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From: Nial Stewart on 23 Jul 2010 08:01
> It's a complex subject and a dying art....change anything and you need to re-needle. start here :-
> http://www.sucarb.co.uk/


Yes I know, that's why I want to be able to determine what I need by measuring
the A/F mixture along the current needle.


Nial.


From: Nial Stewart on 23 Jul 2010 08:14
> The tops of the free floating pistons have a vacuum applied to them via
> a hole to the venturi. That vacuum is what allows the pistons to rise,
> as the surface area being acted on is much greater than that of the part
> in the carb bore.
> I don't know how you could actuate a sensor via this piston without
> introducing a leak into the upper chamber.

If you unscrew the daspot damper from the top of the carb you can get
at the top of the piston, this is separate from the sealed chamber.

If I drill a hole in the damper top I should be able to get a rod down
to rest on the piston (I haven't thought through securing it yet).


> You should be able to check for correct mixture by manually raising the
> piston about 1/8 to 1/4 inch. The engine should die when it is raised.
> Many SU models have a small small plunger on the side of the throttle
> body for just that purpose.

That just tells you if your idle mixture is about right.

> Keep in mind that the weight of oil in the damper rod cylinder affects
> mixture also. The heavier the oil, the richer it runs. I think I used
> something like 2 or 5 weight oil...like sewing machine lubricant. Flush
> all used oil and fill up to the top with new oil. The extra stuff will
> be forced out when you replace the damper piston.

No, the oil in the daspot affects _transient_ mixture. When you put your
foot down the damper stops the piston rising to the new equilibrium
position as fast as it wants so there's temporarily more air over
the jet which richens the mixture.

The opposite happens when you lift off.

The static mixture is dependant on the taper of the needle.

> The needle jet is usually adjustable up and down the needle. In fact,
> the choke (enrichener, actually) works by lowering the jet on the needle
> taper.

I know, and as I said in the OP the fact the car runs better with the choke
permanently slightly out shows the current needle is the wrong profile.

> There may be a variation here, as the 1 inch or so SU is unfamiliar to me.

AFAIK they all work the same, but it's 1 3/4" on my car.


Nial




From: m II on 23 Jul 2010 09:47
On 10-07-23 06:14 AM, Nial Stewart wrote:

> AFAIK they all work the same, but it's 1 3/4" on my car.

Thanks for clarifying some items. One thing I did notice a while back
was the numbering of the carb models. If your carb is one and three
quarter inch, it should have a '6' in the designation.

SU seems to use 1/8 of an inch sizing, so the six becomes 6/8 , or 0.75
.. That is added to the baseline 1 inch. 1 + 0.75 = 1.75

My HIF 8 is therefore 1 + 8/8 = 2 inch.

Best wishes for your project.



mike
From: IanM on 25 Jul 2010 04:51
Tim Wescott wrote:
> On 07/22/2010 11:54 AM, Nial Stewart wrote:
>> I am afflicted with the love and ownership of a classic Mini...
<snip>
>> I have been thinking of making a rig so I can properly measure& log
>> the A/F ratio
>> against the carb piston lift (the needle protrudes from the bottom of the
>> piston into the jet). This should allow an optimum profile to be deduced
>> and the correct needle produced.
>>
>>
>> I can use one of these to measure the A/F ratio....
>>
>> http://www.innovatemotorsports.com/products/lc1.php
>>
>> ..but need a method of measuring the piston lift _fairly_ accurately and
>> without loading the piston too much (the lift is regulated by a spring
>> and too much extra weight on the piston would affect this).
>>
>> I can mount things so that a vertical probe sits on top of the piston and
>> be moved as the piston moves. (I'll try to get a picture tonight).
>>
>> I can used an LVDT but this is expensive and overly accurate for what I'm
>> doing.
>>
>> Can anyone suggest any simpler/cheaper method of measuring this?
>>
>> Thanks for any thoughts.
>
> A linear encoder.
>
> Or hand-wind an LVDT. It'll still be complex, but it'll be cheaper
> (assuming you can read it).
>
For a linear encoder that short, I'd use the sensor head and part of the
encoder strip 'ratted' out of an old inkjet printer. It is normally
held under spring tension but for an active length of ~ 50 mm, if
shortened it should be stiff enough to push/pull through the sensor
without being under tension. You will probably need something like a
drinking straw glued to the piston top for the vertical probe, and the
other end split and glued to the piece of encoder strip. The extra
weight on the piston would be minimal.

This won't get you a 'home' position, but if you track the encoder
continuously from endgine off, you can just measure that.

--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk
[at]=@, [dash]=- & [dot]=. *Warning* HTML & >32K emails --> NUL:
From: Nial Stewart on 27 Jul 2010 12:57
> For a linear encoder that short, I'd use the sensor head and part of the encoder strip 'ratted'
> out of an old inkjet printer. It is normally held under spring tension but for an active length
> of ~ 50 mm, if shortened it should be stiff enough to push/pull through the sensor without being
> under tension. You will probably need something like a drinking straw glued to the piston top
> for the vertical probe, and the other end split and glued to the piece of encoder strip. The
> extra weight on the piston would be minimal.
>
> This won't get you a 'home' position, but if you track the encoder continuously from endgine off,
> you can just measure that.


Thanks for the pointer Ian, all I need now is an old inkjet.


Nial.


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