Spitfire Misfire

[Jonah]

Decided to take my Mk3 Spitfire for a run in the sunshine today, after a winter of much work, lighting problems, steering rack change……..and other fettling

the car was difficult to start and had a noticeable misfire, I checked the plugs which were very black and sooty, after cleaning them the car started but as I tried drive off under load it coughed and spluttered.

Could this be a mixture problem?

i checked static and dynamic timing which seem fine 

thanks Julian 

 

 

[Pete Lewis]

a too slow "fast idle"  will soot the plugs very quickly needs to be 1500 and more on full choke 

do make sure the jets pop back to  the adjusting nut after using the choke 

a quick prod with a finger works to check jet sticking 

and dont use plugs with an R in the suffix 

Pete

[Jonah]

Thanks Pete, I’ll check tomorrow and will report back 

[Jonah]

At fast idle with the choke out it revs at 2,000, when warm with the choke in its circa 850

the jets appear to be free and the plugs don’t have an R in the suffix

Over the last 18 months the car has moved very little, the last time fresh fuel was added was early 2023, is it possible the problem is due to fuel that has become stale?

Would it be a good idea to add fresh fuel to the tank?

[Iain T]

14 minutes ago, Jonah said:

Over the last 18 months the car has moved very little, the last time fresh fuel was added was early 2023, is it possible the problem is due to fuel that has become stale?

Could be stale fuel, do you use E5 or E10? The more ethanol the more moisture it attracts.

Iain 

[Jonah]

I use 3 E5 with additive

 

[Pete Lewis]

additives are really not needed there is years of lead memory in the head and valve seats 

i would save and keep your wallet closed , using E5 is the best move 

if you grind or reface the seats then some memory can be lost 

over the past 20 odd years I have never experienced stale fuel even when left over winter with a odd start up every couple of months . the fuel pump will need priming to refill the float chambers but always started on the button 

Esso 97+ does not have any E  added if that helps 

I have always used sainsbugs 97  E5 always works for me 

Pete

[Iain T]

Needles sticking and need centralising? With the damper plunger removed does the air piston drop easily with a clonk on the bridge?

Are both needles adjusted the same?

Iain 

Edited March 5 by Iain T

[Pete Lewis]

it can be down to the plugs   many made these days do not have the ceramics glazed as fuel injection will not over fuel 

the unglazed insulation gets sooted on choke and it never recovers well even when cleaned the contaminant shorts the spark .

maybe a good blast with a blow lam may help this (not proven)   some makes do glaze Bosch had some but its getting a rare find these days

on many this doesnt give a problem but some cars suffer badly with plug fouling misfires   just some ramblings 

Pete

 

[Jonah]

both pistons do fall with clunk but with engine running when I lift one piston the revs drop but the other one makes no difference

ill also try new plugs

[Pete Lewis]

if raising the other piston does nothing then something is very wrong

thats if you give the piston a good heave up ,do they both sit level when idling ???

the trick is to be very touchy feely, and raise the piston lightly just a couple of mm  

if rich the idle should give a hint of a rise

if lean it will give the hint of a drop

if nothing changes  its about spot on

again these are smal changes you need to be quick at recognising , no big lifts or dramatic changes 

so when you say you lifted .....how far ????

and if setting mixtures you need the filters on and use the lifting pins , with filters off you are proving nothing 

Pete

 

[Jonah]

I used the lifting pins to raise the pistons right up 

 there is a small amount of oil in the inner damper pot which resists when screwing the damper in, the manual says fill the dash pots to within 1/2 inch

[Iain T]

29 minutes ago, Jonah said:

used the lifting pins to raise the pistons right up

Only lift the piston 2mm or so. As I said are you sure both carbs are set the same including the butterflies. As Pete said on tick over check the air pistons are the same height ie gap between the piston and bridge.

Good luck

Edited March 5 by Iain T

[Jonah]

Thanks Iain, I’m not sure if the carbs are set the same, I had assumed they were, but now…..

I think I’ll spend the evening reading the Haynes manual on carbs

[Jonah]

Meant to say that there is a noticeable misfire/cough when I increase revs, timing using a strobe seems fine.

ill put new plugs in tomorrow and as Pete suggested I’ll try some gentle tickling 

[Iain T]

2 hours ago, Jonah said:

Meant to say that there is a noticeable misfire/cough when I increase revs, timing using a strobe seems fine.

When you increase the revs can you see the timing mark moving and giving more advance? 

 

[Jonah]

Yes it does move giving more advance 

[Jonah]

Spark Plugs were the route of my problem, thanks everyone for your help

i managed to get some Accuspark plugs that were supposed to be an equivalent to the N9Y plug…..but they didn’t work at all and made the engine very unhappy, got some N9Y’s today and all is good!

the engine was also running a little rich

i have one final question for you, I fitted a new rocker cover gasket but oil still seeps out….any suggestions?

thanks Julian 

[Pete Lewis]

on orig metal covers check the cover gasket face is quite flat , its easy for the gasket to roll out of place from the groove it sits in .   only solution is to apply sealer and  tape it in place till it sets

after all the years they do get distorted especially the top where over tightening has deformed the cover,  easy to knock out on the bench .

if you have an alloy cover then you need a flat neoprene lifetime gasket 

Pete

 

[Jonah]

Thanks Pete 

[Pete Lewis]

did the accuspark plugs have an R in suffix ??

have to say obscure look a likes from where ever are best avoided  

there can be enough troubles with tried and tested using a china copy is asking for headaches 

our low HT will always struggle with any Resistive plugs , 

do you still have std plugs or some electronic unit ???

every classic forum  ( all marques)  has poor accuspark reports 

they look the part but can be worse than troublesome 

Pete

Edited March 7 by Pete Lewis

[Jonah]

They are called ‘AC9C non resistor spark plug’ I looked in the Accuspark website and they state it’s an equivalent to the N9Y…….as you say Pete buyer beware

i I had an old set of standard plugs which fired up on first turn 

[Iain T]

55 minutes ago, Jonah said:

I had an old set of standard plugs which fired up on first turn 

Time for an Italian tune up🏎️ to clear it's throat!

[Jonah]

Cracking idea Iain!

[Pete Lewis]

i fear i keep coming across all sorts of problems locally and on here with this make of electricals 

off to test one  on saturday....yes its got a acus dizzy fitted only fires on 3 of the 6 

dial up problems and google shows all marques have problems seems it is a std feature .

if they work thats  fine  a good few have good reliability  but if they dont you join the queue of  headaches

the snag is if you keep sorting out problems you never see the good ones 

Pete

Edited March 8 by Pete Lewis