Spitfire Mk3 carb dilemma – to change or not?

[Graham Ness]

My Spitfire Mk3 has been difficult to start recently, and the limited movement available on the choke cable indicated something probably wrong with the carbs.  On inspection I found that the jets were not moving when the choke cable is pulled.  Assuming that the jet bearings were simply gummed up with old evaporated fuel, I removed the carbs and the jets and cleaned the whole lot up.  On reassembly I find that the jets are still jamming slightly in their bearings and not returning to the fully up position.  A quick push with a finger makes them return, but it is a bit inconvenient.   I’ve also noticed in the past that a previous owner has removed some metal from the front (air filter side) of both carb pistons.  Don’t ask me why; perhaps they thought it might improve performance.  See the attached photo..

 

So, to my dilemma.  I have a pair of old AUD441 carbs that came to me in a box of bits a few years ago.  Research indicates that these probably came from a Mk4 Spitfire.  Should I invest time and money in rebuilding the butchered AUD257 carbs from my Mk3 with new jets, bearings and needles; or should I simply fit the AUD441 carbs and see what happens?  The only change I appear to have to make is the block the engine breather holes in the AUD441 carbs.

 

Alternatively, since the jets seem to move very smoothly in their bearings on the AUD441 carbs, could (should) I remove the jets and bearings from the AUD441 carbs and fit them to the AUD257 carbs.

It should be noted that the engine seemed to run OK in the past with the original AUD257 carbs and their doctored pistons, though I sometimes wonder how well it would run on a properly set-up set of carbs.

 

Thanks in advance for any advice..

Graham

[Pete Lewis]

if SU thought that relief was a good idea to improve fuel atomisation they would have done it themselves

 

the idea that improving the flow under the air piston makes things better is somewhat mis guided and will probably disrupt the draw and atomisation needed from around the needle ,   .   

 

its a some love it some hate it idea , 

 

sticking chokes is always popping up as a problem with many SU,  some have the jet supply pipe a few mm too long ,  some have stuck jet tubes , some get the meccano levers all bent and binding, some have seized  choke cables ,  

 

generally a good clean , a dab of grease  and some realignments make for a happy choke 

 

what you do with the pistons is down to preference but I would ditch them and use the   'as  designed'  set .

 

Pete

[Graham Ness]

Hi Pete,

thanks for your comments - and I can't fault any of them.

But what I was hoping for was some more sage advice from those with fuelling experience about the pros and cons of sticking with the original AUD257 'Fixed' metering needle carbs, as opposed to fitting the later AUD441 Swing 'Biased' needle carbs.  I guess it can be argued that SU considered the later Swing needle carbs as a development/improvement, but what views do other members have on the benefits - if there are any?  I have a pair of '257' carbs with modified/butchered pistons.  I could replace the pistons at whatever price that might be; or simply swap the '257' carbs for the pair of apparently good '441' carbs that I have.  I can't believe that I can simply swap the pistons from the fixed metering carbs with replacement pistons, can I?

Swapping the carb pair seems like an easy solution, but are the later carbs any better?

 

Incidentally, cleaning the jets and their bearings has largely addressed the 'sticking choke' problem.

 

Graham

[Pete Lewis]

the main  advantage of the biased needle is it atomises fuel better from around the frontal needle area where as a fixed needle draws fuel from around the back of the needle which was not as good for emission or economy.

 

I dont know of any reason why the pistons wont interchange , other than a biased needle  does not need the jet centralising which is more critical on a fixed needle   if they fit and slide ok and the vacumm chambers are the same bores theres no stopping a swap over 

 

Pete

[Graham Ness]

Hi Pete

thanks once again.  That was the sort of advice I was looking for.  I'll try some experimentation and see if the car's performance is transformed..

I simply wasn't sure if the pistons were exchangeable; but I'll try (in due course) and see what happens.

 

Graham