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annoying misfire - advice sought

Posts: 209
Joined: Tue Feb 16, 2016 9:06 am
Location: hong kong

annoying misfire - advice sought

Post by Marcus » Mon May 06, 2019 9:43 am

Hi Dart/V8 owners,

I have a slight misfire on my Dart as revs rise. At idle it runs smooth, it is not shaking itself to bits. So far I have changed plugs, coil, leads and checked the points gap. The condenser is new as is the cap. Still not running right.
I do get a bit of oil leakage at the base of the spark plug tubes so to be safe I cleaned all of that up and difference.

I used an inline test lamp to check each firing pulse for each cylinder with the engine running and could see that all 8 were receiving high tension. So I figured maybe a compression problem. Using the numbering conventions in the manual I obtained psi readings as follows:

1l 148, 2l 150, 3l 150, 4l 158, 1r 162, 2r 150, 3r 150, 4r 159.

Looking at these numbers It does not seem like I have a burnt valve, leaking inlet manifold or failing head gasket. Oil and water are not mixing either. However I did notice that plug numbers 2l, 3l, 1r and 4r were becoming sooty at idle whilst the remaining 4 were spotless. That made me think that one of the carbs was running lean or the other rich. The exhaust at high engines speeds is still clean so I assumed the former. However the problem is that when I checked as to which carb provides fuel to 1l,4l,2r and 3r in order to adjust it my hypothesis falls down as it does not reconcile to the combination quoted in the service manual. The workshop manual states that one carb provides the fuel air mixture to 1l,3l,2r, 4r and the other to 2l, 4l, 1r and 3r! So my clean versus dirty plug combination is not logical.

Other thoughts...stuck vacuum diaphram? I was rather hoping to drive up to the 6oth in it but at this rate of problem resolution I will be lucky to get to the 2020 national!

Fuel is not stale. fuel filter checked and clean. Any members have a bright idea as to where to go next? Scrap yard? :)

Posts: 434
Joined: Wed Sep 05, 2018 9:55 am
Location: Ridgewood - East Sussex

Re: annoying misfire - advice sought

Post by classiclife » Mon May 06, 2019 10:03 am

Hello Marcus,

One item you have not mentioned as being replaced is the rotor arm, these are often overlooked and are not immune to issues - especially modern replacements. Many classic owners are using the red rotor arms and these have an excellent reputation.

Hopefully you replaced all of the items one at a time and tested accordingly; this does help to ID the incorrect part. If change everything at once then that makes ID of a defective part more difficult as you now have no idea what is good and bad.

Do not be fooled that a "new part" is in perfect working order, it may well be defective - this certainly applies to condensers and on occasions the coil. Speaking of the latter, does it get too hot to touch when the car has been running ?? If so, it has an issue. I take it you have the correct coil and not a ballast type unit.

Your compressions are acceptable and it is worth bearing in mind that the V8 is a dirty engine - as such you will quite often witness sooty plugs; although I agree with you rich / lean assessment.

Another avenue of investigation is to have a look at the carb fuel bowls and ascertain they are clean inside with no fragments of fuel line / particles from the fuel tank - both as a result of degrading.

I do not know what the ethanol levels in fuel are in HK, but not withstanding that you should now be using fuel line rated at R9 or Gates Barricade fuel lines; these are specifically constructed to deal with modern day fuel issues.

My last bit of advice is to check all your electrical connections and ascertain they are secure and free from muck.

I hope the above assists ??

Good luck.

1968 Daimler V8-250 Saloon
DLOC East Sussex Area Representative.

Southern Classics Society Events Co-ordinator.

Posts: 209
Joined: Tue Feb 16, 2016 9:06 am
Location: hong kong

Re: annoying misfire - advice sought

Post by Marcus » Mon May 06, 2019 10:30 am

Thanks Richard, I’m afraid coil leads and plugs were changed at same time. I’m just heading out to garage with renewed enthusiasm. I’ll report back later. Rotor arm to be changed as first job.

Ian Slade
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Joined: Sat Feb 13, 2016 9:54 am
Location: Akrotiri Cyprus

Re: annoying misfire - advice sought

Post by Ian Slade » Mon May 06, 2019 11:06 am

Check the distributor is giving full advance, the distributor is known for seizing giving your symptoms, remove the cap and base plate then rotate the the spindle by hand viewing the advance weights and ensure they are fully opening.
Owner since the 70's, Genghis is slightly to my left.

Big Col
Posts: 413
Joined: Tue Feb 16, 2016 12:45 pm
Location: Leeds

Re: annoying misfire - advice sought

Post by Big Col » Mon May 06, 2019 9:31 pm

I would be tempted to follow previous posts and be very wary of new parts. The Dissy weights do need checking and lubricating occasionally. If you have put new leads and caps on that are the carbon ones then make sure you have pushed them fully home. Check all the low tension circuit, even to the point of running a direct feed from the battery to the dissy. It probably isn’t at fault but Iwould also check fuel delivery and that the bowls are not full of crud.
I may be slow but I’m rough as well !

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Re: annoying misfire - advice sought

Post by Jez » Tue May 07, 2019 6:44 am

I think your original thought regarding carburettor mixture is correct. If you look at the design of the inlet manifold, each carb feeds cylinders 2 & 3 on one bank and 1 & 4 on the opposite bank. If you search the forum I think this has been discussed before. It does suggest your original diagnosis re mixture is correct and if the plugs are sooting up at idle I would lean off the relevant carb by, say, half a turn anticlockwise of the mixture screw and see what happens.

Cheers - Jez
Jez Stow
1960 Daimler Dart
and a few other toys

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Joined: Sun Feb 14, 2016 11:54 am
Location: Leicestershire

Re: annoying misfire - advice sought

Post by Tony250w » Tue May 07, 2019 10:30 am

Having had the same problem and after checking everything as you have done my problem was found by chance When the engine was hot and switched off there was a hissing sound which turned out to be a carburettor float bowl vent pipe blocked with many years of crud and completely upsetting the mixture. Just another thought which may or not be worth checking. Good luck

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Joined: Thu Feb 18, 2016 10:54 am

Re: annoying misfire - advice sought

Post by Dig-SP » Tue May 07, 2019 11:02 am

Have you checked the plug extensions, take them apart to clean and tighten, often the Bakelite can have cracks, a shrink sleeve can fix that if you don't have new ones, worth a check, new plugs ,?

Ian Hastings
Posts: 104
Joined: Tue Mar 15, 2016 3:00 pm

Re: annoying misfire - advice sought

Post by Ian Hastings » Tue May 07, 2019 11:28 am

Following on from Dig SP's comments if you still have the original bakelite extensions I would suggest you change them to rubber type ones as fitted to some Fords and Hondas.
I had a similar problem eventually traced to the old extensions - I could actually see the spark coming off the bakelite!!
There was a thread about changing them not too long ago on this forum.

Posts: 130
Joined: Wed Feb 10, 2016 11:38 am
Location: south wales

Re: annoying misfire - advice sought

Post by buckfield » Tue May 07, 2019 2:38 pm

Hi, not wishing to add to a already comprehensive fault finding list, I have had a similar misfire on high revs before and have had 6 different reasons why, 1. engine earth strap had corrosion at the bulkhead end cleaned up and fine, 2. a very small current drain on the dizzy traced to a bad earth on the mounting plate, cleaned and copper grease fixed that, 3. float needle valves stuck/sticking after visiting a new petrol station, bad fuel that got past the filter,4. alternator had excess charging rate, new control box sorted that,5. only applying to petronix ignition systems, coil failure and a loose magnetic sleeve in the dizzy. 6. also to do with pertronix was the HT leads that wrongly had solid copper cores and changed them to pertronix silicone firepower leads and that was sorted, touch wood no electrical gremlins since 2012. on the old forum there was a post about the V8 being very sensitive to vibration and electrical stability? but I can't find that post again, might help you if not to check these? Tony

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