If I start my bike up then the oil return is healthy for a few seconds but then reduces to the odd spurt, blowing big oil bubbles. If you turn the engine off, wait a minute and try again, then the same behaviour occurs. This would seem to point to the oil pump being OK, especially given that it is not wet sumping.
At first we thought it might be a problem with the return flow of oil but we're now beginning to suspect that it might be that the oil isn't flowing into the system quickly enough. Hopefully it is not the feed non-return valve as that means splitting the crank cases and replacing it.
There is a good description of the lubrication system on the Classic Bikes web site:
