Carb Tuning Advice | The Z Club of Great Britain

Carb Tuning Advice

Discussion in 'S30 (240Z,260Z,280Z) Engine' started by Geoff-R, Oct 1, 2017.

  1. Geoff-R

    Z Club Member

    Hi all, I’ve tried to search and found a lot of info but a lot of people arguing about the exact procedure. When I got my car I took the carbs off, refurbed them with a kit as the gaskets were in a bit of a state and popped them back on the car. Ideally in the future I’ll have them properly refurbished but for the moment I just want to tune them up.

    The car has a set of round tops on it I’m not sure what happened to the originals, I can only assume the original owner got rid of them as they thought they were restrictive. Anyway, basically I have limited knowledge of carbs, I’ve grown up in an era where carbs have been phased out, this is the only car I’ve owned with carbs so please forgive me!

    I currently have the carbs on the car, it runs okay, revs out on full choke, bangs a little bit though. Once I start backing off the choke it starts to die a bit, I can get the choke about half off before the engine starts to die. So would anyone be able to give me a few pointers on how to tune from the beginning?
  2. TimW

    TimW 1978 UK RHD 260z

    Hi Geoff,

    Are they three or four screw carbs? Not sure if there's much difference in tuning procedure, but I'm familiar with the 4 screw Hitachi and did basically the same, started from scratch. After a few dismantles and about a dozen tunes I eventually got the hang of setup and tuning and can tune them in no time now.

    My mistake was not to check timing, valve clearance and spark plug gaps first before delving into the carbs.

    I'm a bit stuck on time today, but can post up shortly the set up procedure from scratch that I went through if of help
  3. uk66fastback

    Z Club Member

    I think that would be a lot of help for a lot of people Tim, if you could.
  4. Geoff-R

    Z Club Member

    Hi Tim, would appreciate a few pointers if you can find the time. Mine are three screw ones, I’ve checked out a few videos on YouTube, this one looked pretty good:

    I’d assume regardless of whether it’s 3 or 4 screw carbs the tuning principles should remain the same?
  5. Geoff-R

    Z Club Member

    By the way I meant to add I did do the valve clearances yesterday, not a great deal of adjustment required but there were a few valves out a fair way which is probably be to expected. Must admit not done the timing or spark plug gaps!
  6. toopy

    Z Club Member

    That video is for SU carbs, the Datsun/Nissan ones are Hitachi (SU Copies) so differ a bit from those in the vid.
    The tuning principle is the same tho

    Once the cars started to warm up you should be able to pretty much lose the choke assistance.
    If it wont tick over off choke, try turning the small screw close to the throttle spindles/butterflies (There's a long connecting spindle between both carbs), on the inside edge of each carb, towards the manifold. this opens/closes the throttle butterflies, and sets the idle. Very small adjustment each time, literally 1/32 turn of a screwdriver can be 100+/- rpm and of course you have to try and balance both for a smooth idle.

    Make sure the single screw higher up on the inlet manifold where the rest of the throttle linkage is, is backed off and not touching the stop, this is only used for raising/holding the revs when tuning/timing etc. You can of course use it to help initially, but ive found it better to not!

    There's also the mixture setting for the jets to consider as well, but unless you know what your doing, its easy to get it very wrong, hopefully Tim's explanation will enlighten you, possibly me too :thumbs:
  7. toopy

    Z Club Member

    SU carb


    Hitachi SU


    note the different float chambers and fuel inlet arrangement's and the different mixture jet's and linkages

    Attached Files:

  8. Geoff-R

    Z Club Member

    Cheers for that :thumbs: unfortunately I’m busy all week so it’ll just be a case of reading up on this in the week now and putting it in to practice next weekend hopefully! It’s a bit of a learning curve for me. I think I’ll do the gaps and timing first which shouldn’t take long and then hit the carbs. Am I right in thinking the optimal timing for the L26 is 8deg TDC?
  9. smileyinside


    Well Geoff, mine was set by PO to 10 degrees at idle and it ran super hot + gave up a load of power. The fuel was basically finishing the combustion cycle in the exhaust pipe.

    Without Vacuum advance pipe connected, set it to around 30-34 (mine's at 32) degrees at 3000rpm and you'll be good. That equates to around 17 degrees at idle without vac advance connected. You're much better off setting it at the max advance point (over 2500 rpm) than at idle.
  10. Geoff-R

    Z Club Member

    Cheers for that, all good info and much better to get it from people running these engines than from the FSM, I know it's a good source of info but sometimes must be taken with a pinch of salt.
  11. nospark

    nospark New Forum User

    I've got SU's, learned about them and the car now runs better. So my tips are for SU's and not Hitachis.
    Get Haynes Carburettors (workshop manual) ? ; look at YouTube (universitymotorsltd) ;

    1)Take off the air filter(s). Take the tops off the carbs; See the jet flange is slightly below the throttle plate ? Turn the jet adjusting nut so it is flush. MARK ONE OF THE FLATS OF EACH CARB jet adjusting nut so you have a base point for future reference. Now turn the jet adjusting nut 2 turns exactly on each carb so the jet flanges drop equal amounts. You now have consistent settings on the carbs. That is the basic settiong for your air/fuel richness setting

    2) Buy an air flow meter. The Synchrometer ones are great. It is essential that you use these together with each carb throttle adjusting screw to get the same air flow into each carb. Before you do this, it is essential that the linkage bewteen the front and rear carb is disconnected

    3)After getting base points for the air/fuel richness and then the air flow there is the issue of setting the fuel height in the float bowl.

    The main message with twin carbs is consistency. The internal parts of the carb need to be same and the adjustments need to be the same with a few subsequent minor tweaks.

    Again, the above might or might not apply to hitachis. I am also in Cardiff if you get stuck and need a fresh pair of eyes.
  12. TimW

    TimW 1978 UK RHD 260z

    Hi Guys..

    Ahah.. Great video. Love that guy, he has a great way of explaining things.. I found Dylan's video particularly reassuring when first embarking on what seemed a complete minefield. My set up is 260z with 4 screw Hitachi Carbs. Yes, as others have said I would say the principle is the same. There may be some variations on things like interlock clearances or float level settings, but tuning is the same principle.

    I found this book useful, if only for confirming tuning is roughly the same in principle and that I picked up the bit below about setting the initial throttle position which I'd not seen mentioned of elsewhere.

    I was much the same, new to carbs and to me all seemed quite daunting. After a few attempts, coupled with a good few dumb questions and a couple of blind panics all came good. Much of what I know is just hands on and tips from the chaps. I'm sure there are many more with greater knowledge, but happy to put down what I do.

    Guys, if there are inaccuracies or suggestions, say so and hopefully one of our kind moderators will allow an edit so as not to mislead.

    I'll list the checks I make after dismantling the carbs as a starting point. Then list what I do in tune ups. It's been a few months, but here you go.


    Set up checks after a dismantling and before installation

    1. Check condition of banjo bolt and filter. (often missed and a source of poor fuel flow)

    2. Check for signs of throttle shaft wear. (or check on car with carb cleaner)

    3. Check/Centre the butterfly valve with lever components detached.

    4. Check the inlet needle valve seats.

    5. Set the float level. (can be checked on car will supply details later)

    6. Set needle collar position. (the collar should be in line the base of the piston, not the groove)

    7. Centre the jet and check piston fall.

    8. Tighten the mixture nut to its seat, then slacken two and a half turns.

    9. Set the mixture nut limiter against the mixture nut, then tighten the mixture nut one full turn. (the result beinf from step 8 the mixture nut being at one and a half turns from seat position and giving a full turn to enrich if needed)

    10. Check/Set the starting interlock with lever components attached.

    11. Check the jet is still returning to seat freely without any marring as it slides in.

    12. Set the starting position of the throttle screw.


    Other component checks before tuning

    1. Check valve clearance

    2. Check spark plug gaps

    3. Check timing

    Aye, same here. I set to 32

    Basic checks (if not carried out during refurb).

    1. Check needle condition and collar position. The collar should be in line the base of the piston and not the groove.

    2. Remove the dampers, being certain not to mix them up. Check pistons fall freely, with an audible clunk. Any dragging, then clean and check the centring of jet or the jet for signs of wear. If the jet looks oval in shape, replace it.

    3. Check that jets fully seat, are not sticking or marring as they move in and out. If not moving smoothly or returning to seat then check the jet centre, choke mechanism and elasticity of the fuel line from float chamber to bottom of the carb.

    4. Check damper oil level

    Preparing the carbs.

    1. TBH, I'm not sure if this step I do is the same for 3 screw carbs, my experience is only with the 4 screw. Hopefully it will be useful for those with the 4 screw and if applicable to 3 screw, great. The rest should be the same

    With the jet fully seated (choke off), the air filter assembly removed, the dampers removed and the piston lifted reach into the carb air intake with your finger until it touches bottom of the butterfly valve and slowly loosen the throttle screw. You should feel the bottom of the butterfly valve move slightly backwards as you loosen the screw. Loosen the screw until the valve doesn't move any further and the butterfly valve is fully closed. Keeping a finger rested on the base of the valve, start tightening the throttle screw slowly until you feel the bottom of valve start to move forwards. Stop at that point, note the position of the slot in the throttle screw and then tighten to either one complete turn or one and a quarter. With your finger still on the bottom of the valve as you tighten the screw you should feel the bottom of the valve continue to move marginally further forwards. Do this on each carb. This will set the initial idle speed and set a roughly equal throttle base setting for each carb. (on mine with one turn of the screw its around 1,000 - 1,200 rpm)

    After a few tunes ups it becomes easier to lift the piston and reach in with a finger, but it can be a little fiddly at first. If you have difficulty remove the depression chamber and piston temporarily for improved access. (It's worth checking at this stage if not checked before that the butterfly valve is centred and seats nicely when the throttle screw is fully loosened and the choke is off.) Set the throttle screws using the same method and the reassemble.

    (The Haynes manual talks about doing this setting when on idle, adjusting each throttle screw until an even idle is achieved. I've found that if I do the finger method first, I tend to have a good start up RPM and the engine runs evenly from the start.)

    2. Tighten the mixture nut on each carb until it seats, then unscrew each equally one and a half turns. The Haynes manual states half to one turn, other sources say two. I opt for one and a half as a good starting point. (I have tended only to need to lean up slightly from this setting, quarter to a third turn max). When tightening, if a resistance is felt or it feels a little gritty, slacken the nut and spray with carb cleaner to clean out any debris from the spring and washer that may stop the nut seating properly, thus effecting the base setting.

    3. Important. Slacken completely the balance screw and fast idle screw.

    (When tuning, I prefer to disconnect the choke cables. It's not necessary, but I like to ensure there's nothing interfering with the return to idle.)


    1. Start the engine and allow to idle. The revs are likely higher than normal due to the throttle screw setting. As mentioned, mine tends to be at 1100-1200 rpm and the choke not needed. Bring to normal temperature. Ensure the choke is off, adjust the throttle screws equally aiming for an even idle at say 900 rpm. (Haynes manual says 700rpm, but as I aim for a 850-900rpm idle I tend to aim for around 900 rpm at this stage.) If the engine is running a little lumpy you may need to adjust each throttle screw unequally (i.e. one tighten, the other loosen). If so, and the revs are dropping too much I adjust the front carb up slightly and then the rear carb. To drop it down I also start with the front carb. Each adjustment is a fraction of a turn of the screw. If losing your way as I have in the past, or making too many unequal adjustments. Switch off, reset the throttle screws as previously done and try again. I've found using the finger method easiest, resulting in no unequal adjustment of the throttle screws needed at this stage.

    2. Now place a synchrometer in the front carb air intake (closest carb to the fan for clarity) and note of its reading. Place the synchrometer into the rear carb, make note of its reading and slacken or tighten the rear carb throttle screw to adjust to the same reading as the front carb. Check this several times, ensuring that after any adjustment that the throttle is blipped, the engine settles and a further reading is taken. (After doing several tune ups I found it much easier to have two synchrometers, further advantages explained later. The ones I use below seem to make little to no difference in air flow when leaving them inserted in the carb during balancing.)

    3. On each carb, using the piston lifting pin or a flat head screwdriver gently lift the piston a fraction. If the idle lifts for a second or two and then goes back down then the mixture is about right. If the idle drops, this indicates the carb is too lean and you'll need to slacken the mixture nut. If the idle raises and stays raised then it's likely running too rich and the mixture nut needs tightening. Any adjustment on the mixture nut is done at around an eighth of a turn or one flat of a nut at a time. After each small adjustment, blip the throttle, allow to settle and check again.

    Right or wrong, I tend not to bother with this method. Instead I adjust (usually tighten slightly) each mixture nut until I've heard the idle slightly raise with the engine continuing to idle evenly, then loosened the mixture nut just a fraction. Before each adjustment I make mental note of its starting position and if there is no change in idle I assume the mixture roughly correct and return it back to that setting. I've found this to be a good base setting and then I monitor the mixture using other methods over time which are detailed below. Colortune as mentioned below can be handy here to check progress if unsure.

    4. Having adjusted the mixture, the throttle setting will likely need fractional adjustment . Repeat step 2 ensuring both airflow readings are identical.

    5. Next, adjust each throttle screw equally to achieve the preferred idle speed (i.e. both up or both down equally so to maintain equal airflow). I set my idle to 850-900rpm, the Haynes manual states 650rpm for manual, 700rpm auto I think. After each adjustment, blip the throttle and check again with the synchrometer. Repeat until the desired idle speed is set and ensure both airflow readings remain identical.

    6. It's well worth spending the time getting this next step spot on and its where I spend most of the time fiddling. With the engine at idle, slowly the balance screw so that the screw just seats. There should be no change in rpm when seating the balance screw. If so, back off the screw, blip the throttle and set again. Once set, check visually that when opening the throttle, both pistons lift simultaneously. To check the accuracy of the balance screw setting tighten the fast idle screw to hold the revs at say 1300rpm and check that the airflow reading from each carb is identical. (Any difference in readings indicates the balance screw needs further adjustment or if after repeated attempts something is amiss with the linkage.) With the engine still at 1,300 rpm, if necessary adjust the balance screw to achieve identical readings. Now fully loosen the fast idle screw returning the engine to idle. Rev the engine a couple of times, allow it to settle and then take readings again at idle, adjust the balance screw again if necessary. Repeat the process until identical readings are obtained at the higher RPM as well as at idle. I like to do this throughout a series of rev ranges. i.e. set to 1,000rpm check, set to 1,100 check, set to 1,200 check etc.

    This is where I have found the use of two meters very handy. It eliminates the need to adjust the fast idle screw to hold the revs while you swop the meter from carb to carb and then setting it back to idle etc. Instead, I just open the throttle and cycle gently through a rev range, checking readings simultaneously as I go and adjusting the balance screw as needed. Then simply release the throttle to drop the engine back to idle for a reading and so on. I've found it much quicker and easier to see readings throughout the rev range and I think the result is more accurate. Also, dependant on body/eye level position the meter can appear to have a slightly different reading. Having the two meters is just a quick flick of the eyes, whereas swopping from carb to carb the eye level can change... possibly a bit anal, but it is what I do.

    7. When balanced, reconnect the throttle linkage and tighten the fast idle screw leaving a clearance of 1 to 2mm.

    8. Lastly, reconnect and set choke cables. (I re-check choke settings later on using the synchrometers when the engine is cold)


    Having done the tune up, there are a couple of checks I do shortly after.

    1. Choke adjustment. I check/re-adjust the choke on the next first start from cold. This involves a one off removal of the air filter cover and filter, then with the filter housing back still attached I insert the synchrometers into the air intakes. Start the car with the choke and check/adjust each choke so that meter readings are identical. I've not read this anywhere, but it seems to work and the car runs smoothly on choke.

    2. Checking the mixture setting. I'll take the car for a good drive. Park up at home and immediately switch off the engine. When cold, I'll check spark plug condition. I look for a nice even browning across the two sets of three plugs. If there are any signs of being too rich or lean then I adjust the relevant mixture nut a fraction. I repeat this a couple times checking that air flow readings are still reasonably in line after any minor mixture adjustment. TBH any adjustment needed has been minor. When re-checking the airflow, I just remove the filter cover and filter, pop the synchrometers into the air intakes via the filter back box and cycle through the revs.

    When first delving into the mystery of carbs I used Colortune. It was pretty good and gave me the confidence I needed. I'll use it occasionally to spot check mainly out of curiosity as it's quite fascinating to watch throughout the rev range, but now in general I go by spark plug condition.

    There's some good videos on Youtube about Colortune.

    One thing I would say, is that after having the odd blind panic and with the garage full of smog I'd would to go back to step one of basic checks, re-setting the throttle screws and mixture nuts to start position and then trying again. Each time, each step becoming a little clearer and easier.

    I'm sure more can add, but hope it's useful. Guys, any thoughts appreciated.. Be gentle!

    I'll post up the bit about checking the float level with the carbs in situ when I've found the pics. Got me a new laptop, darn it, files all over the place atm.. Last laptop 8 years old.. I don't like change.. Windows what!.. haha..
  13. Geoff-R

    Z Club Member

    Cheers for the input guys.

    Tim, that write up is something else, thank you for taking the time to put it all down, I'll get on to this on the weekend. Hopefully I can get some reasonable results!

    Sent from my ONEPLUS A3003 using Tapatalk
  14. slaphead

    Z Club Member

    Nice write up Tim good to see you use the same failsafe method as me on my hs6 carbs plug Colour even though I've got the colour tune kit I'd still rather trust plug colour over time.
  15. toopy

    Z Club Member

    What he said! :thumbs:
  16. Geoff-R

    Z Club Member

    Tim, I followed your write up today with some positive results. For the first time I managed to run the car without the choke which is a huge step in the right direction. Whilst I didn’t have hours and hours to work on the car I did manage to do the spark plug gaps, ignition timing etc and then I hit up the carbs. I did hit a few problems in my limited time, I think this is mainly down to the fact that I have round top 240z carbs on my 260, I have no idea what happened to the originals but I’d say the conversion has been done as a bit of a bodge, the throttle interconnection is not the best and I had to really tweak it to open both butterflies at the same time. There must be something missing from mine as there are 2 springs running from the interconnection down towards the manifold, I’ll try and dig a picture out. I think this has a knock on effect as using a syncrometer I can’t really get both carbs below a draw of 11 and then to top it off when I lift the piston in the front carb it doesn’t really have an effect on the idle of the car but the back one does, slight rise and then fall as expected.

    Still all that said for a first attempt tuning the carbs I’m quite happy. I didn’t get time to use the colortune I’ll give that a crack next weekend, using that might have given me a better idea of the fuel mix.
  17. toopy

    Z Club Member

    This is the preferred set up, the 260z flat top carbs are a pig to keep in tune when old and worn and spare parts are scarce if not unobtanium! 240z carbs are far more reliable

    This is correct, these are the throttle return springs
  18. Jimbo

    Jimbo 1978 260z in yellow

    i hate to say it but the reason you can't balance them and keep them in balance is you might have worn throttle spindles
    i had exactly the same setup and had exactly the same problems
    however that said you should get a reaction if you open the one front slide???
  19. TimW

    TimW 1978 UK RHD 260z

    Hi Geoff, that's good news on positive results. It took me quite a few attempts to get the hang of it.

    As Toopy says, 240 carbs are best and the throttle return springs are normal. At first I thought something not right with my springs so tried others, but stuck with the original ones in the end. Post up some pics when you get a chance.

    Also as Jimbo mentions, check the throttle shafts using carb cleaner, plus squirt a good load of carb cleaner around the gaskets at the back of the carb where it meets the inlet manifold. I've heard of peeps having them missing. Worth a check I reckon, if only to eliminate

Share This Page