Another misfire - aargh!

[1969Mk3Spitfire]

We went to Tatton Park today, exceptionally well attended by both exhibitors and visitors. Uncharacteristically glorious weather for south Manchester.

It’s less than 10 miles from home but includes some dual carriageway so tried to give the engine an Italian tune-up. Any good this may have done was subsequently lost by queuing at idle for half an hour to park at the event. Car performed very well.

On the journey home we took an alternative route, single carriageway country lanes. Almost home and the pesky misfire raised its ugly head. Same symptoms as “first” time, that is “instant” miss and “instant” recovery.

This is a picture of plug 1, 2 and 4 (no 3 is an absolute pig to remove as it hides behind the dizzie). Mileage on 1 and 2 is around 25 since last brushed clean.

No 4 combustion looks good. No 1 is still probably a bit rich. No 2 makes me want to thrash the car with a big stick. Still can’t rationalise why it gets so sooty, so quickly. More ideas most welcome.

[Wagger]

Assuming that you have twin carbs, that is either front carb too rich, oil getting in, lack of compression at speed or lack of sparks. If it was lack of sparks, the misfire would be more apparent. Try leaning off the front carb first. If no joy, then investigate with a vacuum gauge on each manifold if possible. Leaky or sticking valves usually show rapid fluctuation in vacuum.
 

Probably repeating other posts, but, does the exhaust smoke, and are you noticing oil consumption? This is as mysterious as my number two plug on my Vitesse. I share you frustration and fully sympathise.

[Pete Lewis]

why sooty   well it can take 10 miles of steady running to clean the choke developed soot off the plugs 

any ongoing acceleration will make a richer mixture so that can contribute to soot deposits 

you need a nice long cruise at 50-60 then pull a plug 

and sooted plugs do not seem to recover   a hard hic up and recovery sounds like a plug breaking down under pressure 

but it does look like its is far too rich i would lean it of 1/2 turn 

what air filters do you use ???

 

[1969Mk3Spitfire]

Thank you, both.

I said in an earlier post that I had just leaned the front carb by one flat. Lifting the do-dah causes the engine to slightly increase in speed, then slow down. the rear carb exhibits pretty much the same characteristic but, from looking at the plugs, it’s obvious that 3 and 4 pots have better combustion than 1 and 2. I will lean the front carb by another flat and try again.

How did you have in mind to configure a vacuum gauge in the manifold? What should I be looking for?

The exhaust is not smokey. It hasn’t done a lot of miles since preseason service so no measurable oil consumption.

1/2 turn sounds a lot, Pete?

I’ll change No2 plug if you think there’s a possibility of it breaking down, albeit it’s only done a couple of hundred miles.

Air filters are new (ish) and are OE. The car is unmolested, original, albeit restored.

Noted that a decent run is at the speed of sound (there’s certainly a sonic boom in the cabin) for a good few miles, then coast into a lay-by to pull the plugs.

I appreciate your help in trying to resolve the mystery. It’s frustrating and annoying in equal measure.

[Badwolf]

Looks very similar to the problems that I had when the front carb choke was sticking on..ironically also when en route to the same Tatton Show about 4 years ago. Think I mentioned before, choke linkage was bent and not pulling off, but I think that you have checked yours.

[Wagger]

I had a vacuum gauge on my Vitesse 6 in 1975 and it started wobbling frantically when an inlet valve spring broke. It had a manual describing what its capabilties were. I have an unused servo outlet on my 2500 engine and can use that. I have not done that test yet. I am trying to work out how to do it on each carb after the butterfly. Waiting for my son to arrive in four weeks with all of the carb tuning kit that he has used on Spitfires and Six cylinder models.

I will be doing more testing soon, so will keep you posted.

[Pete Lewis]

i/2 turn may seem excessive  but you want a result  so a tickle  needs to ne more a prod 

have you checked the needles are flush with the air piston and measured the jet drop with a instrument of sorts ?

if not  level it to the bridge and drop each  jet  2 full turns  and test 

if the lifting pin 1mm gives a hint of increase then its rich  needs a very light touch and you get a couple of seconds of + 50 rpm   

you need to lean the jet to get no hint of change on a lift  

its not a yank it up   all very touchy feely must be done with filters on ( any manual that says remove and lift with a screwdriver will be very wrong as soon as you fit the filters   ...that  idea is  ...Nuts 

Pete

[1969Mk3Spitfire]

Really appreciate input from all.

I removed the carb tops this morning to measure jet drop. I remember a couple of years ago setting them both to flush - 2 turns.

The front carb appeared now to be set much leaner than the rear.

I also disconnected the choke cable and there’s no doubt that there is more friction and an occasional binding in the front carb. I’ve further slackened the screw holding the lever to the bottom of the jet. I’ve also given all components a good clean.

With the front jet reset to flush - 2 turns I started the engine. By the time I’d set mixture with the pin, air filter off, I had leaned one full turn, pretty much from where I started.

I refitted the air filter for final check.

Sadly, it’s not possible to have two consecutive days of sunshine in Manchester and today it’s raining so the car is staying in the garage. Free revving, it sounds very willing.

It doesn’t seem right that the jet position on the two carbs are so different but appear to be set against the pin technique? Any views?

Unlikely to be able to do more and/or road test for a week or so as I have a sailing break with a couple of pals next week, hoping to get to Belfast, weather permitting.

 

[Badwolf]

With my choke linkage, one of the metal connector thingies was ever so slightly bent, so little that it was very difficult to see. It was only by playing around with it for some time that I noticed where it was binding and a quick squeeze with a pair of pliers straightened it and ended 30 years of grief. It was the same with the boot lid. It wouldn't 'drop close' for 30 years again, then I remembered the kink in the boot lid when I got the car (lid was replaced all those years ago) and suddenly, for no reason wondered...if someone backed the car into something to kink the boot lid, did they push in the back panel as well? Two minutes with a length of wood and a jack solved the problem. Sometimes it's the simple things..like life!!!

[1969Mk3Spitfire]

17 hours ago, Badwolf said:

With my choke linkage, one of the metal connector thingies was ever so slightly bent, so little that it was very difficult to see. It was only by playing around with it for some time that I noticed where it was binding and a quick squeeze with a pair of pliers straightened it and ended 30 years of grief. It was the same with the boot lid. It wouldn't 'drop close' for 30 years again, then I remembered the kink in the boot lid when I got the car (lid was replaced all those years ago) and suddenly, for no reason wondered...if someone backed the car into something to kink the boot lid, did they push in the back panel as well? Two minutes with a length of wood and a jack solved the problem. Sometimes it's the simple things..like life!!!

That's an interesting comment and one which I shall investigate upon my return.  Many thanks.

In general, is is "normal" to have the jet position / screw set fairly different between the two carbs to seemingly achieve the same mixture, as measured by the lift pin method? 

Fearing the wrath from some, is it worth employing my 45 year old colourtune? 

[PeteH]

The colourtune is just another "useful" tool, I have/had 2. Useful for basic setting On a Vittesse (1980`s) and the Beamer (1990`s).

Have you tried Swapping the plugs?. I had a batch of 4 for a Punto, brand new, a few years back one of which was "faulty" straight out of the box. Putting 2 into 3 (say) will confirm if the issue is the Plug or confined to that Cylinder?.

Pete

[Badwolf]

PeteH - in all the time that I have had my car I have never actually considered that one (or more) of the plugs might be faulty. Sad isn't it. Carbs, yes, plug leads, yes, rotor, yes but never thought of having a faulty plug. Who would have thought it. I stupidly considered them 'bombproof'.

[Pete Lewis]

replacing a plug is the simplest and cheapest trial 

obviously jet and needle settings between carbs can vary a little thats why they make it all adjustable 

the flush needle and jet down 2 turns and idle screwed in 1.5 turns  is a base setting and expect some adjustments from that 

to accommodate tolerances in all the parts involved 

Pete 

[Iain T]

When I had to set different front to rear carb jet heights it was due to wear. Air was ingressing on the front carb making a leaner mixture. I had them refurbed at carburettor exchange and they are now set the same and the plugs are the same colour. However it could be a simple gasket change could solve the imbalance. 

Iain 

[1969Mk3Spitfire]

The curmudgeon in me usually disapproves of people using mobile phones when at a motorway service station but I’m now on the M74 with a a revised opinion 🙂

Thanks to you all for your valued input.

I’ve had a single cylinder sooting issue for a while which now seems to have manifested itself into a misfire.

I’m 110% sure that it is not a plug issue. I’m not at all sure about pretty much everything else. I’ve measured dizzie play and it is tiny, tiny. I’ve replaced all of the ignition service items except condenser (club shop condenser was rubbish and went into the bin).

There is at least one full turn difference in jet position of the two carbs and this intuitively doesn’t seem right to me albeit both now exhibit the correct mixture setting when using the lift pin. Otherwise, the car runs and accelerates smoothly, no hint of air entering the system through excessive wear. Car has 54,000 miles only.

There could be mileage in the earlier comment from Mr Badwolf to further investigate the choke linkage. It does seem incredibly crude that the link swivels/articulates based upon a slack screw at the base of the jet. I guess that’s part of the appeal of these cars.

 

[Pete Lewis]

colour tune will give you a pictorial view of the flame 

your ears will also give you a good idea of where the best running setting is 

other things that affect mixtures is air filters, and the piston spring rate 

giving the jet a prod with the fingers to make sure its returned after choke use is mandatory with the crappy SU linkages

on Strombergs the only real snag is on CDS+ where the choke discs wear /clogg/corrode 

spark plugs are the life and soul of the engine and having a set swap is relatively cheap fix 

a nice  hic up when under load is normally a plug breaking down with a pronounce Miss

Pete

[Badwolf]

Lady BW and I will alway remember our first run out after the recommision. It's detailed here somewhere. We coughed and bounced and kangarooed all over the place. Got to a roundabout (just) with a Merc on my tail. Took the roundabout in 2nd, no power to pull out. Down to 1st, not much better. Choke out, it picked up and coughed and spluttered all the way to the pub. Plugs out..yeuk. Jet black, oily, horrid. Cleaned them up started home later, running well, then here we go again. Limped home...well at least the car has been out under it's own power for the first time in about 25 years. Just proves what the mot was worth (yes, I know!!).  Plugs out, jet black... again. Who're you gonna call...yes, this forum. Chapter and verse on carbs, plugs and ...well...everything. Now you can't get rid of me, well not unless you pay Kevin loads o'money to chuck me off. Thanks everyone...again.

[Pete Lewis]

bet the smokey Jag on the platinum procession had sooty plugs did no body push the choke in ????

Pete

[Badwolf]

Doh...er... what's a choke ...er...?????

[Wagger]

Passenger hung her handbag on it, like all our girlfriends did!

[Badwolf]

Ah, yes, I knew that it was there for a good reason. 

[Pete Lewis]

and  it may have a marmalade sandwich inside   

Pete

[PeteH]

10 hours ago, Pete Lewis said:

body push

Oh yes they did, they pushed it down the Mall, 3 blokes in Hi-Viz.😂 Marmalade is for putting on Toast, here. Mind we also have "treacle" on our Yorkshire Pudding!.

Pete

[Josef]

On 06/06/2022 at 07:20, 1969Mk3Spitfire said:

Fearing the wrath from some, is it worth employing my 45 year old colourtune? 

I like the Colourtune for an extra data point, and confidence in what my less experienced eyes and ears are telling me about how the engine is running. 

[PeteH]

Nothing wrong with colour tune. But like all "tools" including computers, they should never be regarded as "god". Big mistake made by too many in todays world. Mk1 ears, eyeballs and nasal passages are the first line of defence.

Pete