Am I sporting a vac leak?

[AK260]

 

Time for some collective thinking please oh wise heads.

 

Problem: the car feels like it's "missing" constantly but randomly (I.e. Not always on the same Cyl). Mainly on steady throttle or idle. Goes like stink but tries to die when idling. Plugs all look the same, like they are firing and chocolate brown if taken out after hard acceleration and ign off + neutral. It feels like it would, if a carb was way out of balance or a cyl not firing correctly.

 

 

 

Engine: L28, 10.5:1, 270 degree high lift cam, Dynamic Compression c. 205PSI across all 6, pertronix flamethrower coil, 8.5mm HT leads, NGK BPR6ES plugs gapped at 1.1mm , 123 ignition. Z therapy SUs / SM needles.

 

When did it start: I fitted the 123 ignition, stupidly following the instructions and the no.1 plug position on the cap was not correct against where the rotor was pointing - timing 120 degrees out. It resulted in a condition technically referred to as "car-no-startie" followed by a baaada boom!!!! Biiig bada boom, bada big boom. Goodbye baffles, hello shiny new straight through silencer! :D

 

 

 

Coincidentally, later I found the magnecore leads had breaks that came to the fore after the dizzy change, resulting in intermittent ignition issues and on power/wot popping - according to the condescending chap on the phone at magnecore this failure is my fault for not unplugging them often enough!!! Not sure how that is possible as my HT leads are on and off like a tart's knickers!! No matter, at least I know what not to buy next time. So that made me think HT was to blame.

 

 

It seemed to go away of it's own accord but it's back again.

 

What I have done so far: replaced coil, leads, two different distributors (280zx 12-80 and 123 ignition). New plugs, Cleaned plug holes and threads for a strong Earth. Checked all low tension connectors are clean and tight. Checked all HT leads for breaks and connector stability when on.

 

New caps and rotors on both dizzies. Engine Earth checked for loose connections. Interestingly the thick ground cable from battery to starter body has 5ohms impedance (eeeew) so new one on order (but it ran fine previously with the same cable so it can only get better with more volts). To prove it's not the 40 year old wiring, I even ran direct cables from battery to coil (with a fuse of course, which is very much needed to stop the engine - a condition technically known as "car-no-stoppie").

 

Carbs balanced to near perfection and previously RR tuned with needles profiled to match the engine. Using the lifting pins makes the revs go high briefly then settles back down to where it was so I think the mix is OK.

 

Timing on 123 set to:

 

 

800 rpm is anti-stall and it works well.

 

She was flying like a hell cat the other day but then after idling for a while it started again and won't go away even when fully cooled down.

 

When on power or WOT I can leave black lines everywhere so the power is healthy.

 

I'm strongly tending to a Vac leak now. Could the backfire through exhaust and intake have blown a hole in the manifold gasket or the brake booster Vac lines? Could it have damaged the carbs? I checked the butterflies and they are still round ;) ... and flat.

 

Brake booster seems OK as I can pump the pedal 3-4 times after ign off before it starts to fight back.

 

So if anyone has seen such a thing and has any advice beyond my current trouble shooting, then I'm all eyes and thumbs! ;)

 

 

 

 

 

 

 

 

[Zed Head]

1 hour ago, AK260 said:

Problem: the car feels like it's "missing" constantly but randomly (I.e. Not always on the same Cyl). Mainly on steady throttle or idle. Goes like stink but tries to die when idling.

It feels like it would, if a carb was way out of balance or a cyl not firing correctly.

Engine: L28, 10.5:1, 270 degree high lift cam, Dynamic Compression c. 205PSI across all 6, pertronix flamethrower coil, 8.5mm HT leads, NGK BPR6ES plugs gapped at 1.1mm , 123 ignition. Z therapy SUs / SM needles.

When did it start: I fitted the 123 ignition, stupidly following the instructions and the no.1 plug position on the cap was not correct against where the rotor was pointing - timing 120 degrees out. It resulted in a condition technically referred to as "car-no-startie" followed by a baaada boom!!!! Biiig bada boom, bada big boom.

It seemed to go away of it's own accord but it's back again.

When on power or WOT I can leave black lines everywhere so the power is healthy.

I'm strongly tending to a Vac leak now. Could the backfire through exhaust and intake have blown a hole in the manifold gasket or the brake booster Vac lines? Could it have damaged the carbs? I checked the butterflies and they are still round ? ... and flat.

Some time frame would help.  Did all of this happen in one day?  You said it went away but came back.

[AK260]

Good call that man.

 

So, installed 123 last summer and blew the back box the same day. Compression tested and all well.

 

Found HT lead issues a couple of days later

 

Replaced HT leads a week later and the world became a better place but unsmooth idle.

 

Changed dizzy back to 280zx - power better but still issues with steady throttle. Changed cap and rotor - much the same.

 

Changed back to 123. Changed plugs and coil also a week later. No difference.

 

Slept for winter.

 

New cap and rotor on 123 6 months later (last week) all good.

 

Retuned carbs on Friday last week. All good right until I pulled the plugs for the last time to check colour. Plugs back in not smooth anymore on part throttle or idle. Changed plugs again in case I damaged them, no difference.

 

Checked all electrical connectors again etc. No difference.

 

Changed back to 280zx dizzy last night still the same. But interestingly no vac advance at idle just at fast idle. Also stalls with 280zx one at idle but not on 123 (due to anti-stall timing). So it made me think about vac leaks.

 

Does that help?

 

 

 

 

 

 

 

[Mark Maras]

 I'm going to go the other direction and suggest that it may be running rich at idle. Is it possible it's as simple as a stuck nozzle or choke out of adjustment?

[jonathanrussell]

Sounds like you really know your way around these cars so I am guessing you have the carbs tuned well. But, I have found that these cars like to idle a bit rich. If you try to set the mixture too lean, you get (I get) terrible idle performance and backfiring through the carbs.

If you apply a bit of choke while idling does it straighten out?

[AK260]

Thanks guys, all great insights.

 

I did look at chokes before but I haven't recently - so I will undo the cables and make sure the jets are returning fully. John, I will try what you suggested before doing that.

 

Will also get vac gauge out and let you know of any leaks.

 

Next up, I need to get these bad boys on and head for a RR tune again. But I do want to find the root cause of this annoyance first!

 

 

 

 

[AK260]

Quick update as promised:

 

I fitted the Z story exhaust and in the process found that on the original header gasket there were several exhaust leaks and a small vac leak on cyl 6.

 

With new header and gasket, the car runs just fine. Amazing how much fuss a little air leak can cause!

 

 

 

 

 

The Z story system is a thing of beauty and performs well. Sean the supplier is also a very decent chap who has been most helpful post sales.

 

 

 

 

And finally something to scare and entertain you with:

 

 

 

 

[grannyknot]

That's a nice looking Z.  Interesting car stand, kind of a gear cranked teeter totter, did you buy it or make it?

[AK260]

Thanks dude. She's a labour of love and really getting there. A few photos below:

 

 

 

 

 

With the new race/sport exhaust fitted, she managed 232bhp @6400rpm and 211lbft at 5000 on the dyno last week (corrected power).

 

The engine power was still climbing but the AFR was going too lean (high 13s) so the tuner backed off. The curve was a little bumpy low down for my liking. She is sporting very modified SM needles but I am doing a whole lot more needle work at the moment. Once sorted I will write it up properly for the benefit of others wishing to go down the SUs on modified L28 path. I'm pretty confident I can iron out the curve and release a few more top end ponies with better fuelling.

 

The tilting lift is made by CJ Autos in the UK. I started a thread on it and there was a lot of controversy on where to position it on the car. I still haven't had the balls or the need to take it to it's max height. [emoji33]

 

https://zclub.net/community/index.php?threads/car-tilting-lifts-cj-autos.25075/

 

Given mine is a '77 260 it has the longer fatter floor / chassis rails (like the 280) that go all the way back. You can lift the car on the cills instead of you wish as the lift is width adjustable but I felt where I have it was the strongest less damaging place to go - especially given the wide weight distribution across the car.

 

It is my number one favourite Best Buy tool!!! I don't have space for a post lift in my garage. With this tool, except for a tranny swap there is nothing I can't do on the car now that requires underside access.

 

 

 

 

[grannyknot]

That's my new wallpaper for the next month, gorgeous.  Gotta love a deep dignified blue.

 

[AK260]

That's my new wallpaper for the next month, gorgeous.  Gotta love a deep dignified blue.
 

You are very kind sir! Thank you! [emoji106][emoji106]

[AK260]

So gang this has been driving me nucking futs!!!!! It keeps coming and going so I got down to business after homeschooling the kids today!

 

I got a propane torch and unlit, I went 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!

 

So next I got a length of 18mm ID rubber hose and listened around the carbs because the rear carb I thought was whistling loudly. Well 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 to 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 AFR that previously would not budge past 15 now goes 18+ on the overrun when in gear. The idle is sitting at a lean 16 and cruising steady throttle AfR is around 15-16. WOT when passed the reversion part of the range sits doggedly from 11.5-12.5, the latter at around 6.5k rpm. The rear wheels now squirm in 2nd at 4K rpm.

 

So for now I am quietly confident the issue may have been resolved and will try to find a better solution with some plumbing rubber O rings.