AK260

And that was before you found the leak!? :P Seriously though - that’s quite a lot!! Here’s a screen shot of the FSM

That is strange indeed, typically one side or the other will be darker. Some other thoughts and forgive me if it’s already been said. Let’s eliminate potential electrical issues: 1. Swap 3 and 4 with say 1 and 6 and see if you get the same effect. Could it be a faulty pair of plugs? And are the gaps the same (although that shouldn’t make a huge difference). 2. If 3&4 soot up again, try rotating HT leads from other plugs that aren’t sooted up. 3. Check the 3&4 electrodes on the inside of the distributor cap to ensure they are not corroded. Looking at the mechanical side: 3. Have you checked manifold gasket for leaks? 4. Have you checked valve clearances lately? 5. On your comp test, what was the difference between 3&4 and the other cyls? 3&4 aside your plugs look a little too hot to me, how have you adjusted your mix? I have found that idling for a long time with a lean mix on a hot day, the car starts to misfire as if there is an electric problem, but it’s a heat problem.

Not today but the weekend ... https://youtu.be/C1clNk-Mq24 Best savoured with headphones.

Cap and rotor

So I wanted to share one more thing for interest, feel free to shoot me down. Given that a lean mix needs more time to fully burn, it stand to reason that an oxygen starved over rich mix also needs extra time to burn as fully as possible. To put this to the test a few weeks ago, I used the 123 ignition to conservatively add a few more degrees at the dips building up to the troughs and then back again where the engine comes to life again. Ignoring the anti-stall peak at 800rpm -which works brilliantly BTW - I call this my twin peaks timing advance (but sadly no Sheryl Lee - am I showing my age again!?) ... The butt dyno told me that no longer did I feel a dip in power but I felt the dips behave more like flat spots. The flaw in this of course is that it is fixed regardless of throttle position or engine load. So at part throttle, I would have more timing than would normally be there - hence the conservative approach and not whacking it up by 5 or more degrees (above trend) at each point. But I like the fact that it worked. Why? Because I’m a middle aged man in suburbia with 2.4 children, living the dream! Some men have golf, I have my carbs and my timing curves!

Great post!! Thanks for sharing - I will remember this if mine ever fails in action. Also I found the same parts on eBay at a fraction of what 123 have been charging. I have to say; with my recent antics with mix / carbs / torque holes I’ve grown to like my 123 again. Being able to tune a couple of degrees more on specific parts of the power band and experimenting with the TUNE function has been enlightening, I wish they would offer some form of data logging though! And you are right - NOWHERE on my instructions did it say to check with a timing light (although I did). And still doesn’t ... http://123ignition.nl/files/manuals/123manual_TUNEPLUS.pdf

What a great question! I would imagine all carbed cars will have some level of this due to the high velocity charge causing shockwaves bouncing back from the intake valve as it shuts. In fact all Engines will have the intake shockwaves. Even modern ones. Some use a Helmholtz resonator to capitalise on it - almost like a spring storing energy and returning it at the right point. However cam overlap combines with this effect and magnifies it especially with performance cams. Exhaust velocity has a lot to do with it too. So if the exhaust is free flowing / has large primaries and the engine isn’t producing enough gas flow, the exhaust gases / pulses expand too quickly and lose their energy / speed and reversion becomes more prevalent. (God! That sounds like I almost know what I’m talking about!) I’m sure more wisened heads than mine can provide a better explanation.

Thank you sir! Loving the body of knowledge on this forum! [emoji106] I need to read a lot more. Will likely buy his tune tips books to better understand the principles. (Even if I’m not running triples).

So chaps, here’s a thought to chew over: The video I shared has gotten me thinking and yes it hurts ;) The intake velocity is high, the intake valve shuts rapidly, the energy of the intake air bounces against the back of the valve and sends a pressure wave back up the inlet tract. With the balance tube on the SU manifold, the shockwave not only affects the carb that was supplying but travels through the tube to the other carb which potentially disrupts the airflow of the other bank. On DCOEs, it spits harder through one inlet tract, say cyl 1, but cyl 5 starts to breath through an undisturbed tract. So would this be one good reason why DCOEs behave better at the top end beyond the sheer size of each carb feeding each cyl? Except for sacrificing low end smoothness what would happen if the balance tube was blocked? Surely that would be the same as The DCOE scenario? But without an idle circuit? Also I think I better understand why foam filters on carb mouths have been known to disintegrate under the shockwaves and / or even catch fire when the carb spits [emoji33]

Utter genius!!! And so on point!!! [emoji1787][emoji1787][emoji106][emoji106]

Thank you sir! That was a brilliant read and good food for thought. I definitely saw an increase in the dips after going to a more free flowing header / exhaust. It also seems there is hope to tune out, or at least minimise the dips. I do think I can get a few more ponies out across the mid range and the top end once the needlework is done. The torque curve quite consistently follows the AFR curve and drops off at the top as soon as the AFR starts to lean up (lean being relative - below 14 but above 12.8). Ordering needles today, so the weekend should prove to be interesting. In the interest of science, I plan to sort out the carb tuning to work well with the existing header, power run, then re-install old header / exhaust and do a power run to see the difference in the mid range and top end. It will cost me another 250GBP but as the man in Dirty Harry said (for those old enough to remember) - “I gots to know”, so it will be worth it! And for those not old enough to know it ...

Thanks buddy, it’s quite fun experimenting and learning! That thread you shared is very interesting and certainly a testament to commitment and tenacity! [emoji106] I will read again in detail and absorb / learn from your experiences. On the hybridz there’s a thread that asked people to post their dyno sheets. That and goes if googling showed me that nearly all the “tuned” L series engines seem to have those exact two dips +/- 500 rpm from mine (I’m guessing cam timing dependent). https://forums.hybridz.org/topic/123487-compare-your-dyno-sheets-here/ So it seems it’s not a just a carb induced factor - having said that a wise man with 40+ ears of technical Z experience was just telling me about his experience of variations in inlet tract shape / length etc and the airbox / filter housing having a big effect on these conditions. For example (and let me quote very badly) open trumpets makes the inlet tracts hugely different due to the shape of the inner wing and the distance to the mouth. Then there is reflected pulses etc and the Helmholtz effect. Never-the-less (as you say) I believe we are blessed with this effect on our engines and what needs to be done is to spend time and effort in minimising it but I’m not certain we will ever eliminate it. My plan is to sort out the mix as best as I can then do a power run with existing header / exhaust and another with the old one for some back to back testing to see if the primary diameters can tune this out a little.

Thanks chaps. Being in the UK the weather has been atrocious so the next video may be a while :(. Just had a month of rain in one day!! Yes the springs are indeed available here and I will definitely experiment. Some think of this as a bit of a faff but for me it’s an enjoyable part of my Z ownership journey. I love spirited driving and tinkering in equal measure! ;) Wow that is a serious mixer!!! I thought my 36 channel mixer was complex [emoji33]

Thanks chaps and yes I will do a much better video / show full speed version - not sure why the sound didn’t work but this was a first trial run with the web cam. I need to shield the timing light pickup and cable as it is OK for tuning but clearly it suffers with noise when all the cable is under the bonnet. The VERY long version of what I’m doing is on the link below: https://zclub.net/community/index.php?threads/su-carbs-will-work-perfectly-on-my-modified-l28-and-give-dcoes-a-run-for-their-money.25839/ Suffice it to say it all started with not being happy with the RR tune and the modified SM needles so I started working things out for myself. I have two torque dips at 2.1-2.5 Krpm and 3.1-3.5 k rpm where the AFR heads into the 9s and 10s. I’ve been suspecting reversion of exhaust gasses combined with over rich needles at that point in the rev range. No stumbling, just bogging down before breathing fire and scaring the life out of modern performance cars between 3.5-6.5krpm! [emoji12] But I want to smooth out the power band as best as possible with needles. I’ve modelled what I am aiming for on a spreadsheet and have shortlist of two needles (DX and KD) that may improve the situation around the torque holes. Also the RR showed it gets lean over 6000 going from mid 11s and 12s to 13.8 on the AFR. So once I’ve dealt with the low-mid range I will be very slowly polishing the last few stations to get the very top rich enough for track days. Dotted line is the average of the two SM needles the tuner modified. The unmodified SM is on the chart for reference and judging how lean I want to be at cruise ... However, I wanted to get some “real” data on where the piston sits at various “cruise” RPMs as well as seeing evidence of reversion - hence the video. Like the idea of the washers, had similar thoughts on that [emoji106].

So I got a little time to play and this happened. You can clearly see the reversion through the carbs!! [emoji33] Unfortunately the video quality reduced on the upload so you can’t see the piston markings on this version of the video clip. I may try stronger carb springs to increase intake velocity and see if that reduces it. Alternatively I can make some weights to securely sit on the top of the pistons ;) https://youtu.be/uedh4L32OkA Oh I just realised - I need to get a life!

Just a teaser for now without much explanation but this could get very interesting! Will do a proper write up as to why and what when I finish the work ... 1/8” increments ... Webcam pointing into carb mouth Nice view ... Timing light with rev counter fixed in field of view for analysis, need to improve lighting ... Let the fun begin!! Ah, no! Children’s activities stopped play at the crucial moment! [emoji22]

I took the advice and bought some banjo fittings and used 5mm clear pipe ...

Ok dude, electric fans won’t stop fouling plugs. The first thing to look and adjust would be your mix. But before any of that, go for a drive with a pair of cloth gloves, towel and a spark plug remover socket in the car. Re-created the scenario. As soon as it happens, turn off engine, pull over, pop the bonnet, lay the towel on the wing and pull your plugs (which will be hot) and lay them on the towel. Now look at what the plugs look like and if the ones from the front and rear banks are different in any way. I also experienced similar symptoms but with fuel vaporisation as the header was heating up carbs and everything carrying fuel above them. A decent heat-shield works wonders. Good luck chap.

I had that header on mine, it’s an absolute peach and will make your engine sing beautifully!!! It’s a very well balanced size and length of primaries. Ps. Keep the stock manifold - with prices shooting up for originality someone will pay good money for that one day! )

Does this count?

This is what upsets me - I bought the Z as a hero halo car I can drive hard and enjoy, not as an asset that I worry about breaking. But having the “inferior” 260z I doubt I need to worry too much :p

I have garage envy now!!!! [emoji106]

Safe to say no warranties offered then! I envy you guys in the US. The $1000 car would never be found over the pond at that price and is probably worth more than that in wings alone (if not paper thin with rust that is)!

“Dad joke” alert!!!!! [emoji1787][emoji33][emoji8]

I would echo this sentiment - this is one of my absolute favourite / friendly car forums and why I lurk a lot. Lots of very lovely people here. Read Jason’s words in the spirit he meant them. Knowing him in person, he’s a very decent chap with a no nonsense attitude. He’s also a total connoisseur of Datsuns and Bimota bikes!!