If you do get into needle selection a good site is mintylamb.co.uk, you can compare three needles at once.

Update: I changed my dashpot oil from Marvel Mystery oil to a conventional 30wt oil. The difference was fairly dramatic. Unfortunately, the stumble when I stab the throttle hard for a downshift after a long period of deceleration is still there. It did change, however. Perhaps I'm putting too fine a point on it, but it seems as if it's gone from a crackle and slight backfire to more of a deep-throated "phift." My guess is I went from an overly lean condition to an overly rich condition when I bip as a result of the thicker oil. I think I'll leave well enough alone. Mixture-wise, I'm good at idle, middle and top end. I'm not messing with chasing needle profiles. When I get my O2 sensor up and running I may have a better idea of what's going on, but I'm not hopeful for a solution without going down the needle and spring rabbit hole.

30wt oil may be spec'd for new carbs, but they do wear, so that would probably be a reasonable starting point. Add in the climate your mini lives in - Canadian spec'd minis had an electric dashpot warmer to compensate for cold weather.

You should consider that SU specced 30wt for all Mini engines ..from the 32HP 850 to the 75HP 1275....

I'd be very interested to hear what BMC Comps used in their comp engines ....but it hasn't seemed to be a topic worth pursuing...

If its rabbit holes you're after............

Cheers, Ian

Wow. I had the lean/rich vs. oil thing backwards. This makes sense, actually. I began to experience the stumble about the same time I switched from 30 wt. to Marvel. Don’t recall why I switched, probably my tendency to fix things that aren’t broken. If Haynes, Vissard and SU tech docs mentioned this, I missed it. 

I’ll switch back to 30 wt. today and see if my stumble disappears. Don’t need to wait for my O2 meter for that. Thanks!

 

How the accelerator pump effect works:
When you open the throttle, vacuum is applied to the dashpot, lifting the damper and opening the throat of the carb at the bridge. This allows more air to pass through. It also pulls the needle out of the jet to admit more fuel to match the increase in air flow. Unfortunately there is a delay in getting the fuel to flow adequately, resulting in a temporary lean condition. To combat this, the dashpot has the damper to slow its rate of opening, with the result that the air flow is delayed but the velocity across the bridge increases, drawing a bit excess fuel. This gives the squirt of fuel for the accelerator pump effect.

If the dashpot oil is too light or low, you get the lean mixture stumble or lag. If the oil is too heavy you get a rich mixture stumble. So, to combat a lean condition, use a heavier oil, and vice-versa.

Yeah, that was my thought as well, the ARF gauge will tell me if it's spiking rich or lean on the bip. If it's going overly rich I'm thinking heaver dash pot oil, overly lean and I'll try lighter oil. Am I on the right track? I'm hoping to solve this with a change in oil; I'd rather not go chasing different needles or springs—I don't need another rabbit hole to fall in to.

Unfortunately, my O2 meter stopped working when I upgraded to a new iPhone. There should be an app update soon. Will advise…

Wideband article: [link]

Dashpot oil is linked to the accelerator pump effect, hence asking what your AFR gauge is reading when you are stumbling.

The grades of oil used in the SU dampers cover a wide range.  I've seen publications that say 20W, 30W, John Twist recommends 90W gear oil, and some of my books just say "engine oil".  

Burlen (SU) lists their oil as 20W (link below).
//burlen.co.uk/dashpot-oil-light.html
So that's what I carry in the boot of my British cars. 

3-in-1 in the blue/white bottle is 20W and the bottles are small and convenient.
https://www.3inone.com/products/motor-oil/

To each his own.

IMHO the whole issue of dash pot oil is something of a "first world problem".  My SU book (which has an entry for 1071s - so must be mid 60s) calls for proprietary "SU dash pot oil" and specs this as 30wt.

My experience is that if you forget the oil the engine will be a pain to start but will run OK once you get going.  

20W50 is "normal" Mini engine oil so it makes sense to use .. its probably readily available..  All you need is a stable and reasonable viscosity. 

I would suggest not trying to second guess the designers....

Cheers, Ian

Seems to me that Calver likes 20W Redline for all but stock applications.  That's what I use and like it.  But this is a can of worms because no two engines are the same and thus require a different combo of dashpot oil, springs, needles, timing, etc, etc.  

I have seen owners use a lot of different oils in dash pots from 3 in 1, mystery oil, auto trans fluid, 20w, 30w, and 20w50 engine oil.
When you have a tuned Mini Rosebud it is going to take a little trial an error to find what works best.
SU service department used Duckhams 20w50 fwiw.

If the backfire happens from coasting in gear, in addition to the above look for leaks in the exhaust system.  

A thicker oil would help to slow piston rise. I did a tune up on a old 1100 in a early mini last week. There were a
number of problems I had to chase. Once I changed plugs, point plate, points, condenser, cap and wires the steady
state flutter was gone. When bliping the throttle there was a slight hesitation. Having cleaned the piston and bells
and added a couple flats richness hesitation remained, I removed the springs and changed to springs the same
length and number of coils with .002 greater wire cross section. I had already topped the carb oil with 3&1 oil. Issue
was gone and off it went. Steve (CTR)

What does the AFR gauge tell you as you blip - does it go rich or lean ?

Whenever I bip the throttle for a downshift, the engine hickups a bit, sometimes with a little backfire. This most often occurs after a short period of coasting preceding the bip. The carbs are dialed in, the timing is fine, the plugs look good and the mass O2 meter is within range. I'm thinking of trying a thicker dashpot oil. I'm using Marvel Mystery oil, which is pretty thin. Am I on the right track?