The 'E' Type Forum

I’m struggling with a frustrating problem and would appreciate any advice.

My car starts from cold easily; I leave the fuel pump on for a few seconds until you can hear the return one-way valve operating, then full “choke” and accelerator down. The problem is that usually one spark plug isn’t firing. The engine runs quite smoothly on 11 cylinders but you can tell it isn’t quite right.

It is usually 5A that is not firing (but has been 6A). I use my timing light to verify that the other plugs are firing (the timing light doesn’t flash when a plug isn’t firing). When I pull out the plug in question it is visibly wet.

To get round this I usually run the car up to temperature and then change out the wet plug. I leave the plug out for a while and then install a replacement plug. Sometimes this can take a few goes until I have all 12 cylinders firing. Then all is good.

I have the Reopus system installed (and have a spare amplifier and all ignition parts). The HT leads were all replaced a few years back. I change my plugs every year. I have spent a lot of time rebuilding the carbs and the car runs really well (eventually).

I wonder if anyone has had a similar issue and what I should troubleshoot.

Dear Nigel,

The only common factors here are plugs + plug leads OR the right rear carburettor being too rich at start. Since you swap the plugs out and it'd be easy to swap the 5A and 6A plug leads onto the other bank, I'd look to see whether you can either lean out the right rear carburettor a bit at idle, or work out whether the lion's share of the extra choke fuel goes to the right rear manifold.

kind regards
Marek

....or a slightly sticking inlet valve on that cylinder?

MarekH wrote: ↑

Wed Feb 15, 2023 5:46 pm

OR the right rear carburettor being too rich at start.

That's where I'd be looking first.

Thank you all for your replies, these really help. I did some more investigation and noticed that the strobe light shows that the 5A spark plug starts firing on the overrun. It was very clear, turn the throttle up (using the top of the throttle pillar) to circa 3k rpm (no spark) and then let it drop quickly. The sparks appear and look very good, not intermittent. However they disappear again very quickly once idling.

Marek, I think you may be right that there could be something happening with the enrichment device which pushes too much fuel to the right rear carb on idle. I have had the carbs off before (which isn’t difficult) so will investigate further and report. I wonder if it could mean that the enrichment path to the front right carb is blocked? Or somethings else….

Congratulations on a nice bit of troubleshooting with the strobe light input. It'd be lovely if moving the plug leads around moved the problem, but that's a bit a long shot and was included for completeness. I expect you have been overfueling the three rear A cylinders and it manifests on the richest one, which ought to be the shortest run. (Think Bernoulli...).

If there is any scope for adjustment, try making the A side have less overall choke.

I have a spare lambda sensor and gauge you can stick down the tailpipe if you want it. I pass by your way most Wednesdays and can drop it off.

kind regards
Marek

Doddsy wrote: ↑

Wed Feb 15, 2023 4:23 pm

My car starts from cold easily; I leave the fuel pump on for a few seconds until you can hear the return one-way valve operating, then full “choke” and accelerator down.

Just a comment on your cold start method, I know nothing about V12's, but on all cars I've owned pressing the accelerator full down with the choke on would lead to flooding.

Pumping the accelerator prior to turning the key in a car with SUs does nothing.

Holding the accelerator down while cranking probably just leans the mixture off, counteracting the effect of the choke.

Carbs like Webers have a mechanical pump that squirts fuel into the induction path if you pump the accelerator. So to start my MGB fitted with a Weber I'd pull the choke on, give 3 pumps and turn the key.

abowie wrote: ↑

Fri Feb 17, 2023 7:06 am

Pumping the accelerator prior to turning the key in a car with SUs does nothing.

Andrew, wouldn't he have Strombergs if it's a series 3 V12? Does the same thing apply to strombergs?

malcolm wrote: ↑

Fri Feb 17, 2023 9:38 am

abowie wrote: ↑

Fri Feb 17, 2023 7:06 am

Pumping the accelerator prior to turning the key in a car with SUs does nothing.

Andrew, wouldn't he have Strombergs if it's a series 3 V12? Does the same thing apply to strombergs?

Yes he does, and no I don't know whether Strombergs have an accelerator pump or not.

Anyone enlighten me?

The Z-S CD 175 does not have an accelerator pump.

Doddsy wrote: ↑

Wed Feb 15, 2023 4:23 pm

I have the Reopus system installed

This is where I would start . One bad ferrite bar in the disk could lead to weak spark on that cyl...

I have no faith in OPUS or REOPUS.

Thanks to all for all the tremendous advice on where to go looking. The weather was kind here yesterday so I had my car out and had a play.

I was particularly intrigued by the comments from Allrand and Abowie about my starting technique. I’ve only had the car for 40 years and have always used the choke full on throttle wide open. It worked before so I never thought to change (even with the significant exhaust smoke!) . Anyway I decided to give the plugs a good clean and tried with the choke out just one indent, no throttle and she fired up, a bit sluggish at first but soon came to life with a bit of throttle. Timing light showed she was firing on all cylinders and so took her out for the first run of the year. She ran really well.

It was 13C here so not cold. I want to see if this repeats in a cold spell (still very likely to happen) before I am convinced it is sorted out but I am hopeful. It does make me wonder how others cold start their cars and is this the preferred method.

jagwit wrote: ↑

Mon Feb 20, 2023 3:58 pm

Doddsy wrote: ↑

Wed Feb 15, 2023 4:23 pm

I have the Reopus system installed

This is where I would start . One bad ferrite bar in the disk could lead to weak spark on that cyl...

I have no faith in OPUS or REOPUS.

FWIW, I'll elaborate on my opinion about suspecting OPUS/REOPUS.

I believe your method of replacing a visibly wet plug with a dry one is treating the symptom, not the problem.

"Flooding an engine" is AFAIK, a term used on engines with a carburetor which has an accelerator pump (which the Strombergs do not have). Excessive pumping of the accelerator pedal on carbs with accelpump results in excessive amounts of liquid fuel being dumped into the intake, which is then drawn into the combustion chamber and then thoroughly wets the plugs to the extent that the spark is literally killed off WITH FUEL. Then the engine is considered "flooded". Remedy is to floor the pedal, keep if floored and continue cranking to dry out the combustion chambers until another attempt can be made at starting the engine, without flooding it, again.

It could very well be that a similar situation is happening with your car compliments of OPUS/REOPUS. That particular cylinder appears to be "flooding" (but it isnt!!) due to inferior spark.

Here is how:

1. Let's ASSUME for now, that you do have a very weak spark on 5A - ONLY on 5A. Let's assume that the spark is so weak that applying choke causes the very rich mixture (most likely also containing liquid drops) to that cylinder (and all the others) to be so rich that the weak spark is unable to ignite the mixture on 5A.
2. Repeated inhilations of rich start mixture (as the engine turns / runs) causes more fuel to be drawn into that cyl, ensuring that the spark plug becomes thoroughly wet, as if it is being flooded and there we have the wet "flooded" plug.

So how is it possible that you could have a weak spark ONLY on 5A?

The way OPUS/REOPUS works, is that the ferrite rods in the rotating disc plays a HUGE role in how well the coil is charged for a spark. My own car was fitted with REOPUS before I bought it from my friend and we researched ignition systems for the V12 extensively at one point. We found two issues with the OPUS concept:
1. If the ferrite rod inside the disc is broken somewhere along its length (inside the disc! - as we indeed found), it results in a weaker spark, due to the coil charge time being reduced;
2. if the ferrite rod inside the disc is located further away from the edge (closed to the dissy shaft), it results in a weaker spark, also due to the coil charge time being reduced;
In conclusion, with OPUS/REOPUS it is QUITE POSSIBLE that one or more weak sparks could consistently be fed to a particular cylinder(s). Cold starts is when a particularly potent spark is called for.

BTW: It should not be necessary for you to press the accelerator pedal down when starting the engine. Doing so does not materially change the amount of air being drawn into the engine while cranking! This is because the air valve, which is opened as a function of the air depression over the bridge forming between the air valve and the butterfly is not opening during cranking. There is not enough air flow to cause a depression over the bridge and thus the air valve does not lift (during cranking). Also, if the choke mechanism has been set correctly, pulling the choke, in effect is also stepping on the accelerator on your behalf!

So, my recommendation is to replace the (RE)OPUS ignition system with a proper modern system like Lumenition or Pertronics (using part no 9LU-1122A Ignitor II - as for OPUS dissy). While you're at it, replace the vacuum retard module with vacuum advance JLM519 for much improved fuel economy, more part throttle torque, and less stress on the cooling system during part throttle cruising conditions.

I’m pleased to report that I have finally found the source of the problem. I had not set the “Fast Idle Setting” correctly (19.15.02 in the Repair Operation Manual). The problem bank had the setting screw screwed right in and the gap B was as large as it could be. The other bank was set to 25 thou.

When the choke is applied the enrichment device opens and extra fuel is sucked from the float chamber and enters the carb between the air valve and the throttle butterfly valves. There is also a cam which opens the throttle butterfly valve additionally when the choke is applied. The adjusting screw rides against the cam and sets the amount the throttle butterfly valve opens to allow additional air flow.

This lack of adjustment resulted in the severe flooding that I was experiencing. It was very easily resolved although I did take the carbs off to see what was happening. Thanks all for your advice here which really helped. The difference is amazing and I now have a car starting and running so much better. I have also changed my starting technique and don’t apply any throttle! And the sun is out….

Doddsy wrote: ↑

Sun May 21, 2023 6:47 am

I’m pleased to report that I have finally found the source of the problem

Very pleased for you.

When everything is set right and works right.... It works.