Lagondaforum

Subject: Intermittent firing with my engine

---

Hi All Lagonda 3 ltr Engine.
I am having intermittent firing with my engine. I clean the plugs and the engine runs very well with lots of power. I can run this for two or three days then the engine will only fire on four or five cylinders. The plugs are a nice light gray colour and set to 22 thou I have also opened the gap to 30 thu for a cleaner burn. I have changed the diss cap and the six plug wires. I have checked the timing, points and capacitor and replaced all the plugs. I have had the carbs professionally tuned. It could be a sticking valve/s (has anybody any experience with oil additive to help clear sticking valves).
This car has only covered 500 miles in the past twenty eight years.
What I can’t understand if it is a sticking valve, why the car runs so well at any speed with loads of power then drops a cylinder or two after leaving the car over night. Any thoughts would be most welcome or do I have to resort to the BIG HAMMER treatment!

Peter

---

Subject: Re: Intermittent firing with my engine

---

Hi Peter

so it is not firing (no ignition) on one or two cylinders after 2-3 days use and a night in the garage ? or is the ignition on all cylinders always good ?

For checking the ignition I would unplug each igniton cable one after the other, there should be a drop in engine speed if the cylinder is ok. If it is not dropping on one this one is bad but it can be the ignition on this one or something else.
Other test could be to unplug the ignition cable for this bad cylinder and test the spark on a separate spark plug or using the capacitive strobe light for the test (no spark - no flash).

You don´t have any misfiring? I would assume that a sticking inlet valve gives misfiring shooting into the carbs (on a sticking outlet I am not shure what it would do).

For finding bad cylinders I also like an infrared thermometer (looking like a laser gun), you can measure temperatures very lokaly (e.g. on the exhaust manifold at each cylinder) and without touching the part.

From what you tell it seems repeatable, how do you get the engine back running well? you say cleaning the plugs?

How are the valve gaps?

If it is a sticking valve this should show in a compression test, I assume

Just some thoughts, no professional checklist

Peter

Subject: Re: Intermittent firing with my engine

---

Thank you Peter for the reply. I have undertaken all that you suggest and when the engine runs it runs very well, it’s most perplexing!
I replaced the distributor cap with a new cap of the correct type and put new plugs in, the car fired up on all cylinders and run like a train for three days. I thought I have fixed the problem but when I went to start her up on Sunday, back to the five cylinders. I was so fed up I had to just walk away and haven’t been back to it yet so It may still be the BIG HAMMER!
One thing I have found is the earth strap from the bell housing to the firewall has come apart and needs to be replaced... Question! Could a bad earth from the engine/gearbox to the firewall cause the engine to become intermittent?
It would seem that once I lose a cylinder it never returns without cleaning the plugs, the cleaning of the plug may be just my imagination who knows.
I intend to do a compression test which should resolve the sticking valve theory one way or the other. If it is a sticking valve why will the engine run so well over all speed ranges and never miss a beat (That is when it decides to run on all six cylinders).
I do have the flashing plug adaptors and they are all flashing which should mean the plugs are firing.
Any other suggestion/theories are most welcome.

Just a little frustrated!

Peter

---

Subject: Re: Intermittent firing with my engine

---

Hi All Well the story continues! I changed all six plugs this evening, I cleaned up another set of six plugs and opened up the gap to thirty thousandth of an inch (for a cleaner burn) Put the clean plugs into the engine and without a moment’s hesitation she fired up on all cylinders. After worming her up, I have tried the old trick of spraying Redex directly into the carb’s intakes. I have run the car for an hour and she’s still on all cylinders. I thing I will have to take her out for a good long run after I do an oil change and see if I can clear the head, inlets and outlets of any residue carbon.
My understanding is that this engine is prone to fowling plugs? Any comments on this are welcome. The Plugs I am using are NGK C7HSA. What are others using? any comment re the type of Plug.
The car has been standing for twenty eight years but before she was put away the bottom end of the engine, the valves and other items had all been reworked including the head.

Peter

---

Subject: Re: Intermittent firing with my engine

---

Hi All. After talking to various people over on the AMOC site (Aston 2-4 has the same engine) It would seem I’m using the wrong type of plug. They recommend NGK C6 HSA which is a hotter plug than the NGK C7 HSA (the plug I’m using) and is better suited for the standard engine. I have ordered a set of C6 HSA and will let the forum know how I get on.

---

Subject: Re: Intermittent firing with my engine

---

Hi Peter,
From your descriptions we think your problem is with the coil and/or the distributor cap.

Since 1950 (the year after my birth) we have had in the family three 2.6 litres and two 3 litres, 290/1.
One of the 2.6's gave trouble not dissimilar to yours. A new coil provided an instant cure.

We suggest
- check that the wiring to the coil is correct. ie on a Lucas coil SW (secondary wiring) is power IN
CB (contact braker) is to the distributor points. If this order is reversed the engine will still run but the power of the spark delivered to the plugs will, on warming up, diminish rapidly.
- Distributor cap. You mention you have changed caps, was it for a brand new or second hand cap?
The caps will, after years of service or simply with age, develop non visible hairline cracks which, on heating, open up and allow the spark to take a short cut to earth.
If your cap is new, check with the old one that the terminals are in exactly the same position relative to the locating step on the original.

Is your car a 290 or 290/1?

Regards,
Mark and Jim

---

Subject: Re: Intermittent firing with my engine

---

Hi all,

May I reinforce the sage advice from Mark and Jim. I have never owned a DB Lagonda so my comments here are based on running clapped out modern cars for daily use - keeping my pennies to spend on ‘proper’ cars.

I once had a series 1 Vauxhall Viva Van for my daily journey to work. The engine power got worse and worse. It seemed to be on its last legs and a work colleague offered me a free engine if I removed it from his scrap car. How could I refuse?

Once fitted, the van went like a rocket.

I passed my old engine on to somebody else, warning him it needed attention. Some weeks later he greeted me cheerfully and thanked me profusely for the wonderful engine - all he did was CHANGED THE DISTRIBUTOR CAP!

Therefore I completely agree with Mark and Jim’s advice regarding ignition components. May I also suggest this might be a job for a ‘Crypton Tune-up’? In the 1960’s I was mighty impressed by the ‘white heat of technology’ associated with the impressive array of dials and screens that such a unit offered an ‘all thumbs’ mechanic. It also got results, and something like this would pick up the kind of problems we see here.

This is even more desirable with a V8 or V12 – who knows if all cylinders really are working?

I look forward to the next instalment of the saga.

Colin M34

(Born 4 years before our family’s 1953 Rover 75 was made. I learnt to drive on this car….)

Subject: Re: Intermittent firing with my engine

---

Hi All I will take on board what everybody has said and go over the ignition side again.
Mark-Jim: She is a 190/1. I changed the old diss cap after discovering the cable screws holding the ignition cables into the top of the diss cap had melted welding the cable to the very TOP of the hole. When checking these seemed tight & secure but were in fact well away from the brass contacts, inconsequence the spark had to jump a great distance. I thought I’d discovered the problem after changing the cap for a NEW-OLD stock cap and she ran well for three days but as I said above Sunday back to five cylinders. My next move is to change the capacitor/points check the wiring to the coil maybe change the coil also. I’m very reluctant to put one of those new Holden diss cap’s on as I’ve had such bad reports as to their reliability but if need must so be it. > The saga continues

Peter

---

Subject: Re: Intermittent firing with my engine

---

Continuing saga of my miss-firing engine.
Looking through some box’s of bits in my workshop the other evening I discovered 6 unused KLG P Ten 80 sparking plugs, these are the very spark plugs used by Lagonda in this engine. (I remembered buying them on e-bay a few years ago). My thoughts were these plugs should decide if it is faulty or wrong plugs I have been using that is causing the engine to fire on five cylinders. I set each plug to 22 thou, put them in the engine and started her up. Without a moment’s hesitation she fires on all cylinders, I then took her out on a five hours drive without the slightest problem. I’ve changed the oil and used one of the patented oil additive which are supposed to help with cleaning any carbon deposits that were remaining.
I have run her every day for the last week covering lots of miles and she has started first time and on all cylinders, I’m now starting to have a little more confidence in her. Let’s hope this has solved what was becoming a very irritating problem.

Unless the Gods of misfiring engines have other ideas!

---

Subject: Re: Intermittent firing with my engine

---

Continuing saga of my miss-firing engine.
I have now covered well over 500 miles with the car since changing the plugs and she has never missed a beat. Oil pressure starts off about 45 LBS (classic 20/50 oil) dropping to 25LBS when she reaches normal running temperature and stays at that pressure at all speeds except idling when it drops to almost nothing. Touch the accelerator and she’s up to 25 LBS again.

Any observations, comments, suggestions, welcome.

---