Setting the carburettor float levels

[Robert 352]

I have checked Jonners Wiki on tuning carburettors and I have read the “Bible” according to Jereon (various comments elsewhere and e-mails to me) and I have reviewed the older thread “How to Build & Power Tune SU Carburetors by Des Hammill” viewtopic.php?f=4&t=8037 but nowhere can I find someone who can tell me what the float level should be set at.

Yes I realise that the float level will need to be adjusted to get the level sitting correctly in the jet but there must be a nominated starting point which is roughly correct. Then I presume you add or remove washers under the float needle assembly to adjust or fine tune the level.

I have purchased a couple of new float needle assemblies but I fear they are quite wrong for in their shut off position the float is nearly touching the cap and is sitting much higher that what is shown in this picture.

20130906-4347Ptw Sprint float valve assembly.jpg (92.74 KiB) Viewed 5058 times

Jonners, Jeroen can either of you two, or is there somebody else out there who can, offer a suggestion.

Thanks, in anticipation

Robert

[MIG Wielder]

Hi Robert, I've had a look through the Sprint and 1850 workshop manuals and there is no mention of setting the float height so I reckon it must be "fixed by design" as they say. But I also have an Autobooks 1500 manual here and that describes float level setting on the HS4 by using a 3.0 to 5.0 mm drill shank across the centre of the cover in the inverted state and bending the float at the hinged point until it the float body just touches the drill shank.
The early MGBs have the setting given as 1/8 in for their HS4 s.
HTH
Is that right Jon ?
Tony.

[Jon Tilson]

The float level on later SU's HS type carbs isn't adjustable....
Anything with the plastic float with the raised moulding on it basically.

All you can do is raise/lower the float valve by sticking washers under it. If it aint right with just one or maybe 2 washers....mine all have none
then you have the wrong float or valve.

Jeroen made some interesting points with his slopey diagram, but I've always managed to get any slant running reasonably without altering float levels.

The crucial thing is to have unblocked jets, float valves that work and no air leaks on the pumping side so the pump can actually deliver more fuel than
the engine can use, so the float valve actually has to work, and an unblocked pump filter.

Then tune each carb and be sure its working individually. Not much more I can add....except most later slants seem to be set to run too lean at the top.
ERO works well on stock early TR7 HS6's and standard 1850 exhaust plumbing and is about the only one Ive looked at recently that isn't lean at the top.
HOH was also good as well, but havent seen it for a while....Jeffers could you tell us what needles are in HOH if you see this?

Jonners

[soe8m]

All Su's have the fuel level about 7-8mm below the venturi bridge. You can easily check by pulling the jet down and the fuel must be flush. Make sure you have a float needle holder with a groove in it. That is the bigger type and must be in a sprint.

Jeroen

[geeksteve]

All Su's have the fuel level about 7-8mm below the venturi bridge. You can easily check by pulling the jet down and the fuel must be flush. Make sure you have a float needle holder with a groove in it. That is the bigger type and must be in a sprint.

If I pull the choke all the way out on one of mine it sits a little above the end of the jet... I try not to use full choke as I'm pretty sure that'll result in petrol finding it's way out!

S

[soe8m]

All Su's have the fuel level about 7-8mm below the venturi bridge. You can easily check by pulling the jet down and the fuel must be flush. Make sure you have a float needle holder with a groove in it. That is the bigger type and must be in a sprint.

If I pull the choke all the way out on one of mine it sits a little above the end of the jet... I try not to use full choke as I'm pretty sure that'll result in petrol finding it's way out!

S

That should be about ok. You can pull the jets down and measure with a caliper and set to 7mm and then compare the fuel level. I think the jets go 9-10mm when full choke so slightly above is ok.

Jeroen

[geeksteve]

That should be about ok. You can pull the jets down and measure with a caliper and set to 7mm and then compare the fuel level. I think the jets go 9-10mm when full choke so slightly above is ok.

Ah that's good to know. Petrol must be getting out somewhere else then

Cheers.

Steve

[dursley92]

Moss have quite a good video on their YouTube channel about floats and setting levels.
Might be worth a look.

[straylight]

Thanks Russ, useful videos.

http://www.youtube.com/watch?v=Z6Le_HkrkKQ

I'm going to go out on a limb here and suggest that the level of the fuel in the float chamber isn't that critical. Not because I've just seen it described as "close enough is good enough" in the video above, but because I think a few mm of gravity feed petrol into the carb is so small compared to the suction of fuel. Even at idle, the carb is generating quite manifold vacuum and drawing the fuel from the float chamber in. I don't think the float level is critical to tuning the carbs, once it is high enough, it is fine.

This is supported by the comments above on where the level sits in relation to the needle, only on full "choke" does the level of the fuel in the float go above the level of the venturi bridge, normal running means the fuel is drawn into the low pressure chamber rather than gravity fed. A 5mm head of gravity pressure is enough to create a dribble, hardly enough to supply an engine with enough fuel to pull 5k.

I think the only problem with the float valve settings is as Jonners says. It is more important that the float valve is doing its job and stopping the float chamber overfilling (when fuel then exits out of the overflow pipe on top of the float chamber lid to find its way, in Oz cars, into the carbon anti pollution filter). It can only do this if the pump/fuel filter/fuel lines are clear and working well.

If the float chamber level is set too high, on standing the excess fuel will be gravity fed and bleed out of the float chamber into the venturi. Makes starting a bit harder, as the "choke" has to wait until the float chamber is full again before it gets its dribble.

On the sprints, the rear float does sit lower than the front, but they aren't interconnected except by the supply line. The rear float shuts off when the chamber is full in relation to the rear carb, then the front shuts off when it is full, in relation to the front carb. The front float chamber can't fill up the rear chamber.

As I said, out on a limb here and the number of times I've stuck my oar in and been wrong is embarrassing, but I'm just not getting the whole float chamber height being responsible for performance tuning these engines, other than allowing a gravity dribble when under full choke.

stu

[soe8m]

I'm not going into this all again but it's importand to have the fuel level in the jet at both carbs the same so the engine can suck the same rate of fuel, especially at idle and starting. Otherwise it is not possible to fine tune the carbs properly. Setting the float hights by "the book" does cause a too high in the front carb and too low in the rear carb.

If you adjust the co at idle the carbs should have about the same jet heights. If not you get in trouble when cruising or high speeding that front or rear are too off with eachother. A difference in fuel level in jets there's a difference of easyness of sucking the fuel out at idle and hot starting. If that is not ok one jet is higher or lower at idle when adjusted the mixture and there's your trouble when starting to take off and drive.

Jeroen

[straylight]

thanks Jeroen,

but,

the rear float chamber is attached to the rear carb and fixed relative to the jet in the rear carb.
the front float chamber is attached to the front carb and fixed relative to the jet in the front carb

so why wouldn't the float valves be set to the same height relative to each chamber, so that they end up the same relative height to the jet in the respective carb ?

and just for reference, 12mm of water creates a pressure of 0.01804 psi. the inlet vacuum on an SU carb at idle is more than 8psi. The effects are almost 3 orders of magnitude apart. The extra ease of sucking in the fuel due to 0.018 psi extra pressure is negligible.

stu

[Jon Tilson]

Its a tough call is this but I tend to think the jet level of fuel is quite important but it isnt about the differential in gravity fed pressures, which
as Stu has said is about 3 orders of magnitude lower in significance to the manifold vacuum.

Its about how easy the fuel can vapourise, and the nearer the surface to the passing air flow, the more gets to vapourise. Its also of course down to the size
of the hole through which fuel passes, controlled by the needle. Dropping the jet for cold starting mostly just means more fuel gets through a bigger hole as the jet hole drops down a tapered needle.

When the level is too high, as happens when the float valve doenst shut off if blocked by dirt, the engine will always run far too rich. The level is too high and much more fuel can evaporate, even if it doesnt get as high as the venturi bridge.

On a Sprint engine it sits at a slight angle so the front is higher than the back. Jeroen drew some nice exaggerated diagrams to illustrate this and the science and practise looks sound. But the angle is very slight, and the difference caused by it cannot be great, probably sub 1mm.
I would be more inclined to make the engine level by sticking spacers under the gearbox mounting, and our cars dont all suddenly run out of tune when going up hills commonly of far greater angle than the slope of the motor. I often go up a long 1 in 20....the engine sits at nothing like that.

I have never found this difference great enough to upset the ability to nicely tune any slant engine. But then again I dont have a rolling road or exhaust gas analayser. Its just purely on feel, how the car goes etc.

I think it could be worth fiddling about with float valves and thin washers, but I have never needed to. Jet needle changes have always been enough to get mine
near enough...

Jonners

[soe8m]

Found it:

viewtopic.php?f=4&t=20838

Jeroen

[Jon Tilson]

So given the tolerance is 6 - 8 mm below the bridge....

what angle does the engine have to be at to allow a difference greater than 2mm in level front to back carb?

I'll do the trig maths myself....but you can all try it too...

I'll stick my android phone spirit level on the Sprint when I get the chance....

Jonners

[straylight]

Thanks Jeroen,

actually not such an exaggerated diagram really Jonners.

The angle of the engine is about 6 degrees, 1 in 10. Measured top of dashpot to top of dashpot, 22mm in 220mm.

the centre of the carb is 65mm from the centre of the float chamber. Since the rear carb has the float chamber "downhill", it will be 6.5mm lower at the position in the jet than the level in the float chamber. Likewise, the front jet level will be 6.5mm higher at the jet than in the carb. These add up to a difference in jet level of 13mm.

This isn't caused by the front carb being higher than the rear. It is caused because the float chambers of the front and rear being "uphill" and "downhill" of their respective carbs and at an angle. If the rear float is set to the same position as the front, it could be placed underneath the front carb and still work if both carbs were horizontal, but they are not.

I'll stick by my comment that 13mm of head pressure makes little difference compared to 8 psi of vacuum, but I'm not sure about the effect on starting. I am wondering though, if the SU float chamber designers took such care in ensuring the level in the jet was so precise (within mm) why would they tolerate 13mm difference in levels ? Maybe once the engine starts cranking and the suction process starts, it makes little difference ?

The front float would need to be set 6.5mm lower than the mid level, the rear 6.5mm higher. Seems like a lot. Certainly a lot of washers to fix something that is not apparently designed to be adjustable.

Thanks for the diagram Jeroen !

stu