pretty convinced that leaning out the low end should minimise the effect and give me a smoother BHP line. The fact is, I absolutely love the way Sean’s header has opened up the top end and want to stick with it.
Sean, thank you for sharing that curve. Interestingly, I’ve noticed that most modified engines with wilder than stock cams / shaved heads have the two torque dips at pretty much the same revs and the same rev separation of about 1k rpm. Mine has the same bumps but way deeper dips and much higher peaks, which I’m hoping the next set of leaner low / mid range needles will straighten out a bit. Quite a few dyno charts to look at below for comparison. Obviously every engine / carb / exhaust combo is different but you’ll see what I mean about how common those dips are ...
https://forums.hybridz.org/topic/123487-compare-your-dyno-sheets-here/
The thing that is missing from most dyno charts is the CO2 plot which would show how rich or lean the mix gets throughout the rev range. Mine shows it going super rich at the torque dips (where on the web cam we saw the reversion effect). Hence why I think leaning up the earlier part of the rev range and sacrificing a little low end power may be a good solution. Although I’m not kidding myself that it will ever be fully cured. Most performance engines / setups have some compromises somewhere.
Thanks dude, me too. Need to solve this one as it bugs me.
Very interested to see how you will do that, please feel free to share when you have it worked out. I did contemplate using a miniature proximity sensor drilled into a spare dome I have and using an Arduino to log mm piston lift, vacuum and revs but given the time required, the thinking stopped right there!
Very interested to see how you will do that, please feel free to share when you have it worked out. I did contemplate using a miniature proximity sensor drilled into a spare dome I have and using an Arduino to log mm piston lift, vacuum and revs but given the time required, the thinking stopped right there!
So to recap where I got to:
I checked the float levels using banjos and they were dead on, thanks to ZTherapy!
The engine developed an intermittent and random “miss” - I spent DAYS of my life and £££s troubleshooting the electrical system and replacing components.
The 123 ignition plasma ball turns out to be a rookie error. In the 70’s with lower voltage coils it was OK to unplug leads and crank the engine, but with the flamethrower pushing out 40Kv it’s going to find a path to ground and this test method is not recommended.
I then started to read about vac and air leaks and set about finding the issue.
This is an interesting read: https://mechanicbase.com/engine/vacuum-leak-symptoms-causes/
Here’s the text of another post I did on the subject of finding this leak:
With the engine running, I used an unlit propane torch around all the areas where it could possibly be leaking. Nothing!
Then WD40 on the same areas. Nada!
Then water mist sprayer and guess what? Zip!
Next, I got a length of 18mm ID rubber hose and listened around the carbs and manifold area because the rear carb I thought was whistling loudly. Well, I found that both carbs are doing it and it was mainly at the mouth where the two small holes take air pressure to the piston. Hole 5 below. As I understand, SU carbs do make a high pitched sound without the air cleaner on.
Next up, I took the domes off to see if anything obvious was amiss. All I could think of was that somehow the domes are leaking.
I checked the bottom of the domes for flatness and found the rear carb in particular to be less than flat. So I got some 3mm closed cell foam sound deadening material and made a couple of gaskets.
This made it way harder to centre the needles and pistons but a bit of patience and carefulness while tightening down did the trick.
I went out to “buy some bread” and it’s a different car! The wobbly engine is a shed load smoother. The idle has also dropped over 100rpm which is a classic sign of plugging an air leak.
The AFR that previously would not budge past 15, now goes to 18+ on the overrun when in gear and cruising downhill. The idle is sitting at a lean 16 (previously 13.5-14) and cruising steady throttle AfR is between 14.7- 16. WOT when out of the reversion part of the range sits doggedly between 11.5-12.5, the latter at around 6.5k rpm. The rear wheels now squirm in 2nd at 4K rpm. This is all with the air cleaner off for troubleshooting. It would richen up a little when I put that back on. But boy does she sound angry with it off!!!
For now, I am quietly confident that the issue may have been resolved and will try to find a better solution with some plumbing rubber O rings.
The whistle though is now even louder - is this normal operation or do I have another leak that is now sucking harder? I’m erring in the side of the former but watch this space. Here’s a video of the sound ...
With COVID, my testing is now restricted to when I have to go get any shopping - especially the items not available at the local “Little Waitrose” that require me to go up the dual carriageway to the big Sainsbury’s.
I should be happy but I am itching to try out the MC needles with jets at around 1.1mm down and the original 2oz springs and the 4oz red ones I am currently using.
Moral of the story? Check intake and manifold leaks religiously before blaming other things.
BTW, I really did go out to buy bread from the next town!!!
I checked the float levels using banjos and they were dead on, thanks to ZTherapy!
The engine developed an intermittent and random “miss” - I spent DAYS of my life and £££s troubleshooting the electrical system and replacing components.
The 123 ignition plasma ball turns out to be a rookie error. In the 70’s with lower voltage coils it was OK to unplug leads and crank the engine, but with the flamethrower pushing out 40Kv it’s going to find a path to ground and this test method is not recommended.
I then started to read about vac and air leaks and set about finding the issue.
This is an interesting read: https://mechanicbase.com/engine/vacuum-leak-symptoms-causes/
Here’s the text of another post I did on the subject of finding this leak:
With the engine running, I used an unlit propane torch around all the areas where it could possibly be leaking. Nothing!
Then WD40 on the same areas. Nada!
Then water mist sprayer and guess what? Zip!
Next, I got a length of 18mm ID rubber hose and listened around the carbs and manifold area because the rear carb I thought was whistling loudly. Well, I found that both carbs are doing it and it was mainly at the mouth where the two small holes take air pressure to the piston. Hole 5 below. As I understand, SU carbs do make a high pitched sound without the air cleaner on.
Next up, I took the domes off to see if anything obvious was amiss. All I could think of was that somehow the domes are leaking.
I checked the bottom of the domes for flatness and found the rear carb in particular to be less than flat. So I got some 3mm closed cell foam sound deadening material and made a couple of gaskets.
This made it way harder to centre the needles and pistons but a bit of patience and carefulness while tightening down did the trick.
I went out to “buy some bread” and it’s a different car! The wobbly engine is a shed load smoother. The idle has also dropped over 100rpm which is a classic sign of plugging an air leak.
The AFR that previously would not budge past 15, now goes to 18+ on the overrun when in gear and cruising downhill. The idle is sitting at a lean 16 (previously 13.5-14) and cruising steady throttle AfR is between 14.7- 16. WOT when out of the reversion part of the range sits doggedly between 11.5-12.5, the latter at around 6.5k rpm. The rear wheels now squirm in 2nd at 4K rpm. This is all with the air cleaner off for troubleshooting. It would richen up a little when I put that back on. But boy does she sound angry with it off!!!
For now, I am quietly confident that the issue may have been resolved and will try to find a better solution with some plumbing rubber O rings.
The whistle though is now even louder - is this normal operation or do I have another leak that is now sucking harder? I’m erring in the side of the former but watch this space. Here’s a video of the sound ...
With COVID, my testing is now restricted to when I have to go get any shopping - especially the items not available at the local “Little Waitrose” that require me to go up the dual carriageway to the big Sainsbury’s.
I should be happy but I am itching to try out the MC needles with jets at around 1.1mm down and the original 2oz springs and the 4oz red ones I am currently using.
Moral of the story? Check intake and manifold leaks religiously before blaming other things.
BTW, I really did go out to buy bread from the next town!!!
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uk66fastback
Club Member
Superb bread, errr thread. Following closely - you are proving you can master these SUs. I've got my ZTherapy ones sitting on the shelf awaiting refitting - which I will get round to soon as I am furloughed so as soon as I have done some of the million other jobs before the Z (courtesy of the missus), I'll be full-time in the garage ...
I will go through this thread from the start again I think - I am thinking of getting the 123 distributor as well.
I will go through this thread from the start again I think - I am thinking of getting the 123 distributor as well.
Thanks dude, a rookie getting there VERY slowly! There are positives to furlough! I have fixed so much around the house.
The 123 has really grown on me as a mostly plug and play solution - I wasn’t convinced at first but as I’ve lived with it, I have found the usefulness outweighs the niggles. Just wish with a simple bit of code they would make the app log data while you drive.
Here’s a link to my experiences with it in case you haven’t seen it already ....
https://zclub.net/community/index.php?threads/123-ignition-for-datsun-l6-on-my-l28.25467/
The 123 has really grown on me as a mostly plug and play solution - I wasn’t convinced at first but as I’ve lived with it, I have found the usefulness outweighs the niggles. Just wish with a simple bit of code they would make the app log data while you drive.
Here’s a link to my experiences with it in case you haven’t seen it already ....
https://zclub.net/community/index.php?threads/123-ignition-for-datsun-l6-on-my-l28.25467/
An interesting read Ali. Are you absolutely sure that the dome was seated properly and the surfaces were clean. I find it hard to believe that the face was machined 'less than flat' and has been that way for 45 years. How much 'less than flat' was it?
I realise that the piston wouldn't rise and fall correctly if the dome was not seated properly but could that have been your problem?
I realise that the piston wouldn't rise and fall correctly if the dome was not seated properly but could that have been your problem?
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Great point Rob - not too sure as I always do the drop / clunk test before and after the final tiny bit of turn. I also have removed, cleaned and re-seated them several times.
I am suspecting that they have never been right in my ownership - could someone had dropped them at some point? But to contradict myself, I don’t think Z therapy would have missed it.
The amount of off flat was very small, I did it by a straight edge, eye and light coming through not any measurement device. But the problem was always intermittent and for worse when warmed up or after a heat soak.
Will keep a track and report back.
I am suspecting that they have never been right in my ownership - could someone had dropped them at some point? But to contradict myself, I don’t think Z therapy would have missed it.
The amount of off flat was very small, I did it by a straight edge, eye and light coming through not any measurement device. But the problem was always intermittent and for worse when warmed up or after a heat soak.
Will keep a track and report back.