I have a 14 HP Briggs & Stratton OHV engine on a Toro zero turn mower. When I have the shroud off, the mower runs smoothly. When I reattach the engine shroud, even without having inserted the filters, the engine begins to run rough. Inserting the filters exacerbates the problem. I have new fuel, a new fuel filter, have readjusted the valves, with little success in improving performance. Any help would be deeply appreciated.