Dash pot oil
 

I know that the book recommends ATF or similar but what effect would it have on performance using a heavier, engine oil for example, or a lighter oil ?

a heavier oil would make the piston rise slower restricting power and increasing fuel consumption .

Stromberg carbs should use ATF

SU carbs should have SAE20 oil but I find 10w40 works well. Slightly thicker oil will give more enrichment is acceleration, something these engines tend to need.

That was my understanding. Thicker oil slows the rise of the piston under heavy acceleration and momentarily causes a richer mixture.

ATF is more stable across a range of temperatures

ATF is more stable, because it is single weight. Straight SAE20 weight oil is ideal really, in my experience, and as recommended by SU/Burlen: http://sucarb.co.uk/technical-carbur...gs-chamber-oil

I found some single 20w oil for much less however (I think it was for a lawnmower, can't remember).
10w40 etc would change characteristics as the engine warmed up.

I always used 3-in-1 oil as dashpot oil for a pair of SUs fitted on a 60s 997S engined mini - it was quoted as an SAE20 oil.

I will stick to ATF used in my cars including rally stuff and in my commercial repair business for 40 + years..but basically its personal choice.not wishing to get into a pi##ing competition. No offence to anyone as if you ask 100 people they will all give well intentioned advice

Experience always counts in these matters - as far as I'm concerned it always comes down to personal experience/expertise and what you know works for you and the machine in front of you at the time and how you want it to perform.

The biggest mistake I ever made was changing the SU needles for richer needles and using a lower 10SAE oil giving a faster drop/drop on pair - trying to balance them was a nightmare so eventually they replaced them with a pair of Webers on a smoothed & re-profiled inlet manifold, but that was a wholly different balancing act!

AS;

Don't take this wrong, but where do I start...based on what did you change Metering Needles and Damper oil viscosity?...if for any reason less than full range dynamometer motor tests, where engine response and exhaust gases are checked under all load (steady-state and transient) conditions, I expect that explains half of the tuning problems...

From: https://www.sw-em.com/su_carbs.htm
"...this is exactly why wrong Metering Needles get into carbs: ...he says: "...you can kinda look at the chart....I think I'm going to go to an XXX needle". What kind of a system for picking a needle is that? None! Metering Needles for a particular configuration of engine (mostly influenced by camshaft, manifolding - intake and exhaust flow) were arrived at after countless hours on a manufacturer's motor test stand (dynamometer), running at various RPMs, loads, throttle settings, and with exhaust gas analyzers... very few casual mechanics or even shops are equipped enough (mentally or hardware) to get this any better than the factory!...but they think they are!...so they'll try...and that's why incorrect needles are found in carbs! Do not change Metering Needles unless you can put motor/vehicle on a dynamometer to scientifically check the results...like so! (see Reference: B18 on dynamometer) ...and if you feel the need to change the mixture slightly (for instance seasonably), adjust the Mixture Adjustment Nut only...no more!"

...and Damper Oil should be 20 weight (and because that is what ATF is, and because it has a wide operating Temp range, and is available inexpensively in quarts, it is a popular alternative)...and BTW... Dashpot Drop is not dampened...only Rise!
https://www.sw-em.com/SU%20Damper%20Valve%20markup.jpg

I doesn't surprise me that you gave up in the end and went to Webers...I wonder how that went...

Cheers

The recommended "fix" at the time to get that increased the volume of fuel/litre of air on the induction stoke was to go the richer needles, by reducing the SAE, the piston rose more quickly so theoretically leaning the mixture to the point of not running overly rich; the real effect was as you state above - which compounded the nightmare of balancing the twin 1¼ SUs. The Webers held their tune much better but needed some modification to the bulkhead to make them fit, that said I always thought that although some of the improvement was due to the carbs ability to increase air flow much of the benefit was derived from the better air flowed manifold itself.

I eventually sold the Weber set-up and went back to SUs with 20/50SAE in the dash pots (Duckhams Q worked best) using twin 1½s on a flowed manifold which seemed to work better with the longer dwell Janspeed cam. In real terms it was over tuned for a road car, but was an absolute delight to drive...it was just waiting for the car in front at traffic lights to get 20 or so yards ahead before you let in the clutch that was a pain!

I also thought the same, and used engine oil in my twin SUs on my Maxi.

On my stage 1 tuned Mini, I used richer needles and still got it to pass emissions.

Emissions weren't a major factor when I broke my Cooper S up in the summer of 1975, there was far too much to repair in terms of sills/rear subframe/bootfloor, rear floor panel, front inner sills & etc. most of the non-fibreglass panels were riddle with the rot and at the time I had neither the time nor the money to replace the body............sold the engine for more than I'd paid for the car and all the "go faster" parts over the preceding 7 years, which paid for a four year old Ford Corsair V4.

The only car I've driven which even comes close to the handling of that 997S is my son's Japanese grey import '92 Eunos/MX5 - it produces the same grin factor when you start it up and slot it in gear...sadly, (nah :rolleyes:) it is sat on our drive and I am forced to drive it every week or so to stop the brakes sticking/seizing.....the C70 almost does it but can't quite get the hang of this new-fangled thing called aircon in a cabriolet, but then the Eunos also has it - still can't figure out why anyone would need air-con in a ragtop!

I miss my custom mini pickup... No electronics, just clean pure fun..

https://farm5.staticflickr.com/4310/...d44a6f3a_b.jpg

I have used a special 'v8250' home brewed mix for many decades now, that is thinner than what SU/Burlen Fuels suggest. It allows, combined with correctly spec'd springs, much quicker take up, excellent idle and excellent stability throughout the rev' range.

A 50/50 blend of 10w/40[or 20w50] with 3in1 will do very well for most road applications. It's really worth adding this book to this years birthday or Christmas present list...

https://www.amazon.co.uk/Build-Power.../dp/1901295141

ps I use something altogether different and sometimes have the fastest off-the-line SU's...the recipe's tip-top secret but does include super thin sowing machine oil ;)

I used air tool oil years ago because the garage had loads of it and it cost me nowt.

AS; "the richer needles" implies there are a limited number available...in fact, I have no less than 3 8 1/2" X 11" pages full of dimensional data for metering needles for 0.100" Jets, the sizes of which are given in ten thousands of an inch accuracy (!)...determining which is "the one" for your engine configuration takes some engineering, and trial and error, accompanied by solid research and practical load tests, not just a quick look at the list...

Forum; Dashpot oil is indeed crucially important to be present, but viscosity 20W +/- is just not that critical for anything but where its necessary to get the absolute best out of the engine (and this applies only to transitions ie throttle response!), consequently a lot of people play with viscosity and swear they get a noticeable result...I have to question that...now if I had a lightened Flywheel and Intake and Exhaust flow which allowed a much faster throttle response, I might be tempted to try varying the viscosity (if I had a dyne or a lot of track time to check it).

Cheers

Thank you for your informative post; the last time I played with SU needles was the summer of '68

Access to the data to which you refer was rarely available in those times, most BMC/Leyland dealers only carried standard needles, indeed even locating richer/weaker needles was a question of "ringing around" the county (Kent, Surrey, & Sussex at the time) to find alternatives. Those I found came from a now defunct small engineering/racing engine builder some 60 miles away.

Much of what was achieved was by trial and error and I assure you there were many errors; in stage 3 tune it was running a tuftrided racing cam (Piper's seemed less prone to wear than Janspeed's at the time), triple valve springs, coupled with a Lotus derived duplex timing chain with the associated gear mods, tuftrided fully balanced crank & lightened flywheel, plus a competition clutch - the engine was redlined @ 10,200rpm on 5* fuel meaning only one gear change was necessary to hit 60mph in a shade under 6.1 seconds - not bad on the 10inch wheels - it literally was a very hot little motor and would blow away most sports cars from a standing start, leaving many an Elan in its wake. Various heads were used over time, the best coming from a frogeye sprite which had a little more metal to play with when it came to skimming & smoothing again compression ratios of 10.6 to 13.1 were used over time - the best set up was achieved using 12.3:1, anything more than that just didn't work as road car + five star fuel aka the old leaded 101 octane became increasingly difficult to find at the roadside hence the sale of that wonderful engine in the summer of '75.

The twin 1.5" SUs on "normal" needles and Duckhams Q 20/50 made for a much more tame drive, but still enabled a very fast take off without the thirst of the Webers.

The point being that the SUs were designed for an SAE20 oil.

Ron, as usual I agree with most all you say, except the part about the need for a dynomometer to get it exactly right when f**king around with different needles and other stuff e.g. oil, ignition, flow alterations, etc...

I recently traded my 16' B20-powered speedboat for, among other things, an old working top-of-the-line Snap-On Exhaust Gas Analyzer (tailpipe "sniffer") and an Ignition/Fuel Injection Oscilloscope (way overkill for our simple cars).

Looking at CO readings from tailpipes of modified cars is something I've always wanted to do, and is a definite gas.

Best, from the chilly North

Which is why I bought mine from a well respected tuning company who made the inlet and exhaust manifolds also, and had already done the hard work selecting the right needle for my camshaft etc.

Which is exactly the same reason why I bought from "a now defunct small engineering/racing engine builder some 60 miles away".

nNH;

Many have tuned carbs using the "seat-of-the-pants / road-test technique"...instrumentation and a dyne clearly take away a lot of the variables to make it a more scientific process...but in the end, the driver needs to be happy with the response, so often, the MN and baseline settings arrived at on the dyne might get adjusted/tickled as a result of the drivers input anyway...I think we understand each other...

Cheers