EM003 MAC ENGINE

Yes the old smoking motor story - can anyone help me with this one?
MAC 1954 Eng no MAC20412 History- Top end rebuild about 6 years ago but to my knowledge the bottom has never been touched.
Symptom-
The bike has been unused for about 2 years, so the sump had filled with about 1 cup of oil in that time. After draining this and starting the bike the engine smokes for about 2 minutes then stops smoking and runs cleanly for appro another 15 minutes then resumes smoking quite heavily. After re inspecting the sump I find about 2 cups of oil unscavenged back to the oil tank.
I have been told the Velo pump is fairly reliable a gives little trouble, I have investigated all the simpler possibilities but to no avail. Checked and washed filters in the circulating system. Plenty of oil is getting to the top of the motor etc Checked and cleaned return lines to tank,no blockage found. Checked that the oil tank is not pressurising (and ran it with the cap off there is a constant flow) Checked if overflow was blocked or kinked. OK
Has the pump had it ?,and were do I find a replacement in Australia. or and I hope,could it be that the gear/shaft of the pump has warn a groove into case endplate and on heating up the gap increases far beyond spec (which I don't know) an the pump fails on the return.
If this last point is a common problem could somebody let me know,and would refacing the casing on glass with lapping paste solve my problem. - Graham White 13/6/2002

My thoughts are that the characteristics you describe do make poor scavenging the most likely cause.
it is quite possible that the return side of the pump has worn out, even though the feed side is still OK. This can happen if there has been damage or major wear inside the engine - it's the return gears that pass all the debris, which is removed by the filter before the oil comes back to the feed gears.
I think you'll probably need to completely overhaul the pump, but it could be mainly the base plate I think base plates are available from the dealers who advertise in Fishtail. I can't think why resurfacing the plate wouldn't be OK if you can do it to sufficient complete flatness, but so many Velo components have dimensions that are critical for stiffness etc that I'd hope for an informed view first. It would be much easier to just mail-order a plate! Velocette specified only that you should lap down the pump body until there is no end-float on the gears. They made it clear that there should be almost zero clearance of the gears in the body and zero end-float, and no scratches Perhaps someone will let you know where you can get a pump overhauled in Australia.
Three other reasons that your scavenge side may not be working:
The oil-pump housing may have been scored during a removal of the pump - this can have a major affect on scavenge efficiency.
There may be an air-leak around the suction filter plug, or even a crack in its housing.
The rod that goes down the oil filter chamber may have had too long a thread on its lower end. (A batch of such were made in stainless a while ago.) So the rod screws too far into the adaptor and blocks the hole for the oil to get into the filter chamber.
Otherwise, I'm afraid the prime suspect is indeed a worn out pump. - TW 14/6/2002