L28 Ignition trouble headscratcher

Faster Behr

Club Member
Anyone who has been following my thread on my Fairlady Z escapades may recall that it was originally fitted with a Holley carb and a Bob sharp Racing manifold. I have set about replacing that with a FAST EFi retrofit fuel injection system in order to get it on the road for the summer. During this work I found that the 280ZX distributor (the one with the E12-80 module) that was fitted with the car had a faulty vacuum advance mechanism. I sought to replace this and iron out any timing issues before setting about finishing off the set up of the EFi. After lots of reading and researching - including a very informative thread here which I re-read many times - I decided to give the 123+ distributor a try.

This is where my troubles really started and where I need some help/fresh ideas/emotional support..

Ok, so I fitted the 123+, added resistor plug leads and resistor plugs (NGK BPR6ES). I couldn't get the car to run at all. I had sequential spark across all 6 plugs except for a weak spark/missing spark on #2 and 5#. Timing was checked and rechecked and rechecked. Compression 175psi +/- all round. Cam timing checked. All back to basics. I tried all different permutations. I removed the original 280Z 0.9ohm coil and tried a 1.5ohm coil (all of which I measured with a good multimeter). No start. I tried changing settings in the 123+ app. I checked all ground points and wiring. No joy.

I put the original distributor back in, timed it up and no start. I hot wired the coil direct to overcome any voltage drop through the start circuit. No good. I could not get the car to run. It should be said that on multiple occasions I pulled the plugs and they were wet with fuel. I cleaned and regapped them many many times.

My starter got weaker and weaker to the point where I had to fit another. RIP.

I changed the HT leads back to standard leads, no joy. I refitted the original BP5ES (hotter, non-resistor) plugs and boom. It started and went straight to a clean idle like nothing had ever happened. I was gobsmacked. There were nearly tears. I reckon I had about 25+ hours of time just into this issue.

What I can't understand is why the plugs made such a dramatic effect. It's either a case that it wants hotter plugs or that it is having a real problem with their being resistance in the secondary ignition circuit. Unfortunately I don't have any other plugs on hand to try to diagnose this.

This all begs the question of what is really going on here. And this is what has me stumped and looking for good advice. The issue was the same for two different distributors and two different coils. I hotwired the coil supply ruling out any errant voltage issues to the coil. It just looks like something doesn't agree with their being resistance in the secondary HT ignition system.

PS. much kudos and thanks to @jonbills who was very helpful with heping me on this.
 

jonbills

Membership Secretary
Staff member
Site Administrator
The difference in the bp5 plug being hotter only comes in to play after the engine is running, preventing it from fouling.
I think there may be something wrong with the bpr6 plugs - maybe fouled or the gaps closed?
 

Fairlineguy

Club Member
Why would you want resistor leads and plugs ?
last year I took a spare set of plugs with me to a rolling road session .
Half way through we took the plugs out to check them .
whilst they where out I thought id put the new ones in which where resistor plugs
it wouldn’t start
Took them back out and put the old ones in and it fired up straight away
 

Faster Behr

Club Member
Why would you want resistor leads and plugs ?
last year I took a spare set of plugs with me to a rolling road session .
Half way through we took the plugs out to check them .
whilst they where out I thought id put the new ones in which where resistor plugs
it wouldn’t start
Took them back out and put the old ones in and it fired up straight away
Resistor plugs were recommended by FAST for their EFi product and were very explicit about it. There was even a reminder in the box from Zcardepot. Otherwise I wouldn't have gone to the trouble.

Interesting that you couldn't get your car to run with resistor plugs.
 

Huw

Club Member
Anyone who has been following my thread on my Fairlady Z escapades may recall that it was originally fitted with a Holley carb and a Bob sharp Racing manifold. I have set about replacing that with a FAST EFi retrofit fuel injection system in order to get it on the road for the summer. During this work I found that the 280ZX distributor (the one with the E12-80 module) that was fitted with the car had a faulty vacuum advance mechanism. I sought to replace this and iron out any timing issues before setting about finishing off the set up of the EFi. After lots of reading and researching - including a very informative thread here which I re-read many times - I decided to give the 123+ distributor a try.

This is where my troubles really started and where I need some help/fresh ideas/emotional support..

Ok, so I fitted the 123+, added resistor plug leads and resistor plugs (NGK BPR6ES). I couldn't get the car to run at all. I had sequential spark across all 6 plugs except for a weak spark/missing spark on #2 and 5#. Timing was checked and rechecked and rechecked. Compression 175psi +/- all round. Cam timing checked. All back to basics. I tried all different permutations. I removed the original 280Z 0.9ohm coil and tried a 1.5ohm coil (all of which I measured with a good multimeter). No start. I tried changing settings in the 123+ app. I checked all ground points and wiring. No joy.

I put the original distributor back in, timed it up and no start. I hot wired the coil direct to overcome any voltage drop through the start circuit. No good. I could not get the car to run. It should be said that on multiple occasions I pulled the plugs and they were wet with fuel. I cleaned and regapped them many many times.

My starter got weaker and weaker to the point where I had to fit another. RIP.

I changed the HT leads back to standard leads, no joy. I refitted the original BP5ES (hotter, non-resistor) plugs and boom. It started and went straight to a clean idle like nothing had ever happened. I was gobsmacked. There were nearly tears. I reckon I had about 25+ hours of time just into this issue.

What I can't understand is why the plugs made such a dramatic effect. It's either a case that it wants hotter plugs or that it is having a real problem with their being resistance in the secondary ignition circuit. Unfortunately I don't have any other plugs on hand to try to diagnose this.

This all begs the question of what is really going on here. And this is what has me stumped and looking for good advice. The issue was the same for two different distributors and two different coils. I hotwired the coil supply ruling out any errant voltage issues to the coil. It just looks like something doesn't agree with their being resistance in the secondary HT ignition system.

PS. much kudos and thanks to @jonbills who was very helpful with heping me on this.
Sorry to hear your having such difficulty with the 123 set up. The rotor arms that come with the 123 have a resistor builtin. Could it be that the additional resistance in the resistor leads might be too much? I’m running with BPR6ES-11 plugs standard leads and 1.5ohm coil (123 recommend coils above 1ohm).
 
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Faster Behr

Club Member
Sorry to hear your having such difficulty with the 123 set up. The rotor arms that come with the 123 have a resistor builtin. Could it be that the additional resistance in the resistor leads might be too much? I’m running with BPR6ES-11 plugs standard leads and 1.5ohm coil (123 recommend cools above 1ohm).

I really don't mind all of this. The benefit is I am learning all the time. That's one of my main motivators to be honest.

I have been thinking about this this afternoon. The 123+ rotor arm measures 4 kΩ. The coil lead is 3.7 kΩ and each lead averages 6 kΩ. So 13.7 kΩ across a cylinder. The resistor in the plugs add 5 kΩ. It's bizarre to this the additional 5 kΩ causes a no-start situation. All six plugs are bang on the same resistance and are mintier than a new born spring lamb so I can't see they are alone at fault.
 

Fairlineguy

Club Member
I really don't mind all of this. The benefit is I am learning all the time. That's one of my main motivators to be honest.

I have been thinking about this this afternoon. The 123+ rotor arm measures 4 kΩ. The coil lead is 3.7 kΩ and each lead averages 6 kΩ. So 13.7 kΩ across a cylinder. The resistor in the plugs add 5 kΩ. It's bizarre to this the additional 5 kΩ causes a no-start situation. All six plugs are bang on the same resistance and are mintier than a new born spring lamb so I can't see they are alone at fault.
Makers of digital electronic ignition systems (or Sparx regulator/rectifiers) often specify that 5 kOhm resistance is required. So you either need resistor plug caps, or resistor plugs or resistor wires.
 

Rob Gaskin

Treasurer
Staff member
Site Administrator
It's funny how we don't believe what happens and continue to find another reason to fit in with our thinking.

I've heard that cold plugs can cause cold start problems - or perhaps that's just cold running problems. In the back of my mind I think some of the old race cars used to be started on hot plugs and then changed?
 
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Faster Behr

Club Member
Well I put some BP6ES in today and it runs like a champ. So my issue is resistance not plug temperature.

I will stick with these. EMF be damned. Unless the ECU catches fire or planes start falling from the sky round my way, then I’ll move on to worrying about something else.
 

Faster Behr

Club Member
It's funny how we don't believe what happens and continue to find another reason to fit in with our thinking.

I've heard that cold plugs can cause cold start problems - or perhaps that's just cold running problems. In the back of my mind I think some of the old race cars used to be started on hot plugs and then changed?

I think that’s driven by experience. We do the same thing many times and we expect the same result. I’ve never had an issue with resistor plugs, or any plugs - other than fouled or knackered.

In my case I made many sweeping changes to the car at once including drilling the head for EFi, fitting efi and changing ignition control. The last thing I would have expected to cause a plain no start issue would have been an additional 5k of resistance at the plugs. But that’s just me.
 

jonbills

Membership Secretary
Staff member
Site Administrator
I think that’s driven by experience. We do the same thing many times and we expect the same result. I’ve never had an issue with resistor plugs, or any plugs - other than fouled or knackered.

In my case I made many sweeping changes to the car at once including drilling the head for EFi, fitting efi and changing ignition control. The last thing I would have expected to cause a plain no start issue would have been an additional 5k of resistance at the plugs. But that’s just me.
I still can't believe it. I mean, I do believe you, but I can't believe it..
 
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