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SU HS2 carbs and fuel/float level

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Poundingsand Avatar
Poundingsand Silver Member Peer Ebbighausen
Los Angeles, CA, USA   USA
I know this has been discussed multiple times in the past, but in all my searching I am unable to find definitive answers, or the information seems to contradict.

Some literature (or comments) say the fuel level in relation to the jets is critical…Others say the fuel levels are only critical to each other.

And what are the ramifications if the level is too low or too high, beyond the obvious (flooding and/or overflowing if too high)? Would the fuel starvation be obvious if too low (cutting out until the bowl refills), or would it act like a constant lean mixture?

If critical to the jets, where should it be? Just below the jet with choke on or choke off? How far below?

Hoping someone can enlighten me, or point me to some literature that goes into detail regarding float/fuel level…

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NOHOME P P
O, ON, Canada   CAN
1967 MG MGB GT "Maggie (GT From Hell)"
With the bell removed.
Looking down the jet holder
with the jet screwed down two full turns from being even with the bridge
With the jet pulled all the way out via choke cable or manually

You want the fuel level in the jet holder to be just over the top of the jet.

To test empirically, shut the choke off, and using a piece of small rubber tube, blow a puff of air straight down the jet. You should see fuel jjuuuusssstttt bounce back and burble over the top of the bridge. If it floods, fuel level is too high, if it does not come over the top, then it is too low.

I used to hate SU carbs until someone taught me these tricks and I realized that the way of setting the fuel level as described in the manual has little chance of getting the job done right.



Pete

Poundingsand Avatar
Poundingsand Silver Member Peer Ebbighausen
Los Angeles, CA, USA   USA
Thanks Pete. Very interesting method I hadn't heard of before. I intend to try it.

And what would the results potentially be if the level is too high or too low?

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NOHOME P P
O, ON, Canada   CAN
1967 MG MGB GT "Maggie (GT From Hell)"
If the level is too high or too low, you get the expected lean or rich running.

What I used to think was that it would not really matter as I could compensate by raising or lowering the jet. But it is a bit more complicated than that. I suspect that the needle profile and jet are meant to work with the fuel at a certain level and the gap between the two at a given "home" relationship that is established two turns down from the bridge.

I have used the puff of air down the jet trick to help trouble-shoot several cars that were "just not quite right" Two of them ( both Healeys) had floats that were stuck CLOSED!! Would have taken me a while to figure it out had the puff test not shown no fuel over the bridge.




If you know that your timing and valves are all set properly, dont hesitate to have a vacuum gauge hooked up to the manifold as you set the jets. You can fine tune for max vacuum.

Pete

dlrhine Avatar
dlrhine Dave Rhine
South, Carolina, USA   USA
I've tinkered with my HS2s trying to set float levels per several different instructions, but have never really been satisfied with the results...
I'll try this method & see what happens.
Thanks for the info smileys with beer



If it ain't broke, I'll fix it 'til it is! winking smiley

Poundingsand Avatar
Poundingsand Silver Member Peer Ebbighausen
Los Angeles, CA, USA   USA
I intend to try it as well, but I have one concern.

Pete, you mention "You should see fuel jjuuuusssstttt bounce back and burble over the top of the bridge. If it floods, fuel level is too high"...

I'm hoping I can tell the difference between a burble and a flood! spinning smiley sticking its tongue out

NOHOME P P
O, ON, Canada   CAN
1967 MG MGB GT "Maggie (GT From Hell)"
In reply to # 3715086 by Poundingsand I intend to try it as well, but I have one concern.

Pete, you mention "You should see fuel jjuuuusssstttt bounce back and burble over the top of the bridge. If it floods, fuel level is too high"...

I'm hoping I can tell the difference between a burble and a flood! spinning smiley sticking its tongue out

I hear you, and had the same concern. You have the same issue when trying to describe a "puff" of air down the jet. The real work is really done when you set the fuel level to just over the jet. The puff test is just a quick way to make sure that it is right.

Pete

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Poundingsand Avatar
Poundingsand Silver Member Peer Ebbighausen
Los Angeles, CA, USA   USA
Pete, did you mean fuel level just UNDER the jet?

NOHOME P P
O, ON, Canada   CAN
1967 MG MGB GT "Maggie (GT From Hell)"
In reply to # 3715220 by Poundingsand Pete, did you mean fuel level just UNDER the jet?

No, with the jet pulled all the way down via the choke cable, the fuel should be over the top of the jet. This is not the easiest thing in the world to judge but is doable. I have a syringe with a long tube that I use to poke down the jet and pull fuel so that I an kinda see how it fills in over the jet.

Previous to doing this, the jet should have been adjusted to be two turns down from the bridge with the choke off.

You are still going to set the float height as specified by the manual. (cause you gotz to start somewhere) This will give you an 85% chance of being right. The rest is just to make sure that the factory method translated to the correct level in the jet.

Pete

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Poundingsand Avatar
Poundingsand Silver Member Peer Ebbighausen
Los Angeles, CA, USA   USA
Ah, with choke ON, that makes sense.

I had a small window of opportunity to inspect my carbs yesterday, and noticed that both jet bearing aren't flush with the jet bridge - one sits about .5MM too low. I imagine this is preventing me from accomplishing that last bit of fine tuning I am looking for, correct?

To remedy this, is it as simple as loosening the jet locking screw, sliding the jet bearing to the proper height and retightening the screw? And can this be done with the carb "in situ" and all components still attached, or is some disassembly required?

pinkyponk Avatar
pinkyponk Gold Member Adrian Page
Berwick, NS, Canada   CAN
From SU: http://sucarb.co.uk/technical-su-carburetters

Float chamber fuel level

The fuel level on an S.U. is not critical, and need not be treated with meticulous accuracy-the normal level is 3/8 in. under the rectangular inner facing known as the jet bridge, but this is rather difficult to observe even with the suction chamber and piston removed and the jet fully dropped. However, a simple mechanical check can be made, and this consists of sliding a certain diameter of check rod between the lid face and the inside curve of the forked end of the hinged lever when the needle valve is in the 'shut off' position. The size of this rod for both the 1 7/8 in outside diameter smaller float chamber and the larger one of 2 1/8 in outside diameter is 7/16 in. On the HS type of float chamber a 5/16 in. rod is used with a brass float, and a 1/8 in. rod when the hinged nylon float is fitted. If the hinged lever fails to conform within 1/32 in. of these check figures it must be carefully bent at the start of the fork section, in the necessary direction for correction, taking care to keep both prongs of the fork level with each other. It must be emphasized that it is not advisable to alter the fuel level unless there is trouble with flooding; and although a too high level can cause slow flooding, particularly when a car is left ticking over on a steep drive, it should be remembered that flooding can also be caused by grit in the fuel jamming open the needle valve, or undue friction in the float gear, or excessive engine vibration, or a porous float.


Being a constant velocity carb the jet is always at the same depression. Other carbs have varying depression at the jet. The fuel doesn't "fall back down" the jet tube in a running SU at low engine speeds.

Float level is not a tuning parameter. If the level is too low you'll run out of fuel under hard acceleration/cornering and if it's too high you'll have flooding.

Adrian



Home built Eaton M62 Supercharger with 9psi boost, "stock" high ratio rocker arms, 8:1 compression, Piper 270 cam, ported head, matched manifolds, CB Performance computerized ignition.



Edited 1 time(s). Last edit at 2018-04-12 11:01 AM by pinkyponk.

NOHOME P P
O, ON, Canada   CAN
1967 MG MGB GT "Maggie (GT From Hell)"
In reply to # 3715385 by Poundingsand Ah, with choke ON, that makes sense.

I had a small window of opportunity to inspect my carbs yesterday, and noticed that both jet bearing aren't flush with the jet bridge - one sits about .5MM too low. I imagine this is preventing me from accomplishing that last bit of fine tuning I am looking for, correct?

To remedy this, is it as simple as loosening the jet locking screw, sliding the jet bearing to the proper height and retightening the screw? And can this be done with the carb "in situ" and all components still attached, or is some disassembly required?

The nut at the bottom of the jet ( jet adjusting nut) holder raises and lowers the jet in the holder. When doing initial settings on an SU, start by screwing the jet up level with the bridge by turning this nut. Then two full turns down. Next, set the fuel level in the bowl. Use the book method as a general guideline, but don't trust it.

In order to verify that the fuel level is set right, have a peek down the jet and give it the puff test. If the book method worked, all should be good. If not, you will save a lot of frustration when things don't run right but you think the float bowls are working properly.


Poundingsand Avatar
Poundingsand Silver Member Peer Ebbighausen
Los Angeles, CA, USA   USA
Hi Pete. That part I am familiar with. I'm talking about the jet sleeve that the jet rides in. One is flush with the bridge, the other sits slightly low. Can it be adjusted by slacking the jet locking nut (labelled in your diagram) that cinches it in? And can it be done with the carb in place?

Poundingsand Avatar
Poundingsand Silver Member Peer Ebbighausen
Los Angeles, CA, USA   USA
I'm not sure if you can see it in the picture, but here t is...would the carrier height affect mixture, or is it unaffected since the jet itself dictates the volume of fuel/air that's able to pass through?


Attachments:
Carbs on bench.jpg    43.5 KB
Carbs on bench.jpg

pinkyponk Avatar
pinkyponk Gold Member Adrian Page
Berwick, NS, Canada   CAN




Home built Eaton M62 Supercharger with 9psi boost, "stock" high ratio rocker arms, 8:1 compression, Piper 270 cam, ported head, matched manifolds, CB Performance computerized ignition.

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