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'd thought of doing something similar to this using a pencil, with graduated marks, in place of the stem and "twisty bits".

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!

 

  Couldn't access the video. My guess is the "it" is a stumble or lag in the transition from cruise to acceleration. Normally one would increase the oil viscosity to eliminate the stumble if the carbs are tuned properly. Crazy idea but instead of weights attached to the piston, one could buy the appropriate sized (steel?) flat washers to sit under the springs. In theory, one could add weight and shim the springs at the same time.  My gut feeling is increasing the weight of the pistons may help a lean stumble but it's going to lean the mixture in all RPMs.

 

The video worked OK for me.

Cool video. After you fix your rev counter, can you post the next video both in slo-mo and at regular speed with sound? Would be cool to seem them both and might help figure out what's going on.

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].

19 hours ago, AK260 said:

Timing light with rev counter fixed in field of view for analysis, need to improve lighting ...

 

Brilliant! Excellent application of the low cost modern tools we now have. How they dreamed of these tools in the 60's!

 

Beatles Mixer

 

Craigslist Today

 

 

For "simple" carbs, there's always another unique approach to improving their operation! The Des Hammill "Power Tune" book mentions the array of springs available for the true SU carbs. Being in the UK, you might have access to these to see if they will work with the Hitachis as well. Keep us posted -- eagerly awaiting the release of your next video!

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]

 

 

 

Hi AK260,

 

Blodi and I tried for over a year to address the AF dip once you get into the throttle.  You can see the pain, suffering, blood, sweat and tears in this thread. https://www.classiczcars.com/forums/topic/56375-weber-selection-and-initial-jet-tuning/#comments We tried many etubes and completely disabled circuits but no significant changes to the rich dip were observed.

The experiment still is not over however, it is interesting that you also see this on single circuit SU's and we see it on 5 circuit DCOE's.  There is also a video of chaps down-under tuning a Z with DCOEs and then with EFI and they get the same rich dip.  I  did some digging and a fellow who tunes bikes had the same problem and found it to be caused by the exhaust system.

Our recent thoughts for addressing this are:

 

1. Putting tuning reflective points in the exhaust

2. Putting anti-reversion traps in the manifold

3. Adjusting the mechanical cam timing with the 3 points on the cam sprocket

 

If you have no luck shaping the needles, I would be grateful if you did some runs for each of the 3 cam settings to see if A/F changes in this rich area. It may possibly be tamed by valve timing as we have these odd non-cross-flow heads.

 

Again, great work! Very impressive.

 

 

Hi AK260,

 

Blodi and I tried for over a year to address the AF dip once you get into the throttle.  You can see the pain, suffering, blood, sweat and tears in this thread. https://www.classiczcars.com/forums/topic/56375-weber-selection-and-initial-jet-tuning/#comments We tried many etubes and completely disabled circuits but no significant changes to the rich dip were observed.

The experiment still is not over however, it is interesting that you also see this on single circuit SU's and we see it on 5 circuit DCOE's.  There is also a video of chaps down-under tuning a Z with DCOEs and then with EFI and they get the same rich dip.  I  did some digging and a fellow who tunes bikes had the same problem and found it to be caused by the exhaust system.

Our recent thoughts for addressing this are:

 

1. Putting tuning reflective points in the exhaust

2. Putting anti-reversion traps in the manifold

3. Adjusting the mechanical cam timing with the 3 points on the cam sprocket

 

If you have no luck shaping the needles, I would be grateful if you did some runs for each of the 3 cam settings to see if A/F changes in this rich area. It may possibly be tamed by valve timing as we have these odd non-cross-flow heads.

 

Again, great work! Very impressive.

 

 

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.

This motorcycle tuner had similar problem with rich dip at the start of acceleration.  He could not tune it our with carbs so he set up reflectors in the exhaust to deal with the issue.

 

http://www.drdyno.com/AIM_2006-06.html