Depends on how far your timing is off. Chances are that youll just get bad mileage and have the diesiling, but theres always a possibility of burned valves. My advice, grab a glass of water, fill it and place it on your intake. when it stops shaking or shakes the least, thats when your timing is closest. that'll give you time to get another gun. Also, remember, #1 spark plug, not #1 in the firing order.