Husaberg knocking when starting

Hi everyone,
We have been racing husabergs in enduro sidecars since 2005 and have had our share of problems but now we have a new one.
We recently had an engine rebuilt and running well for three days racing before being taken out to save for the main race of the year.
When we put it back in the sidecar a week before the race it knocked when started (kick or button) until you give it a little squirt of revs and then it wouldn't do it again until the revs drop right down when braking, then again a little squirt would sort it.
We took it out and stripped it down thinking it was the balancer knocking on the sides until a rev would make it run true? As we stripped it down we found the little stop had fallen out of the cam sprocket for the auto decomp, luckily no damage to rockers or cam. It didn't seem like it was related to the knocking therefore we kept looking.
We got to the balancer and found the bearing had a lot of play and the leading edge of the balancer had a burr from were it was knocking. Rebuilt it and put a spare cam assembly and it ran great for two and a half days with no knocking. But then it knocked the stop out of the other cam sprocket and went around with the cam and caused the inlet valves to meet the piston.
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We then put two new valves, cam chain and the original cam assembly with no auto decomp stop as people say to take them out and let the decomp stop against the bolts holding the sprocket on.
when we started it its making the same knocking it was before (only on start and very low revs as before)
We are starting to think that the balancer had nothing to do with it although it needed changing, could it be the fact that the stop on the cam sprocket is missing? or something completely different?
any advice would be very much appreciated.

Thanks very much for the replies, we will have a look at the decomp spring over the weekend, it does seem the most likely as its the only thing that's the same as before.
Time will tell, and I will let you know how we get on.