I’ve synced and adjusted my twin SUs and I think I’ve finally got them dialed in. The motor idles well, pulls strong and the plugs look great. There’s just… one… little… thing. I’m getting a slight hesitation—a flat spot, so to speak, right at the bottom of each shift under light throttle pressure. Most people unfamiliar with my car probably wouldn’t even notice. The hesitation lasts only a second and doesn’t occur at all under hard acceleration. Perhaps I’m noticing now only because the carbs are truly in tune, mixture-wise.

Seems to me I read somewhere that this kind of flat spot is most likely related to the dashpot oil. I can’t recall if I should try increasing or decreasing the viscosity. What do you guys think? Thanks!

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Michael, Santa Barbara, CA
. . . the sled, not the flower
https://www.facebook.com/PoserMotorSports/

Increase viscosity......

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Cooper S Mk2
Cooper S Mk1
ex Cooper S Mk2, various Morris 1100s, 1300s, 1500s, Leyland Sherpa!

Ditto BBY755
Slightly heavier oil slows the piston from rising too fast from idle. Rising too fast allows sudden rush of air (gas) in faster than petrol (liquid) can be drawn out of the jet - hence a temporary lean spot til the petrol catches up.
Just check there is no binding when lifting the pistons up off the bridge - like dirt or an off-centred needle.

I get it, increased viscosity allows a richer mixture at the bottom end. I guess my next question is what's causing the flat spot—over lean or over rich condition? I'm somewhat familiar with what to expect from of an over rich condition and that's what I think I have. What would an over lean condition feel like at the bottom end? Until I know what's causing the flat spot I don't know if I should go with heaver oil or lighter oil. Thanks!

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Michael, Santa Barbara, CA
. . . the sled, not the flower
https://www.facebook.com/PoserMotorSports/

Usually it's a lean spot but there are exceptions. I had a 1293 with twin HS 4s running #6 needles and Ramflo filters. I always thought it was a lean spot until Graham Russell put his AF meter on and we went for a drive. Yes it was going rich. Problem disappeared when I took both dampers out.
Just sayin'.....
From then on I ran no dampers in summer and ATF in winter.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

The answer is to experiment with various grades of oil from transmission fluid to standard multigrade engine oil and see if anything changes. Bike shops sell fork oil in single grades -10, 20, 30.