I have found that at idle we only have 10 - 12 inches of vacuum in the inlet manifold and if we clamp off the crankcase breather we get 20 -22 inches and the idle smoothes itself out although it still misfires when revved up. We have checked the breather system with no fault found. The misfire tends to be around no 1 cylinder although does affect the others. We also have no fault found on cylinder leakage. With us clamping off the crancase breather, does this mask the real fault or is this where the fault really lie?