One of the Stromberg SU carburators on my TR6 does not hold the oil I add through the damper. I recently had both carburators ovehauled. On one I feel the resitance when I insert the plunger. On the other I feel the resistance right after I add the oil but as soon as I drive the car. The resistance is gone. I'using damper oil on both. Some one suggested I'd use a heavier oil. Any suggestions?