setting up carbs

[Davemate]

i have removed my short trumpets along with the AAT needles,the standard needles, air filters and air box have gone back on.Since i refurbed the carbs and fitted the ram pipes it has run like a sack of shi*.Now its all back as standard it would seem to run better,it now pulls like it has never pulled before, until it hits about 3500 -4000 rpm then it seems to drop off a bit in the pulling department.
The engine is standard exept for a 4-2-1 manifold and a larger bore pipe with a cherrybomb box.The carbs realy did need a refurb as things were quite knackered, jets, spindles and the butterflies had the horrid valve thing in.

i set the mixture by setting the jet and bridge level,then i wound down 1mm (set with a verni gaugey thing)
which is what the book said (su carb book by Des hammill) then i had to wind up two flats to stop the enging revs rising when the piston was lifted about 1 mm
However i did find this
  is it something that needs sorting or not
this is the front carb, look at the jet in the bridge it sits flat with the bridge bush

this is the back carb, the bridge bush is sitting low but with the jet wound up to the bridge level, the piston will still sit
on the jet and work as it should ---correct or not

[Pete Lewis]

Dave
in essence yes the jet tube is going to work with its adjustment but you dont want the jet holder to be pocketed like that
is there a odd thickness sealing washer in there  lurking quietly

Pete

[Davemate]

I must admit I did not do anything with that bit,I just done everthing else
I'll have to whip that bit apart and sort it then

[Davemate]

With the piston and needle removed,you can see the fuel in the jet.Is there a recommended level
it should be,or is it just a case of make sure they are both the same.I currenty have one which is about 1.5 mm below the tip of the jet,with other almost level with the top.
To change it I presumably have to put a washer under the float needle valve to lower the fuel level in the float bowl.When I serviced the carbs the float level was about the same when I upturned the lids and measured the gap between lip and float,as the manual says

[junkuser]

The fuel level is set by the float level.
Should be the same in each carbi.
Do you have the instructions for setting the float level?
Is in most workshop manuals.

[Davemate]

They both have to be the same,I'll recheck them.I did think that they both were the same.
But obviously not.On other cars the hinge is just bent to adjust the Float height but mine are plastic
so do you just pack the shut off valve down a bit with washers until correct

[Pete Lewis]

the plastic floats are supposed to give you the correct float /fuel height.

trouble with su its a tight location to fit a socket /spanner on the brass holder.   and a thin shim washer under the valve  makes a load of height change

all a bit of a pain and much fiddling    Pah   you may not achieve a result.

in general the fuel is around 2mm below the top of the jet tube .  

Peter

[tengah spitfire]

One thing to watch for when adjusting the float / fuel level.
I use twin SU's on the DLM, I had to replace one of the covers.
Got it from SU, after numerous hair pulling and blue language it was sorted.

I could get all set up equaly on both but as soon as I put it all back together one carb would keep flooding
I finaly found it to be the new float chamber cap was softer material and when bolted down would alter the fuel level, so just enough torque to stop it coming loose was used.

This is with a set of MU2 carbs with the centre / top clamping bolt.

[Davemate]

i have set the foat levels the same and i have just fitted a new jet bearing kit. Both jet bearings are now  the same,nice and level with the bridge.With both foat bowl levels the same a new jet bearing,as well as the rebuild kit everthing seems wonderfull.All i have to do is set the mixture.

why is it you buy a rebuild kit and you don't get new jet bearings ?
since i rebuilt my carbs they have run a bit rough obviously the jet bearings were worn.

Having only read one chapter of the su book by des hammill i decided to get them running right as standared and then go from there.but now the car is so much quicker and smoother i cant belive what a diference it has made.
so a good tip would be when you order a rebuild kit order new jet bearings as well !
and make sure the foat bowl levels are set the same

[Davemate]

from the previous post ---all i have to do is set the mixture  ;D

i am getting a brassed off now
i have refurbed both my carbs including the jets tubes, i have set the float levels to withing about .5 mm of each other.
with the jets wound down 1 mm which is 2 full turns i thought it would run ok(ish).
With the car set as above i went on a 100 mile trip yesterday,the car ran so-so but it did not seem to pull as well as it has been,in fact tickover was quite ruff and lumpy when i got home.
I have just removed the plugs and the front 2 are really black and sooty the back 2 looked not to bad,but had a bit of build up on them,both plugs cleaned on the old faithfull wire wheel and put back in.
Useing a scewdriver to lift the pistons to set the mixture i have had to set the back one at 2 full turns down (left it alone)but the front carb i have had to wind up 1 full turn and 1 flat.Now when i lift each piston up by about 1mm useing a screwdriver the engine revs now rise a smidge then starts to drop back and runs very lumpy, so i surpose its set about right !.
Thing is it now ticksover, if set at 750 is REALLY ruff its not even, if i raise it to 1000 tickeover it better which is when i have done the mixture.
When driving it seems to be it bit lazy and if reved and held,as the throttle is released the revs drop to 1000 pause for a second then drop off in little steps until 750 is reached then the ruff tickover starts i have sprayed WD40 all over the inlet maifold and the revs do not rise

the engine is standared 1500 with a 4-2-1 manfold and a cherry bomb,standard air filters and air box and
i only use BP super fuel.
i have fitted electronic points replacment things(this was done months ago)

anyone got any sugestions ???????
would the timming being out cause any of the above it was set a bit gun ho ---set as standared then ajusted until it stopped pinking,
but i have since started to use the bp good stuff and i have not touched the timming !

[Andy Borris]

Trouble is, adjusting one thing, affects the other settings!

And it's all a bit hit and miss too, I'd adjust the timing with a light before touching the carbs again.

Where abouts are you, perhaps someone will pop over with a timing light and give you a hand?

Andy

[Davemate]

1529 wrote:

Trouble is, adjusting one thing, affects the other settings!

And it's all a bit hit and miss too, I'd adjust the timing with a light before touching the carbs again.

Where abouts are you, perhaps someone will pop over with a timing light and give you a hand?

Andy

when the rains stops ill fiddle the timming and see
i am in cheshunt herts

[Andy Borris]

I know that there's a few members near you, so perhaps on Sunday??

Andy

[piman]

Hello Dave,

you need to get your mixture correct first before worrying about the timing. If you time first then alter the mixture (I'm refering to dynamic timing by road testing and adjusting so it just doesn't pink, not book figure timing. A light is useful to determine if the advance mechanisms are working.) the timing will be wrong, as the mixture is weakened the engine requires more timing advance. (the principle on which the vacuum advance economy gain is achieved)

Alec

[piman]

Hello dave,

I just re read your no. 9 post.

There must be other problems if you have the two mixture adjusters so far apart. They should be virtually identical if all else is correct. I know you've looked at fuel level, just double check that it is 2 to 3mm below the top of the jet tube, needle shoulder flush with the piston base, pistons free in the dashpots and drop down all the way, you should hear a click when they stop.
air filters clean, and the air flow balanced ?

Alec

[Davemate]

Needles set correct,pistons free,air filters new and carbs balanced with a gunston balancer.
I'll pull both floats lids and check that they are set the same.

[piman]

Hello dave,

rapidly running out of options. :-(.

Remove the dashpot dampers and run the engine, open and shut the throttle and watch the height of the pistons, they should both rise and fall pretty much the same? You could put a couple of equal length pencils or similar in the top to get a better idea of height if you like?

Alec

[Davemate]

floats are now set almost bang on the same,the jets have had to stay the same on the back one and i have had to lower the front one by a couple of flats so the back one is 2 turns down from the the bridge and the front carb is now set at 1 turn down !
i checked the plugs before i started this morning all 4 look like this

now i have stopped for today(i have to go to work)this is how i have left it,remember the front carb is set 1 full turn apart from the back carb !
click the pic to see the video

when i put the air filters on thats when it runs lumpy so new air filters are needed so just for a couple of days i have fitted
short ram pipes and itg filter socks the tickover is smooth

[Clive]

Just a thought. Are the two float chambers the same?? I know there are different sizes......

[junkuser]

I'm wondering if the front needle has wear on it from running with jet not centered correctly in the past sometime.

[Davemate]

Junkuser i think you may be right
When I get up I will remove the piston covers and check the fuel level in the jet tubes again !
I have a pair of AAT needles I can fit and see if it makes any difference at tickover
As I need new air filters I might bite the bullet and get a pair of K&N's
I have checked that the choke is returning,which it does
I have noticed tonight when I left work that no choke at all was required to start the engine
So to me it must be running rich,as well as the black plugs in the earlier picture

Logic would be if it's running rich and the only way fuel can get out is via the jet tube and it's regulated with the needle,that must be what the problem is.
It's a new jet tube,so it can only be a knackered needle and/or the float level
I am sure the float level is ok
I might swap the needles from the front carb to the back one and see if that makes the back carb run rich,at least that would confirm the state of the needles

[piman]

Hello Dave,

yes try your other needles, the idle dimension on all needles of one group e.g. 0.090", are nearly identical.

Alec

[Davemate]

I got some copper washers from work tonight so no I can set the floats bang on.
Whilst swapping the needles over today I noticed that the back carb was overflowing fuel from the jet,which seems odd as it's the carb that is doing it's thing correctly !
Having swapped the needles around the front carb still had to be leaned off,but only by 4 flats and not 7.The car pulled better tonight so I am getting there !
Tomorrow I will use the copper washers to set the floats bang on and I will swap the standard needles for the AAT's

[thebrookster]

Dave, a quick thought if I may.

What kind of air filters are you running just now, and how well do they fit to the carbs?? Had this problem with my K&N's, the PO had used a thick gasket between the back plate and the carb, and had over-tightened the bolts resulting in the back plate buckling slightly. This meant that when I changed the gaskets for the normal thinner style there was a gap between the back plate and carb, letting air in. Not that much of an issue for mixture, however it meant I could not ever get the two carbs balanced with each other. Made a heck of a difference!! I was quite surprised in the difference when I finally sorted it out, the carbs were not that far apart however it meant 6-7mpg difference.

[Davemate]

i was useing standard paper filters and box,the filters are knackered as they are causeing lumpy tickover so for now i have fitted short ram pipes and foam sock filters(itg).
I think my main problem is worn needles(i have AAT's to fit)and float levels being out and high,i dont have time today, but tomorrow i will fit copper washers to raise the floats to exactly the same(got loads from the gararge last night)i have been useing normal steel washers and i don't think they seal properly and fuel is getting by and overfilling the bowls causing a rich mixture.
i have been trying to set them at 5 mm from the bottom most point of the float to the lip on the edge of the lid,which means they are sitting level, is that about right or should i set them higher --which means the fuel will be lower in the bowls ?


sorry clive yes both the same
real problems have only started since i "repaired" them