choke not working properly

[Fizzy]

this is something ive only just noticed, no idea how long its been like this, could be since ive had the car :o

when i pull the choke cable, the little 'thing' hanging from the bottom of the carb (SU) moves down, and then pops back up when the choke is pushed in. i just noticed today it only does this for one carb, the other side doesnt move, stuck in the 'up' position. however manually pulling it down, it springs back into place.

im under the bonnet because between yesterday morning and yesterday night, it went from running normally to running very rough, coughing and spluttering at anything above 2k revs, spewing soot over my boot. i didnt go out inbetween, didnt touch the car. ive replaced the plugs, and am now going to have a look at the carb linkage, as it seems to be the problem.

what effect would this have had, if any, the choke seemingly only working one carb ? i assume the linkage negates the need for twin choke cables ?

[BrendanD]

Jason,
There's a linkage between the two choke cams that indeed negates the need for two cables!
Choke on only one carb would make the car very rough indeed running on choke....

Most likely the small clamp bolt that holds the 'claw' that goes in a hole in the cam is slipping.
(part 60 on the Canley Classics Mk3 spit carbs page)
regards
brendan

[Brendan The Spit]

First off, what are the chances of being answered by two Brendans?

Secondly, I replaced my choke recently and yes, I can confirm that there should be a linkage mechanism that goes to both carbs, controlled by the one cable. If I remember tonight I'll take a pic of mine, then you can compare and see what's missing in yours.

[Fizzy]

cheers, i have worked a bit of it out.

in the canley diagram here - http://www.canleyclassics.com/?xhtml=xhtml/diagram/spitfireiiicarburettor.html&xhtmlcatalogue=xhtml/catalogue/spitfireiii.html&category=engine&xsl=diagram.xsl

the second choke wasnt engaging because the cam lever (17) had been clamped on the main link bar in the wrong place. although the back of the cam was hitting the screw, it was clamped too low for it to actuate the pick up lever (13), which somehow is what moves the bit at the bottom (jet?)

so its moving now, but it doesnt come down as far as the side that was working anyway, looks like it needs adjusting somehow im not sure. i tried turning in the choke adjustment screw that hits the back of the cam, but it didnt seem to make much difference. ive only had the link off once since ive had the car, years ago. so unless its been slowly slipping, which i doubt, its been like that at least 2-3 years ;D.

also, i can turn the carb pistons left to right on their axis is this right? also one piston seems to drop then slowly descend the last 1/4 inch or so, the other just drops all the way. i'll try and get a video up, reading my post back sounds like gibberish ;D

[Nilfisken]

For the pistons up and down movement have you checked the oil level in the dashpots? Open the small lids on the top of both carbs and see if there is any oil in the damper unit. Not sure about older SU:s but on my car the pistons can only be slightly wiggled in left right rotation since there is a cutout in the piston gripping to a ridge in the bore (unfortunately on the side of the piston lying towards the table here but you should see it at the right end of the flat end of the piston where the needle sticks out from?
Cheers

[Fizzy]

yeah sorry, i didnt put that its more of a wiggle left to right, i cant turn them round or anything crazy like that. ive fiddled a bit mroe with the choke actuating parts, and have now got to the point where they both open at the same time, as soon as the choke knob is pulled. however, i had to take up a lot of play. below you can see where i have had to set the clamps to get this perfect tandem opening, and below a picture of how far the jets move at full choke. bear in mind that the left hand carb used to have about 3mm of movement right at the end of the cable, and the right hand one no movement at all. i also pulled about an 1.5 inches of choke cable through that was 'dead cable', which was slack the knob had to take up before it even began to move the actuating mech.





in theory, ive been having my car running all this time thinking i had choke when i didnt. so unless im muddled up, my car has been running very rich to make up for the fact i had no choke ?

what i dont get though, is before i just 'fixed' it, pulling the choke still used to alter the revs, apparantly without moving the jets at the bottom. is this soley because of the cam hitting the adjusting screw at the back, and that alters revs ?

Nilfisken - yes i have oil, im going to empty and refill them so i know the level is right. im also going to check that groove in the piston,can this wear so it gets wider ?

edit - sorry another question. should the shoulder of the needle be flush with the hole it goes in ? and the hole the needle goes in, standard tune is set it flush then back off 2.5 turns ?

and finally, i've got BO needles in my car. MKIV 1300, electronic ignition, tubular manifold, stainless big bore pheonix exhaust, std filter box. no other engine mods. are these needles right ?

[lancepar]

Fizzy,
Just a bit of info regarding the choke clamps.
Usually these are the last clamps to tighten after tuning the carbs. Make sure the main jets are held right up while tightening the clamps.

8)

[DaveKent]

Fizzy, can't give you the answers you want at mo, but i have to say you are a top man! you've certainly gained some knowledge and confidence since you got that car; Well done!

[Fizzy]

thanks for the help. lancepar, to be honest just to get to the point where the choke is actually moving in tandem is a bonus.

ive just looked at the 'jet holes' or whatever they're called, and in one carb the brass sleeve is 'bevelled' slightly in towards the moving jet hole, the other is dead flat across the top surface. is this a problem ?

[thebrookster]

Fizzy, in answer to your question regarding the engine revving without moving the jets, yes, this is as you say.

You should have two screws that are for settings (sorry, badly worded I know), one is for your idle speed, the other is for your fast idle.

The fast idle should activate before the jet, so that the engine is revving faster (2500 rpm if memory serves me correctly??) before you suddenly richen the mixture. I assume that this is to help prevent the engine from stalling due to the rich mixture, I am not certain.

For the rest, I cannot help, I am still learning this stuff as well :)

[Velocita Rosso]

...and you should have a ball joint at the bottom of your vertical accelerator link bar.....

[Nilfisken]

Hi, for the jet and needles the basic setting is the following (according to most manuals I have seen and it worked for me at least). The needle base should be flush with the piston and the jet should first be set to just be flush with the bridge of the carb body (the surface that the piston rests upon when being in it's lowest position. This means that when the piston is down it just stops before it hits the top of the jet.

This setting is to lean to start the car so next step is to richen the mix up a bit by lowering the jets, since the jets have a fixed size and the needles get gradually thinner the lower down the jets are (and the higher up the needles goes) the more fuel they release to the engine since the area of the top hole in the jet increases when the needle gets thinner (hope it makes some sense...). Then to get a basic point to start your tuning you screw the jets down further from this setting about two-three whole turns of the nut that the jet enters through to get into the carb body. Two is suggested in most guides but I had to go to at least three to have it running properly before tuning further.

Also some carbs have fixed needles that needs to be aligned with the jet while the later ones have spring loaded to be able to adjust themselves a bit. Do check if your needle can be moved up a few mm into the piston and if it wiggles a bit, then it is the spring loaded type. If not I think the jets should be manually centred to the needles using the top nut above the mixture adjustment nut (someone else can probably help you out here). I have never done this but if you search the web for info or consult a Haynes manual you should find some info on that matter.

For the tuning (there are loads of guides out there, search on tuning SU carb and you will find lots and lots of them) I have had some troubles getting the clear difference in idle speed by changing jet height so I have used the spark plugs as an indicator of the mixture and it runs pretty well from that so far.

Good luck!

[junkuser]

"im under the bonnet because between yesterday morning and yesterday night, it went from running normally to running very rough, coughing and spluttering at anything above 2k revs, spewing soot over my boot. "

This sounds like the jet that the "choke" cable moved originally stuck down at that stage Fizzy giving over-rich mixture causing the soot when increasing the revs.
Not unknown for the jet not returning to correct position in SUs. There was a recent thread on this.

(Could have been caused by a float needle sticking though.)

[Fizzy]

mighth ave been, however because of all the dead length in the cable, even the jet that was 'working' was only moving a couple of mm. while i was tinkering, i also noticed one of the pistons was sticking a bit, so i swapped it out for another i had, which is a much nicer, smoother fit.

also, very oddly, one of the BO needles was getting stuck in the jet when the piston was down :-/ i had the needle in the piston so the shoulder was flush, as in the picture above. i swapped the BO's out for some 'M's that were in my spare carbs, and it solved the problem. whether M needles are any good for my car i dont know. what i do know is that the car runs hugely better after yesterdays work, i just need to reset the mix and idle now, and previously it was rich enough to unknowingly negate the need for choke, even in the depths of winter. at the moment it starts straight up and idles at 1100 rpm instantly after starting, so i suspect its well over rich.

[rossybrown]

A question and an offfer Fizzy

1. Did the tubular manifold give you more performance?  You might need to change needles to get the most from it
2. I have a Gunson Colour Tune you can borrow if you want?  I found this the best tool for setting up my Carbs.  Needs two people though (one revving the engine and one looking down the Colour Tune).  Obviously final check was a good blast and see if the plugs were nice and grey.

Ross

[rossybrown]

A question and an offfer Fizzy

1. Did the tubular manifold give you more performance?  You might need to change needles to get the most from it
2. I have a Gunson Colour Tune you can borrow if you want?  I found this the best tool for setting up my Carbs.  Needs two people though (one revving the engine and one looking down the Colour Tune).  Obviously final check was a good blast and see if the plugs were nice and grey.

Ross

[Fizzy]

to be honest, i cant really give a definitive answer on the manifold, as ive obviously only just got the carbs to the point where everything actually works ;D, though it still isnt set right. however, the car is now running better than it ever has done in my ownership (once carb set anyway). previously, the best it ran was during the first year i had it, when i had the cast manifold, and even with my wrongly set carbs, and probably wrong needles, it feels at least as good, so i would hope that once its been properly tuned it would at least be a bit better. since then, the only 'performance' upgrade to the engine has been the manifold and exhaust, so it must have done at least something positive.

thanks for the colourtune offer, i do sort of have access to one if need be. ive used it once before and i thought it was a genius little tool. im going to pick up a balancing tool aswell, and im hoping somebody can point in the direction of the correct needles for my set-up. once i have these, i will thoroughly adjust and tune the carbs.

im quite pleased, ive had running troubles for ages, and ive not been able to work out what it was. farting and spluttering carbs, flat spots and masses of soot up the back of my car. im hoping now ive done a bit of fault finding in the carbs and sorted them, im going to be enjoying the car more than ever. just need to get my overdrive in and i'll be a happy chappy. saying all that, round my mates last night he was surfing pistonheads showing me how much mk3 capri i can get for the money i'd get for my spit.........oh so tempting, saw a lovely little 2.0S, couple of grand, rot free, bit tatty on the inside, and it was white 8)

cheers

edit  - just checked minty lamb needle page, comparing the previous BO needles to the current M needles, no wonder i had bad running. i dont know what the values of each graph axis are, but assuming a higher, steeper line is better, the BO's were totally useless !! im not really sure how to use the graph to decide which needles to get though, any help appreciated. and as it doesnt allow for the different manifold/exhausts etc, not sure how relevant it would be. what should i be looking for to identify a good needle?

AAT needles follow pretty much the same line as M (what i have), but AAT has a much longer line on the graph.
AAQ is again a longer line, and most of the time is slightly above the line for M needles.

[Nilfisken]

Lower doesn't mean worse, it just shows the diameter of the needles along its length so you can see the change in fuel delivered depending on the height of the piston. Every engine needs it's own profile to work well :) Since you set the basic setting around idle speed you need to find a needle that gives you reasonable idle and then feeds more fuel in a way that suits your own engine.
Cheers

[Nilfisken]

Lower doesn't mean worse, it just shows the thickness of the needles along its length so you can see the change in fuel delivered depending on the height of the piston. Every engine needs it's own profile to work well :) Since you set the basic setting around idle speed you need to find a needle that gives you reasonable idle and then feeds more fuel in a way that suits your particular engine combination.
Cheers

[Fizzy]

right so as the BO needles line was shorter, does it mean that it was reaching its 'full open' position sooner than the needles with longer graph lines ? and underfuelling my car at high revs ?

[Nilfisken]

I have only used HS4 carbs so not sure why the line is shorter, maybe the needles are shorter since your carbs have a narrower inlet? What is the diameter of your inlet and is it HS2 or HS4? The M and BO needles start out at the basic setting so with the exact same setting they will idle just the same but the more you open up the piston the BO needles give more fuel for each given piston height (The graph for the BO needle has a higher y-value than M for each given value on the x-axis).

[rossybrown]

Hi Fizzy,

Why not ask Andrew Turner for some needle advice?  I'm sure he mentioned AAQ's to me once for a sports manifold and K&N's, so perhaps as you have standard air filters an AAP might be the ones to go for?

Ross

[Fizzy]

cheers for that. ive just been out on a decent run and when i got back in the car, after leaving it for about 15 minutes, it wouldnt start properly. it fired, but was firing very slowly, and no amount of choke or pedal would change the rev speed. eventually, after much trying and feathering the throttle, it sped up and idles at about 900rpm, then stalled after i reversed out of my space. eventually got it going again, now just letting it cool down before i go investigate. also i stopped at a junction, and when pulling away, nothing happened, other than me creeping out of the junction at a snails pace with very low revs, then after a couple of seconds it picked up.

will see what the plugs look like, and go from there i guess. tuning the carbs seems a bit of a black art to me, regardless of how many manuals i have. :( my head is hurting from all this scratching ;D

[rossybrown]

Sounds like fuel starvation/vaporisation as opposed to just tuning. Check if your fuel filters full.

The set up of my carbs went ok but took several attempts and the inlet plus exhaust are standard so BO's suit.

Have you ever rebuilt the carbs?

[Fizzy]

no not rebuilt the carbs. i was unaware the BO's were std. my fuel filter is about half full, cant say ive ever seen if totally full maybe thats just how it is ? or should it be totally full all the time ? dont think ive ever had a problem with fuel evaporating, ive got the heatshield behind them and wrap on my exhaust, whether they do anything i dont know. the only other thing i changed yesterday was replacing the middle section od bonnet/bulkhead seal, in front of the windscreen. i originally took it out to help cooling, yesterday, thinking the car was sort of fixed (not having cooling problems either), i put it back in to reduce the amount of fumes coming out the top of the bonnet. i might take it out again. i always have the window open anyway ;D

can you check if carbs are worn while they're on the car? can you even see wear with the naked eye? i imagine the tolerances are so small its a case of just assuming if you dont know the history of them rebuild them ?