HS Floats Gap Query

[McMuttley]

I am going through the carbs, cleaning, checking etc (this could take months!!)

 

This evening got to the fuel float chambers.

 

As before, very clean inside, no muck.

 

However, I am confused by the float gap.

 

These appear to be an HS type float - non-adjustable

 

The SU site suggests that the gap between float and lid rim should be between 0.062" (1.6mm) and 0.187" (4.75mm) - quite a wide range ?

 

Mine is double the width of the (useless) HS spanner - so is circa 6mm?

 

I thought that I could turn the screw in for the bit that the float needle sits in, thus raising the needle/jet (?) but this also seems to be non-adjustable.

 

I have yet to check the second. Is the important factor that the two have the same gap, rather than the gap itself?

 

I was expecting the gap to be narrower, thus possibly explaining a high fuel level and thus a contribution to the rich running, but I assume that a large float gap (low float) would actually avoid flooding the carb's jets ?

 

As always, thoughts gratefully appreciated.

 

 

 

[Lebro]

I have been looking into the whole subject of fuel level in the carbs, and is not trivial !

 

The HS series float level setting method in the books (TR manual, & a haynes SU carburetter manual) says to set the gap between the float and the float chamber lid (while holding chamber lid upside down) to be between 3.2, & 4.8 mm. inserting a 4mm drill in the gap would appear to be optimum.

To adjust - some floats have a metal hinge, & these can be bent to give the correct height, if you have a non-adjustable nylon float (as in your photo) then the gap can be increased by adding thin fibre washers under the needle valve body, but to decrease the gap you must remove washers, or if there are non, then the valve body must be machined to be a tad shorter.

 

Except it's not as simple as that !!

 

The nominal height of fuel inside the jets is quoted as being 3/8" (or 9.5mm) below the bridge according to Burlens, they also say the height is not critical.

From measurement of my own engine, the front sits higher than the back when car is on level ground, & the angle is 4°.

If you do the sums (TAN 4° = opposite over adjacent) then if you set the front carb float to be at correct height, then at the jet position the fuel will be 4.7mm higher than nominal. Similarly for the rear carb (float chamber behind the jet) the fuel will be 4.7mm lower than nominal.

I checked this out on my car recently (having previously set the floats to the book settings) & found fuel levels to be 4.5mm below the bridge for the front, & 15.5mm below for the rear carb.

I then set about correcting for this, by adding fibre washer under the front needle valve body, & machining the valve body of the rear carb.

End result was fuel levels at the jets to be within 1mm of the nominal 9.5mm figure. I had to re-adjust the carb mixture settings, and now it is running just about the same as it was before

 

All of which goes to prove that float levels really are not all that critical.

 

Bob.

[Peter Cobbold]

 

 

All of which goes to prove that float levels really are not all that critical.

 

Bob.

Bob,

Yes....the depression over the jet needed to lift petrol 10mm is about 0.01psi.

Here's the pressure needed to hold up a 1inch water column:

https://en.wikipedia.org/wiki/Inch_of_water

Petrol has 80% of water density so its near enough.

 

As far as I can make out the 'constant depression' in a 2" SU is about 0.25psi.

So there's lots of spare depression around. A fuel level a few mm up or down the jet wont make any difference- as long as the fuel doesnt overflow going uphill or in turns.

 

Peter

[oldtuckunder]

 

So there's lots of spare depression around. A fuel level a few mm up or down the jet wont make any difference-

 

As can be seen from the crappy choke mechanisim on the old SU's that waggles the jet up and down, the number I have seen where the jets don't return properly, its no wonder pairs had a reputation for going out of balance.

 

Interesting that whilst the fuel level (provided as Peter said there is no flooding) seems to make very little difference, the distance the jet is below the bridge does seem to. When I was playing earlier this year with some carbs that had adjustable needles and adjustable jets I found that the closer the jet was to the bridge the leaner the mixture I could set on the needle to get good AFR's i.e a 1mm jet lift only required a 0.5 needle lift.

 

I have a theory (yet to be proved datalogged) that the more the jet is depressed from the bridge the more of a pool of petrol is pulled around it before it hits the air stream to get atomised, but if the jet is higher it gets atomised better, of course there are lots of other variables, but I think the later move to fixed jets adjustable needles wasn't without good reason.

 

Alan

[Peter Cobbold]

Alan,

The jet level is another matter: it is important, as you say.

The fuel flow is determined mainly by the area the fuel flows in between the needle and the mouth of the jet. (The length and diameters along its length of the immersed needle also affects fuel flow but this changes only slowly with lift.)

If the top of the needle were parallel than a dropped jet would give the same mixture as one that was the correct mm or so higher. But the top of the needle tapers and that taper allows more fuel to flow past a dropped jet: and we get a richer mixture. The reason a mm change in the jet is pronounced is because there's b8888er all air flowing at tickover. At bigger throttle openings and running air flows the mixture change will be much less.

I am unsure about the pool of fuel over the lowered jet being involved, I think it was a cost-saving measure. At tickover the air flow, low as it is, will be whistling in gap of maybe a mm above the jt and below the piston and it could be fast enough to create a venturi effect. Its doubtful that a venturi depression occurs at moderate piston lifts ( see blog). But at tickover the suction on the fuel could be more than a fraction of 1psi. meaning that tiny changes in the fuel annulus area have an effect on mixture. So the reason why adjustable needles were introduced may well have been to allow less precise needle machining on the production line but still meet tighter emissions. The wobbly needles were later introduced to better accomodate machining errors and wear: errors in the diameter of the up-stream fuel arc have less effect on mixture than a bigger downstream arc. Hence the spring-loaded needle. ( see HP Lenz' monograph cited in blog) . A pool of fuel over the jet wont affect mixture quality at low throttle openings as the butterfly atomises the fuel very effectively:

https://books.google.co.uk/books?id=afYcBQAAQBAJ&pg=PA150&lpg=PA150&dq=atomisation+%2B+butterfly+%2B+carburettor+%2B+sonic&source=bl&ots=CjVso1ZDER&sig=EgXcdZaCvcV8bI-5AlCYqdNUJvs&hl=en&sa=X&ved=0CCgQ6AEwA2oVChMItO6q3qWSyQIVywQaCh2M0wJN#v=onepage&q=atomisation%20%2B%20butterfly%20%2B%20carburettor%20%2B%20sonic&f=false

 

As you say, more hidden variables are waiting to surprise us in the apparently simple SU.

Peter

Edited November 15, 2015 by Peter Cobbold

[Lebro]

Putting all of this highly technical stuff to one side, & to answer MM's original question.

I would aim to set the gap between the front float & the chamber lid to be approx. 9mm, & the rear one to be as close to the lid as possible while maintaining definite closure of the needle valve. This will give the correct fuel height in the jet.

By the way I prefer the rubber tipped needles to any other type.

 

Bob.

[McMuttley]

ahh, rubber tip - that would explain the black tip that the carb cleaner wouldn't remove !

 

Thanks Bob

[BlueTR3A-5EKT]

I have been looking into the whole subject of fuel level in the carbs, and is not trivial !

 

The HS series float level setting method in the books (TR manual, & a haynes SU carburetter manual) says to set the gap between the float and the float chamber lid (while holding chamber lid upside down) to be between 3.2, & 4.8 mm. inserting a 4mm drill in the gap would appear to be optimum.

To adjust - some floats have a metal hinge, & these can be bent to give the correct height, if you have a non-adjustable nylon float (as in your photo) then the gap can be increased by adding thin fibre washers under the needle valve body, but to decrease the gap you must remove washers, or if there are non, then the valve body must be machined to be a tad shorter.

 

Except it's not as simple as that !!

 

The nominal height of fuel inside the jets is quoted as being 3/8" (or 9.5mm) below the bridge according to Burlens, they also say the height is not critical.

From measurement of my own engine, the front sits higher than the back when car is on level ground, & the angle is 4°.

If you do the sums (TAN 4° = opposite over adjacent) then if you set the front carb float to be at correct height, then at the jet position the fuel will be 4.7mm higher than nominal. Similarly for the rear carb (float chamber behind the jet) the fuel will be 4.7mm lower than nominal.

I checked this out on my car recently (having previously set the floats to the book settings) & found fuel levels to be 4.5mm below the bridge for the front, & 15.5mm below for the rear carb.

I then set about correcting for this, by adding fibre washer under the front needle valve body, & machining the valve body of the rear carb.

End result was fuel levels at the jets to be within 1mm of the nominal 9.5mm figure. I had to re-adjust the carb mixture settings, and now it is running just about the same as it was before

 

All of which goes to prove that float levels really are not all that critical.

 

Bob.

 

Interesting issue you have raised about the height of the float chambers on HS carbs. I have the Burlen No Sink floats in my HS units which have a brass hinge that is bendable. http://sucarb.co.uk/float-chambers-spares/floats/hs-floats/hs-stayup-float-kit-hs6-hs8-large-bore-needle-valve.html

 

My engine - according to my iphone app - is 8 degrees nose up.

 

Peter W

[Peter Cobbold]

 

 

 

My engine - according to my iphone app - is 8 degrees nose up.

 

Peter W

Peter, So going up a 1 in 5 hill the engine is 29deg off horizontal Peter

Edited November 15, 2015 by Peter Cobbold

[BlueTR3A-5EKT]

Peter, So going up a 1 in 5 hill the engine is 29deg off horizontal Peter

 

 

I'll go down hill then, it always goes quicker then.

 

Peter W

[oldtuckunder]

Alan,

 

https://books.google.co.uk/books?id=afYcBQAAQBAJ&pg=PA150&lpg=PA150&dq=atomisation+%2B+butterfly+%2B+carburettor+%2B+sonic&source=bl&ots=CjVso1ZDER&sig=EgXcdZaCvcV8bI-5AlCYqdNUJvs&hl=en&sa=X&ved=0CCgQ6AEwA2oVChMItO6q3qWSyQIVywQaCh2M0wJN#v=onepage&q=atomisation%20%2B%20butterfly%20%2B%20carburettor%20%2B%20sonic&f=false

 

As you say, more hidden variables are waiting to surprise us in the apparently simple SU.

 

 

Peter

 

One of the better descriptions of a variable jet carb, which explains why a sprung needle is better than a fixed one, and also correctly describes the operation of the damper slowing descent as well as rise of the piston, if your tuning for performance not economy you just dont want that piston lifting too quick or dropping too fast.

 

I think in reading some of the text that references the performance/atomisation of fuel in carburettors one needs to be careful to distinguish between fixed jet (which I think are being used) characteristics and variable jet characteristics (which I suspect may have far more variables that makes modeling much more difficult).

 

Alan

[Peter Cobbold]

 

Peter

 

One of the better descriptions of a variable jet carb, which explains why a sprung needle is better than a fixed one, and also correctly describes the operation of the damper slowing descent as well as rise of the piston, if your tuning for performance not economy you just dont want that piston lifting too quick or dropping too fast.

 

I think in reading some of the text that references the performance/atomisation of fuel in carburettors one needs to be careful to distinguish between fixed jet (which I think are being used) characteristics and variable jet characteristics (which I suspect may have far more variables that makes modeling much more difficult).

 

Alan

Alan,

Richard Stone's is the only ic engine textbook I've come across that outlines the operation of an SU. The author is Prof of Engineering at Oxford Univ, so would be more familiar with SUs than USA authors. I can recommend the book.

Yes fixed jet carbs are very different - and there are lots of textbooks that cover their operation in some detail. But I have yet to find how much pressure is dropped across the Weber venturis. In a Weber the shape of the 'diffuser' section after the venturis is crucial in minimising this "exit loss". Someone will have measured the pressure loss in a Weber at various air flows but Ive yet to find the data....Anyone?

 

Peter

[Peter Cobbold]

 

 

I'll go down hill then, it always goes quicker then.

 

Peter W

and reverse up the hills

[Lebro]

 

Interesting issue you have raised about the height of the float chambers on HS carbs. I have the Burlen No Sink floats in my HS units which have a brass hinge that is bendable. http://sucarb.co.uk/float-chambers-spares/floats/hs-floats/hs-stayup-float-kit-hs6-hs8-large-bore-needle-valve.html

 

My engine - according to my iphone app - is 8 degrees nose up.

 

Peter W

 

I used a spirit level to ensure my car was level (hub cap to hub cap), then an inclinometer to check the engine using a straight edge over the top of the carbs. If yours is 8° then the fuel level error is double = 9.4mm. So if floats set to the book, then front fuel level = zero WRT the bridge, & rear (if HS types) 19 mm down.

 

Bob.

[Peter Cobbold]

SUs are recommended to have the f/c mounted in front of the jet to avoid leaning going up hills or in extreme acceleration ( and especially in the unlikely event of both simultaneously). But the linkage gets in the way so the rear carb is often 'wrong'

And the centre of the f/c should be in line with the jet along the fore-aft line, to avoid mixture changes in bends.

Peter

Edited November 15, 2015 by Peter Cobbold

[McMuttley]

 

I used a spirit level to ensure my car was level (hub cap to hub cap), then an inclinometer to check the engine using a straight edge over the top of the carbs. If yours is 8° then the fuel level error is double = 9.4mm. So if floats set to the book, then front fuel level = zero WRT the bridge, & rear (if HS types) 19 mm down.

 

Bob.

The hub caps may be level, but what about the rest ! Last year took the dimensions of mine, along both sides, chassis and body work - nothing matched, bounced it up on down on the suspension, measured again, totally different, left it overnight, measured it in the morning and different again.

 

All I could imagine is that in the night it sat in the garage like a 90's F1 Williams just fidgeting about !

[McMuttley]

ps, whereas the float for 1/2 was at least 6-7mm off the lid, the one for 3/4 was about 4.5mm, so a bit lower but nowt significant.

[Lebro]

I don't care about the rest ! as long as front & back wheels are at the same height, then that's the same as being on a level road.

 

Bob.

[Peter Cobbold]

Austin,

Have you sorted the unequal piston lifts? That video posted by PeterW shows a useful test, and very simple.

Peter