Weak dashpot spring
A common cause of flatspots is a weak
dashpot spring. If the spring is too weak
the piston will not fall enough when the throttle is closed during gear changes.
When the power is re-applied, the piston will be too high and the air speed over
the jet will be too slow. The mixture will temporarily go lean causing a
flatspot.
Throttle hesitancy
There are 3 causes of hesitancy:
The oil in the dashpot is too thin/missing. This causes the dashpot piston to rise too quickly under acceleration and the engine doesn't see the rich boost it needs. The temporary leanening gives a momentary loss in power. This flatspot is more apparent when hitting the throttle from idle. Use a thicker dashpot oil.
The dashpot spring is too weak or the oil is too thick. This is 'gear-change hesitancy' and is caused by the dashpot piston not falling enough during gearchanges. When you go back on the throttle the piston is too high and the gas speed is too slow. This leads to a temporary leanening and loss of power. Use a stronger spring and/or thinner dashpot oil.
There is a problem with the ignition, like the plugs are scummed/carbonised or the coil only gives good sparks out when the system voltage gets high. Renew plugs/leads etc.