Jump to content

MK3 Spitfire SU Carbs, Tuning and other oddity


Recommended Posts

Hi all


I had this conversation years ago on the old message board but as it is no longer here it will refresh my memory and hopefully help others.


Trying to set the mixture on my carbs using two methods, 1) Gunson Colortune 2) Lifting Pins.


If I set the carbs via the Color Tune then the lifting pin test fails with an indication of a very weak mixture.


If I set the carbs via the lifting pin to obtain the right engine response then the colour tune fails showing an overly rich mixture.


The two methods do not give an equal result.


No air leaks on manifold, professionally rebuilt carbs, engine only done 4000 miles.


On the road the car ticks over fine with a regular exhaust note, with a single missed beat every 20 seconds or so (electrical maybe)


Driving the car with the colortune setup the car hesitates on light throttle, but as revs rise and heavier throttle its clear and pulls fine.



Just stripped the carbs down and only noticeable things are;


Rear carb piston doesn't drop exactly the same as front, slight hesitation at bottom of travel ( jet alignment? )


Also, examining both pistons, there is a little plastic stop on the underside which keeps the piston raises slightly above the bridge.

One piston this protrudes by approximately half a millimetre, the other piston it is barely above the surface.


I guess these heights must be the same to achieve equal tick over fuel/air mix.


I have looked online at parts catalogues and none seem to list this part or even confirm its existence.


Any thoughts?





Link to comment
Share on other sites

Well I have again today gone through a complete tuning session with the same results.


Setup with colour tune, car dies when doing lifting pin test.


Setup with lifting pin, colour tune is full yellow on tick over and rich.


Re-setup with colour tune and went about a flat richer, took the car out and its a bit kangaroo between 1200 - 2200 rpm then seem ok.


The little plastic stop on the underside of the piston is not listed on any parts catalogue, however I saw one reference to it on a MG forum with someone asking a similar question. Apparently if the plastic thing is missing there is little airflow at tick over and the piston may have trouble lifting at takeoff due to lack of airflow.


Anyway, I dug out the old stop, measured the one from my other carb and made a new one from nylon which now gives the same height as the other carb.

If its the correct height who knows as no information can be found, but at least they are now a pair and the piston is not bottoming out on the bridge.


Next step tomorrow will be to put some fresh fuel in the tank. The current fuel is probably 4 months old but it won't hurt to add some fresh.


So still getting nowhere fast, need the car running sweet for the 19th April as its the New Forest Run down here in the South.



Link to comment
Share on other sites

at idle the piston will be a few mm up, the stop is so the throat is not closed off or you wont start her.

when lifting the pin were the cleaners 'on' ?

you only have to gently feely lift the pin to touch the piston and raise it a couple of mm,  listen for a hint of change down 50rpm weak  up 50rpm rich this only happens for a few seconds , not looking for any dramatic change

make sure all gaskets are not blocking the front face ports 

are the jets f really fully returning to the adjusting nuts after choke is used ?


just some ramblings while the suns not out 



Link to comment
Share on other sites

Hi Pete,


Many thanks for the reply.


Yes at idle the pistons are floating about half a millimetre off of the stops.


Air cleaners were off as per instructions for either method of tuning.


Yes I only lifted the piston about half to 1mm to hear the change.


No blockages with any of the gaskets.


Chokes are fully returning.


When adjusting mixture I always keep my finger on the jet to make sure it is following the nut up when going leaner.


I just can understand the total difference in result from one test to the other.



Link to comment
Share on other sites

Hi Pete,


Re-tuned the carbs again this morning with lifting pins (air filters were off as per manuals) and the car is running a bit better.


No noticable difference on the road with or without filters (standard fitting), but better than yesterday.


Idle exhaust beat seems pretty even now with no real splashy sound.


I think I have the carbs about as good as they are going to get via the pin method, need to run the car and check spark plug colours really.


I have the timing set to 6' BTDC with the engine running (dynamic).


Could this be a cause of the low down hesitation?

The car is not misfiring, just kangaroo at lower revs and medium throttle opening.

Even noticed when slowing for a junction I get the Kangaroo effect off throttle.

Tickover is even.


Cant remember if I need to advance or retard ignition for unleaded, think its retard, but to what I do not know.


As it stands at the moment, in high gear low revs and foot to the floor there is no pinking.



Link to comment
Share on other sites

Well another afternoon with no success.


Re-tuned the carbs with the filters on and using lifting pins.


Checked with Colortune and wasn't really that far off.


Carbs balanced and jets fully home.


Car still runs like a pig on the road but standing still its fine.


Re-checked timing and re-set to 6" BTDC Dynamic with vac disconnected and plugged.


Checked crankcase breather mechanism and diaphragm, all ok.


Changed out the coil from a unbranded to a Lucas.


Changed Rotor arm to an older one I had branded Lucas but for a Delco dizzy.


The car idles pretty good and revs cleanly with no hesitation.


Get it on the road and it's useless.


Just crack the throttle under load and you are ok, give it more and its like a kangaroo.

Give it full pedal and it appears to pull fine when it goes through the kangaroo stage.


Getting down to last resort methods now.

If no other suggestions come along I will rip out the Aldon electronic ignition and go back to points and see what happens.


Run out of things to check and adjust, oh and put fresh fuel in today.


Engine is fully standard with no upgrades, carbs are running standard BO needles.


Ho Hum.

Link to comment
Share on other sites

Rich, where are you?


I have a Gunson Gastester which you're more than welcome to borrow if you're anywhere near North Devon. You could also pop down your local MoT station to see if they'll let you pop the car on their gas analyser, at least you know their equipment will be accurate.


I have used a combination of the Gunson ColourTune and Gastester and generally find that if you set with the ColourTune then it's too rich when looking at the Gastester. Mine has been a bit of a headache, but then nothing is standard so I have a whole heap of other variables.


Alternatively find a local Rolling Road, then you'll be able to see what's happening at more than just tick-over and revs at no load.

Link to comment
Share on other sites

I gues this is spitfire ?

if you have a aldon have you bypassed the ballast coil feed as elec units wont work well at 6-8 volt

you need a 12v supply to the aldon or bypass competely and fit a 3 ohm coil


set the timing back to 10deg and run 97ron


Timing will be different fitting points must re time when you exchange the two methods


make sure your plugs dont have an R in the suffix this will misfire as they need much more HT than we have



Link to comment
Share on other sites

Hi all,


Darren, I am in South Dorset, so a bit far to go. However I have a garage about ten miles away who is good with old cars (into mini's and old D-type replicas). thanks anyway.


Pete, Did not bypass anything as I don't have a resistor. Its a Mk3 Spitfire 1968 with standard 12v coil, don't know regards ohms just the bought correct one for the car.


Fitted the Aldon a few years back and been relatively sweet running.


When the car was running good it kept doing it, but I have always had periods of missing/hesitation all its life.


I changed the plugs out the other day from Resistor ones to NGK BP6ES but the car still runs exactly the same with no change.


I have checked everything about 4 times now, retuned about the same amount of times. no change.


I cannot think its a manifold air leak as I had one years back when the emissions diaphragm went and it was totally undrivable at all rev ranges, this seem to be just low down.


You asked about carb springs, as far as I can remember they have Red coloured ends.


Still cannot work out the cause of the kangaroo symptoms.



Link to comment
Share on other sites

Hi Darren,


Im not ruling anything out at the moment, however I did look in the float chambers the other day and plenty of fuel in them and no visible debris (I do have an inline filter).


My pump is about two years old and has a priming lever. If the car has been stood for a week or so pumping the lever fills the carbs as you can hear the fuel going in up until the floats cut off supply.


So I don't think it is supply related, especially as when I get through the kangaroo phase the engine revs clean at high revs with no hesitation at full throttle.


Thought it might be my air filters clogged, but running the car without them gave the same result.


Its so strange. When you pull away you put on some revs and its clean, take off and about the time you get the clutch fully engaged its kangaroo time. If you can get it past that it seem ok.


Also as said earlier if you have about 3000rpm in say 3rd gear, then down shift to second approaching a junction, as soon as the revs start down to near 1600-1200 ish you can feel the kangaroo effect during off throttle deceleration.



Link to comment
Share on other sites

make sure there's no floating debris ,ends up blocking the back of the float needle,  generally slivers of rubber off the hoses .??


dizzy plate earth wire is sound ??


the R plugs may have cooked the Coil 


is coil neg-  to dizzy and Pos+ to ignition ?




Link to comment
Share on other sites

Hi again,


Pete, will check the float chambers again blow the jets through.

Checked the dizzy earth wire whilst I was in there the other day.

Ran the R plugs of a whole season two years ago and never had a single problem, only done 50 miles with the car last year, but kept it run upon the garage every other week.

Changing the coil yesterday to a new Lucas one with the non-R plugs made no difference.

Checked all wiring polarity and all correct. Nothing has been changed around.


Darren, I have two rotor arms, one with lucas printed on it and another I cannot remember the brand.

Both are riveted arms.

Swapped both over, no change, swapped between the two dizzy caps I have and no change.


Did notice that the coil I swapped out yesterday was extremely hot after driving the car round the block.


Does the earth strap do anything on electronic ignition in the dizzy, as the module has one lead going to the coil + and the other to coil - ? i can understand it when points are employed.


Checked the vacuum advance unit tonight by sucking on a foul tasting tube, and yes it moves so the pipe is clear and the unit is functioning.


The only other thing I can try is to give the Aldon module a more direct 12v feed as suggested in the literature that comes with it, instead of coming off of the coil's +ve terminal maybe.



Link to comment
Share on other sites



All the black rotors with the rivets are rubbish, even the ones with Lucas on. Go to Distributor Doctor and order one of those, they are the only ones on the market that are reliable.


When I had the Aldon on my Dizzy I removed the original earth as it's no longer needed. I always used the coil connections for mine and never had an issue.



Link to comment
Share on other sites

Hi Darren,


I have heard a lot about these red rotor arms and have checked out the link you have provided, thanks.


But the Spitfire Mk3 has a Delco distributor, and the rotor arm has the springy contact on top to match the caps fixed centre contact.


So I don't think that site can help me in this instance.



Link to comment
Share on other sites

delco dont suffer the same problems as the riveted mick mouse lucas ones,    the rivet is too long and grounds the ht 


club shop also sell red ones 


do check the dizzy spindle inst waving around with a worn bush ,but aldon normally copes with this .


with pipe off carb pump out some fuel into a paper towel.


the rubber bits that get cut off float back and forth , when demand comes they block the back of the needle , persistent little swines 



have you done a compression check..


     how about a loose valve guide   ????   now into extreme dreams 



Link to comment
Share on other sites

Well, an update on the saga.


This afternoon I removed the inline disposable fuel filter (about 2 years old) and replaced it with a plain piece of fuel pipe.

Theory being it might be restricting flow and as I do not have another to hand I'll leave it out for the moment.



Took the fuel feed from the pump off of the first carb and using the primer leaver pumped fuel through, proving the pump works fine and flushing any potential dirt. None visible.


Stripped both carbs down. Checked floats, valves, seats and blew them through. Blew through main jets on both carbs. Float bowls had a little black dirt right in the bottom but nothing really to write about.


Re-assembled, primed carbs and started car.


Fired first time and when warmed up checked mixture with lifting pins.

Re-fited air filters (new ones coming next week from TSSC shop) and re tuned with lifting pins again.


Took the car out on the road and it seemed a lot better but there was still a little kangaroo when pulling out of junctions mainly in 1st and a little in 2nd.


Top gear low revs then foot to the floor, no hesitation / misfire and pulls ok ish.


Power does seem to be down though.


Back in the garage, checked timing with strobe and it was at 6' BTDC.


Moved it to about 12' BTDC just for the hell of it and took the car out again.


No change to kangaroo or performance really.


After returning to the garage again I noticed that the ignition coil was boiling hot to the touch.


Is this normal for about 10-15 mins driving?


And if not what would cause this?


Im using non R rated plugs with the Aldon Ignitor ignition.


Do I need an Aldon flame Thrower Coil for use with electronic ignition?


Just thought as well; What timing should I be running for supermarket mower fuel?


Is there a guide or peoples experience on what deg BTDC to set.



Link to comment
Share on other sites

coils do get hot they produce a lot of energy ,some dissipated as heat,   should be hot but not scorching or egg frying


if you have a meter the  primary should be 3 ohms. for 12v feeds 


are you sure there is nothing about the throttle cable which is taut and engine movement is reacting on the throttle opening

espec as this occurs pulling away etc.


make really  sure nothing is behind the float  needle body  (not the fueling jet)


test a rotor  hold ht king lead 10mm above rotor .get it cranked with ign on, if spark jumps to rotor its failed , if nothing its fine 


to get the designed performance you need as close to 100+ octane and book  timing  around 10 static   29-32 dynamic at 3000


if on mower fuel then just retard till pinking stops, but expect lower power and sooty plugs 


vac unit assist fuel consumption on light throttles eg criusing , has no efffect of performance, but on low grade fuel may add too much and get pinking on 95ron  when opening taps  at 50mph



Link to comment
Share on other sites

Ok, here are todays developments.


Checked Rotor Arm for shorting/breakdown - OK

Dizzy Cap - OK

Checked all plug leads and main Lead resistance, all very close so - OK

I have 3 coils, all measure 3ohm so put a Lucas one on - OK


Read on internet about Aldon requiring a good earth, even adding a dedicated earth to the -ve on the Coil.

So I decided to rig up a separate earth cabe from the screw where the flexible dizzy plate earth strap connects to the dizzy body on to the car body to give a better earth than relying on the dizzy to block.


Started up and with air filters on, used lifting pins to set mixture.

Carbs were quite close but got each one to give just the slight raise in RPM.


Took the car out on a 6 mile blast and it ran like a dream.


Back to the garage, took the plugs out to see how they looked and they were as black as black can be.


Put the colortune on and the rear carb needed seven flats to take out the yellow, the front required about two.


Back out on the rad and now the kangaroo is back when pulling away.


So lifting pins make the car run nice but overly rich and colortune makes the car to lean and crap to drive but ticks over ok.


Also with colortune if I snap open the throttle the burn totally disappears, the engine stumbles then picks up so to lean when showing correct colour burn.


Something is not right, either I got a major vacuum leak somewhere that I cannot find or I don't know what the problem is.


I checked the inlet manifold bolts and carb to manifold bolts, on some nuts I put another flats worth of turn on them, but it made no difference.


Another thing I read online is that with the inlet manifold on the spitfire, where there is a cross tube between both inlet tracts, it can mess up the mixture settings as you can end up with one carb over rich, the other over lean so feeding on each other but still get a false positive reading on mixture.

Link to comment
Share on other sites

So now you need two clolour tune or just use ears and senses to judge what improves or worsens


yes a balance tube is part of the manifold, has more effect at idle than full bore


can you measure the depth of the jet, assuming needles are all flush with the piston then the jets should both need the same depth setting and put the colour tune back in its cupboard

did you do a compression check, throttles open plugs out ??



Link to comment
Share on other sites

Hi Pete,


Used to have two colortunes, sold one years ago  :(


Will try to source another it will be handy.


Not visibly checked jet heights but will do tomorrow night if I have time.


Needles are both flush with the piston base, they are the correct ones i.e. BO 


Not done a compression check as I need to source a gauge (another thing on the shopping list)

Hopefully its not compression as the engine only done 4000 miles, but then again, anything can happen.



Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Create New...