Mystery misfire

[phil.dobson@mac.com]

This is the sort of issue that has me swearing profusely and heading for the whisky bottle. ...
Have you connected a timing light onto the offending cylinder with everything screwed in place (I use an old fashioned strobe in series with the plug) . If you are satisfied the light looks the same for this and other cylinders you can hopefully eliminate the ignition side. The only other thing Ican think of assuming the inlet valve does open is that the choked connection on the underside of the inlet manifold is disconnected/open? (although you should hear it hissing like crazy when it runs)
When you undertake a compression test does the pressure build up in a similar fashion to the others or does it take longer. (my thinking is that if the cam clearances are at the absolute minimum you might see pressure but the opening for inlet suction maybe so small as not to be effective.)
Just trying to help.
Best regards and please keep us updated.
Phil D.

[Duckham]

Then they found it had thrown a big-end & the con-rod had got wedged up in the block.

It can't be that, Hugo, as it shows good compression.

There have been a couple of suggestions already you may not have followed through, Jerome:
1. Turn over the engine without starting and pull that spark plug + another to see if they are equally wet. That will tell a lot.
2. Use a Colortune to make sure there is a spark under compression
3. I have no experience of the hisser system but if that could create an air leak to the cylinder it would be top of my list
Joe

[Hugo]

Then they found it had thrown a big-end & the con-rod had got wedged up in the block.

It can't be that, Hugo, as it shows good compression.

Ah yes of course - It's first thing in the morning here in Florida & my brain wasn't fully awake ;)
Well, if you have compression, you have a spark at the right time, you have the valves going up & down and you have fuel, it ought to damn well work!
What are we missing?

[Nigel GT]

I had a similar problem, it was one of the carb float valve stuck closed, so no fuel getting to those cylinders, so worth a check to see if they move freely.
Best of luck
Nigel

[Hugo]

I had a similar problem, it was one of the carb float valve stuck closed, so no fuel getting to those cylinders, so worth a check to see if they move freely.
Best of luck
Nigel

I believe this is a twin carb.

[JerryL770]

You didnt really answer if the inlet valve actually opens.....you said that the tappet goes up and down.....that dosnt mean that the valve opens

I think it does, else maybe you can explain how that could happen.

Otherwise I think that's been answered by subsequent posts.

Cylinders 2 & 3 are working fine so can't be fuel and when the head was returned this week, another inlet manifold has been used, so it's not leaks there.

Now, here's an even stranger symptom. We take the plug leads off numbers 2,3,4 & 5 cylinders and it will start and run on 1 & 6. Then pull the plug lead off 6 and it keeps running

With all 6 leads on, it misfires and when No. 1 is removed it still misfires just the same.

[Hugo]

At the risk of stating the obvious, are you sure it's not just tracking inside the distributor cap? Or my earlier suggestion, that the spark is not tracking from one lead to another where they are bundled together? Did you try running a separate HT lead to the 'missing' cylinder to see if it springs back to life?

[JerryL770]

At the risk of stating the obvious, are you sure it's not just tracking inside the distributor cap? Or my earlier suggestion, that the spark is not tracking from one lead to another where they are bundled together? Did you try running a separate HT lead to the 'missing' cylinder to see if it springs back to life?

Well, everything has been changed at some time in the ignition system, without making a haperth of difference

Maybe will try again, but I will now not see the car again till next week.

BTW, thanks for all the suggestions including yours Phil.

[Hugo]

Including all the HT leads?

[JerryL770]

Including all the HT leads?

Yes.

Plugs, leads, whole distributor, coil.

Is there anything else?

[Hugo]

Let me get this right - it will run on no.1 cylinder, but only when 2-5 plug leads are disconnected? Did I read that correctly?

[mgcjag]

Hi Jerome...back to basics..the heads been off....so valve timing, compression, fuel in cylinder, ignition timing

[MarkRado]

You obviously checked the firing order several times - check it once more;
same happend to me on a V8 Rover.

[chuffer]

Now, here's an even stranger symptom. We take the plug leads off numbers 2,3,4 & 5 cylinders and it will start and run on 1 & 6. Then pull the plug lead off 6 and it keeps running

With all 6 leads on, it misfires and when No. 1 is removed it still misfires just the same.

Sorry, I don’t wish to demean your knowledge but as we are struggling to work this one out........you know that cylinder number 1 is at the bulkhead end don’t you?

Ray

[JerryL770]

Sorry, I don’t wish to demean your knowledge but as we are struggling to work this one out........you know that cylinder number 1 is at the bulkhead end don’t you?

Ray

errr, let me think.

YES

You obviously checked the firing order several times - check it once more;
same happend to me on a V8 Rover.

Perhaps need to check that again but I doubt we are wrong ......... at least i B W hope not

Hi Jerome...back to basics..the heads been off....so valve timing, compression, fuel in cylinder, ignition timing

I will have to get back on that one re: wetted spark plug on No.1. I won't see the car again till next week.

Let me get this right - it will run on no.1 cylinder, but only when 2-5 plug leads are disconnected? Did I read that correctly?

Well, with all plugs connected (without having a Crypton or Sparktune plug) it's difficult to know what No. 1 cylinder is doing but it's very odd that the engine would continue to turn over when started on 1 & 6 only, then pull No. 6

Head scratching continues .......... update next week sometime.

Thanks for help.

[cactusman]

I think like others am mildly confused. I would say removing a load of plugs would seem unlikely to guide one to a solution.

There are degrees of mis fire. No spark will obviously cause a serious misfire while a plug that does spark but not at the tip or with a weak spark due to bad lead connection for instance may cause poor flame initiation....the cylinder will fire...just badly. Poor combustion can also be caused by incorrect fuelling (too little or too much) poor air supply or by air leaks after the carbs that weaken the mixture....among other things.

Complete lack of flame initiation will lead to unburned fuel entering the exhaust and will probably result in pop's and bangs once the exhaust manifold is hot.

I would start by getting the engine thoroughly warmed up....all plugs in. Then remove each plug lead in turn. The one that has the misfire is the one that either makes no difference on removal (if there is zero combustion) or makes markedly less difference to the degree of misfire (if there is poor combustion).

Assuming fresh plugs, leads, cap and coil I would suspect you have poor combustion on one (or more) cylinders caused by a fuelling or air issue rather than a plug/spark issue...unless you are lvery unlucky..

[Hugo]

I believe he said everything was spotless inside no. 1 combustion chamber when the head was off. So total lack of combustion.
I also believe he said that it would run on no.s 1 + 6 when 2-5 were disconnected.
The second paragraph of course contradicts the first, so something isn't making sense.
The only thing that would remotely make any sense of it is if the spark is leaking from one lead to another.

[Series1 Stu]

It sounds to me like you've got tracking between plug leads, probably in the distributor cap. Or a high resistance in the number 1 cylinder electrical supply. Either of these scenarios would explain why no 1 cylinder runs when the other leads are disconnected - there's no lower resistance path for the spark to favour.

Regards

[1954Etype]

I saw something similar on an XK120. Turned out to be the dizzie shaft was worn with lots of play in it.

[MarkRado]

You didnt really answer if the inlet valve actually opens.....you said that the tappet goes up and down.....that dosnt mean that the valve opens

I think it does, else maybe you can explain how that could happen.

Otherwise I think that's been answered by subsequent posts.

Cylinders 2 & 3 are working fine so can't be fuel and when the head was returned this week, another inlet manifold has been used, so it's not leaks there.

Now, here's an even stranger symptom. We take the plug leads off numbers 2,3,4 & 5 cylinders and it will start and run on 1 & 6. Then pull the plug lead off 6 and it keeps running

With all 6 leads on, it misfires and when No. 1 is removed it still misfires just the same.

Jerome,
I apologize for my English preventing me to understand correctly:
The engine runs with leads connected to 1 & 6 only ?
My 3.8 engine (3 SUs) stalls when any two of six are DISconnected (at idle)!
I guess it would need me holding revs high and mixture rich to keep running on two

I am strenghend in my previous consideration that the misfire cannot definitely be assigned to No 1 (although combustion chamber clean).
One would have to judge the misfire live. How strong, how often, how rhythmical?

Knowing from painful experience, my 3.8 needed careful mixture and carb adjustment to idle and run really sweetly (after having gone through all your work mentioned at the beginning - replacing dizzy, coil, leads plugs, plug caps - several times) and still not content

This was my to-do list:
- lap valves; adjust clearance thoroughly
- check chain tension, cam timing, TDC, dizzy timing+gap - accurately!
- sealing compound on every inlet manifold gasket
- carbs/ floats/ jet diaphragms/ return springs ... beyond doubt and adjusted (+aux starting device if applicable)
- adjust mixture:
idle using a synchrometer;
mixture strenght (jet depth with vernier)- iterative plus trying much too lean and too rich

There is no dark mystery (only patience, time and money) - just tick all boxes

May the Force be with you!