Hi Kats,
The pistons can bind if the domes are out of alignment. The way to check is to remove the plungers then lift and drop the pistons with your fingers in the throat on the bottom of the piston lifting it up and down. You should be able to feel any binding. The final calibration check is to lift both up to the top and release at the same time. They should "clunk" on the bottom at the same time. If not, it is possible that the margins have carbon or that the domes and pistons were mixed up at some time (or the springs are not matched or the nylon stop washer is missing in one). Sometimes you can swap the two pistons and the fall calibration will work better. Pistons and domes are matched in the factory but wear and mix-ups do happen.
Binding is usually caused when screwing down the domes. If the dome is clocked (rotated) too much from its natural position, or if one screw is tightened too much at one time (to tilt the dome toward it) then binding will occur. The trick to optimally installing a dome is:
1. Fit the dome w/o screws and rotate it cw and ccw while visually noting the stopping points. You can mark with a pencil. Usually there is ~ 1mm of arc in each direction from the center.
2. Insert the screws to their middle depth (ensure no pressure on dome flange) and repeat the rotating the dome to note the stopping points.
3. Turn all 4 screws down evenly and in a cross pattern (like lug nuts on a wheel) but again, do not tighten so that the dome can not be rotated. You just want the screws to apply minimal pressure and just kiss the flange so that you can still rotate the dome but this time with a slight resistance caused by the screws.
4. Rotate the dome while lifting and dropping the piston. Note that in the center of rotation, the piston will not bind. This is where you want the dome to be.
5. Once the dome is in the sweet spot, Turn all 4 screws down evenly and in a cross pattern (like lug nuts on a wheel). Be sure to give each only a 1/4 to 1/2 turn at a time. Continue lifting and dropping the piston to ensure all goes well.
6. After both domes are screwed down, do the drop test to ensure both fall at the same rate.
7. Add oil and the plunger then repeat the drop test to ensure both fall at the same time.
Finally, when considering an oil, the important item to note is what the damper does: It only slows transitions of the piston to smoothly meter fuel.
Here are the key transitions:
1. Engine vibrations: without the damper, the pistons would shake in the dome and wear it out the edges.
2. Road bumps: without the damper, the pistons would clunk up and down when going over bumps and dips causing the fuel flow to change radically in short bursts.
3. Acceleration: the damper reduces the fuel metering during acceleration by keeping the needle deeper in the fuel jet orifice. Another opposite effect of the delayed piston rising is the increase of fuel flow caused by the manifold vacuum creeping forward of the throttle plate to draw more air over the annular fuel puddle. This also increases the venturi pull of the fuel as more air is now flowing over the bridge and through the smaller opened area above it. The net effect is more fuel during the acceleration stage but not too much.
4. Deceleration: the damper increases the fuel metering during deceleration by keeping the needle higher in the fuel jet orifice. Another opposite effect of the delayed piston falling is the decrease of fuel flow caused by the manifold vacuum being stopped completely in the throat by the closed throttle plate. The result is less air to draw from the surface of annular fuel puddle as the venturi pull of the fuel is low (Less air is now flowing directly over the bridge and through the very large opening area above it. The net effect is reduced fuel during the deceleration stage but not too little.
Ultimately the damper oil you chose will only be noticed during acceleration. Thicker oil will enrich the acceleration mixture, thinner oil will lean the acceleration mixture.