Paul, any timing error must be an exact number of belt-tooth-pitches. The least the belt can jump by, is the distance from (say) the right side of one tooth, to the right side of the next tooth, so the belt pitch is approximately twice the width of a tooth. I suggest you measure that distance on either the belt or the camshaft sprocket, then compare it with how far out you think the timing mark is. If it coincides, I'll post details of how to re-time it. However the re-timing just consists of moving the sprocket in the forward direction by one tooth relative to the belt. It does not verify that you've corrected an actual timing error until you put it back together and repeat the timing check. If you retime it only to find that it was a measurement error and you've moved it from right to wrong, you'll probably be irritated. Remember, at this point you've said that you cleared the carburetor jets, including the tiny idle jet that you can't even see properly, let alone touch, because it is down inside the carburetor. That cleaning job can't be done with petrol and then looking or prodding at the jet, it absolutely requires an actual pressure can of carb cleaner and verifying that the cleaner squirts out from the central threaded hole the main jet screws into, when you remove the threaded plug above the idle jet and squirt carb cleaner down into the hole underneath it. If you dismantle the camshaft because you didn't clean the idle jet properly, you will not end up regarding this as having been a good day.
Please post a picture of the sprocket, belt, cam and rockers taken from "above" (that is, at right angles to the axis of the crankshaft, so the rockers and camshaft are seen in plan view) so I can explain the next bit.
