You can visually check the jet valves and see if any of them aren't returning to full extended height by removing the rocker cover. These things work, but when they don't they really mess the engine up. As SubG has stated, the jet valve system doesn't work on a Weber swap as the auxiliary intake galleries are blocked off by the base of the Weber carb. The jet valves can be removed and the eliminators can be installed without taking the head off. The other issue with jet valve heads are cracking around the jet valve in the combustion chamber roof. An iron/steel insert in the middle of the weakest part of an alloy combustion chamber that expands at different rates. Great idea...