Linear mearurement of +/- 50mm with ~ 0.2mm accuracy?

ratio

A linear encoder.

Or hand-wind an LVDT. It'll still be complex, but it'll be cheaper (assuming you can read it).

"Nial Stewart" wrote in message news: snipped-for-privacy@mid.individual.net...

Buy the right needle..... plenty of places in the UK selling MINI stuff... is the air filter nice and clean ?

You tell me which of the several hundred that are available is the right one and I'll go and buy it!

(The car's modifed a bit and I'm planning a fair bit more).

Nial.

It's a complex subject and a dying art....change anything and you need to re-needle. start here :-

ratio

I have a couple of 2 inch SU carbs in the garage. They're off a 1974 Jag. AKA DriveWay Queen. They're somewhat larger than the Mini version but work the same way.

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.

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.

The carb itself is one of the neatest devices ever invented and when set up correctly, work very well.

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.

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.

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

Later model Strombergs had a rubberized diaphragm sealing the floating piston. They were cheaper to make, as they didn't need the same precise machining as the SU.

mike

ratio

Wind three coils and you have an LVDT where you measure relative signal pickup and no longer care about absolute inductance. Drive coil is in between the two secondaries. Winding all that nice and evenly is what drives up the cost of an LVDT.

Moving core is ferrite, I think, drive frequency about 1kHz when I was doing signal conditioners for them a couple decades ago.

Grant.

ratio

High mu nickel-iron alloy. They're typically designed for a certain drive frequency (eg. 2.5kHz) such that the phase shift is zero.

I think you can make a pretty crappy one and it'll still be good to a percent of so.

Best regards, Spehro Pefhany

ratio

That sounds better, 'cos the core was tapped to fit onto non-magnetic rod that connected to the whatsit being measured. Or spring loaded, many options.

Though I remember the phase being differently expressed, Think we use to zero the thing by adjusting phase control of the sync rectifier with no core inside, then add core to the zero position, then do the displacement each way to calibrate +/- span.

Not much in them, lots of fine wire though, one would need plenty of patience to wind one. I suppose with fast opamps one could use less turns, run at much higher frequency and still get decent signals?

Grant.

re-needle. start here :-

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.

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).

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

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.

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.

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

Nial

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

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.

encoder strip 'ratted'

for an active length

sensor without being

to the piston top

encoder strip. The

continuously from endgine off,

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

Nial.

On Jul 22, 2:54=A0pm, "Nial Stewart"

Dan

Or---

Thanks guys, but I meant while driving (in a remote elecrtonic sensor sort of way).

I suppose I could try to convince the wife to let me tie her to the engine and she could do the measuring while I drive.

On the other hand any other suggestions about tying her up have been rebuked so that probably isn't a goer.

:-(

Nial.