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annoying misfire - advice sought

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Vortex O'Plinth
Posts: 813
Joined: Fri Feb 12, 2016 8:15 pm
Location: Gloucestershire

Re: annoying misfire - advice sought

Post by Vortex O'Plinth » Tue May 07, 2019 4:06 pm

If you pull the choke out does the engine speed at which the misfire starts, rise? If so, it's probably a fuel supply problem - could be dirt in the carburettor jet.
Last edited by Vortex O'Plinth on Tue May 07, 2019 6:59 pm, edited 1 time in total.
Nick

"Open the pod bay door Hal".

Bud
Posts: 222
Joined: Fri Feb 12, 2016 9:06 pm

Re: annoying misfire - advice sought

Post by Bud » Tue May 07, 2019 4:56 pm

re: Ian's post

If you suspect ignition type problems, an old trick is to run the car in complete darkness and look for the "light show" as it will locate insulation failures and carbon trail arc overs most readily.

daimlersteve
Posts: 321
Joined: Wed Mar 09, 2016 11:20 am

Re: annoying misfire - advice sought

Post by daimlersteve » Fri May 10, 2019 5:39 am

Jez and Tony are right regarding carb feeds to cylinder #s . If the carb is rich at idle it will be rich further up the range ( if the needle is org. ) more likely to be electrical . some Plugs once fouled will not self clean up in that rich , now adjusted leaner cylinder use.
cheers , Steve

Marcus
Posts: 202
Joined: Tue Feb 16, 2016 9:06 am
Location: hong kong

Re: annoying misfire - advice sought

Post by Marcus » Mon May 13, 2019 6:16 pm

Thanks to everyone for their replies. Sadly Work and domestic chores have interfered with garage time so I have yet to follow up. I will post the answers when I have the problem resolved.

Ian Slade
Posts: 444
Joined: Sat Feb 13, 2016 9:54 am
Location: Akrotiri Cyprus

Re: annoying misfire - advice sought

Post by Ian Slade » Mon May 13, 2019 7:11 pm

The balancing and richness is controlled on the idle by adjusting the throttle stops and the diaphragm control, the accelerator is then set at about 2500- 3000 rpm balanced using a vacuum gage or bobble type thermometer adjusting the short rods to the carbs, when balance is achieved the richness can be adjusted again on the diaphram control( I found the Gunson Colour Tune spark plug replacement ideal but time consuming to get it right as it needs to be moved to different cylinders to get the best result.) 8 of them is the ideal but 4 gives a very good set up. The final tuning at medium rpm does compromise the idle set up but not much
Owner since the 70's, Genghis is slightly to my left.

KV8
Posts: 209
Joined: Sun Mar 06, 2016 7:05 pm
Location: Cambridgeshire

Re: annoying misfire - advice sought

Post by KV8 » Tue May 14, 2019 9:09 pm

Ian Slade wrote:
Mon May 13, 2019 7:11 pm
I found the Gunson Colour Tune spark plug replacement ideal but time consuming to get it right as it needs to be moved to different cylinders to get the best result.) 8 of them is the ideal but 4 gives a very good set up. The final tuning at medium rpm does compromise the idle set up but not much
I have considered this device for use on my other classic. However can not see how it can be used on DV8 engines where plug is at bottom of tube? Please explain?

Thanks

H

Ian Slade
Posts: 444
Joined: Sat Feb 13, 2016 9:54 am
Location: Akrotiri Cyprus

Re: annoying misfire - advice sought

Post by Ian Slade » Wed May 15, 2019 4:52 am

Actually they're brilliant, the light reflects off the tube and the whole tube glows, far easier to read than the exposed plug.
Owner since the 70's, Genghis is slightly to my left.

timmartin
Posts: 49
Joined: Wed Feb 17, 2016 1:37 pm
Location: Berkshire

Re: annoying misfire - advice sought

Post by timmartin » Wed May 15, 2019 9:48 am

...and the Colour-tune comes with a little extension which is easy to screw onto the plug and doesn’t get in the way.

The view of the spark igniting the mixture and the different colours of combustion are so clear that I have fascinated three generations of my family by showing them. At least, I think they were fascinated ... but they could certainly see it easily.

Don’t forget that the centre two cylinders on each bank (2 and 3) run leaner than the outside two (1 and 4) because of the imperfect design of the inlet manifold. This means that when 2 and 3 are set correctly (ie not too lean) then 1 and 4 have to be a bit too rich. This is why it has a reputation for being a “dirty” engine, and the oil gets sooty so quickly.
But don’t let that worry you, it has been so since before Daimler sold the first one, and we still love them.

The hardest bit is actually avoiding a mistake when deciding which carburettor supplies which inner cylinder, at least it is for me.
Tim
Blue 1964 SP250 in Berkshire

Marcus
Posts: 202
Joined: Tue Feb 16, 2016 9:06 am
Location: hong kong

Re: annoying misfire - advice sought

Post by Marcus » Tue Jul 09, 2019 7:26 pm

Hello all, it’s been some time since I posted the initial query. Thanks to all that posted advice and suggestions. My aim was to get the car to the 60th anniversary rally and given I was running out of time I enlisted professional help in the form of a reputable classic car garage.
They returned the carbs, found faults with the ignition circuit ( 4 condensers fitted and two sets of points) and changed the coil and plugs again before they could eliminate it.
I picked the car up and drove it home, all went well until I encountered heavy traffic. The damned thing stopped 3 times, with symptoms of lack of fuel.
I could only think that there is vaporisation occurring in the heat of the engine bay.
Eventually I made it home and for the sake of my sanity I decided to attend the rally in my wife’s Audi.
When I finally managed to get back to using the car a week later you can imagine how fed up I was to find that sane old misfire had returned. Anyone ever seen the episode of Fawlty Towers when Basil Fawlty beats the living daylights out of an 1100? You will understand my feelings.
I lent on the wing in despair and watched the petrol filling up the glass filter bowel. It was then that I noticed sand like sediment falling into the bowel. I peered closer, and yes, regular deposits were falling in.
As reported earlier, I had cleaned the gauze filter previously and had also fitted additional in line filters but still rubbish was getting through. I took the gauze filter out and was shocked to see how much crud was trapped. Picture of it and a replacement attached.
So now I’ve taken the plunge and purchased a new fuel tank from Christian Demler. I’m hoping that this is the final solution to a most frustrating problem. If it isn’t I’ll be having a dart shaped bonfire at the weekend. Bring a burger!
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Warsash 2
Posts: 273
Joined: Sun Feb 28, 2016 1:53 pm

Re: annoying misfire - advice sought

Post by Warsash 2 » Wed Jul 10, 2019 11:25 am

Hi
A friend of mine had similar trouble with his V8 saloon. Apparently petrol used to be filtered through sand and microscopic materials where left in the petrol as sold. Over a lot of years this precipitated in the tank eventually clogging up the pick up filter and then any other filters. I found this analysis as interesting and pass it on for what it is worth.
If it is vapourisation then insulating the fuel with hydraulic pipe insulation. Particularly relevant if you have S/S exhaust

Regards

Colin

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