Sporty:

Do a careful check, be sure none of the rubber intake seals are dry, cracked, displaced, or leaking for any other reason. Also check the valve clearance, adjust all valves in a consistent manner so they are all as close to the same as you can get them.

Most flat twins, with all the adjustments right, run well.

If you still have an imbalance (that's what makes you lose one consistently) get an infrared thermometer and check the cylinder temps. Again, check each cylinder in the same spot, as nearly as you can. On the Saito twins cylinder balance can usually be reached by playing with the valve settings, set the cold cylinder to minimum clearance and the hot cylinder to maximum. This may be too much, but you will get the idea and can work from there.

But do be sure there's no other fault - you can chase it forever with adjustments when you have an air leak.

Bill.