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Rebuilt YS950S with quality kit from John at Midwest Military. Confirmed correct spring in accelerator pump. Replaced missing check ball and weight. New base gasket. Adjusted idle mixture (it's now about 2 turns out). Confirmed spark advance.
It seems to require choke for longer as it warms (not sure if that is related) and idle mixture screw is further out than before. Vacuum leak?
There is a flat spot, bog, hesitation when you advance the throttle quickly. Does not happen when you move the throttle slowly.
The A1 is much improved from where I started, but this remains an irritant and a mystery. Have seen several similar posts. Thoughts?

Post your Vacuum gauge readings and fuel pressure gauge readings attained since repairs.

Is the accelerator Pump spring on the CORRECT side of the diaphragm?

Is stumble only off idle?

Is tumble present with engine cold & hot?

Have you performed the propane enrichment Vac leak test yet?

Post photos of the two springs you removed from the metering rod and the accelerator pump diaphragms and the the two new springs showing which side you used each on (metering or accelerator).