EM004 MAC ENGINE

I have a 1950 MAC that recently underwent a complete nut-and-bolt restoration by a very reputable builder.
On occasion, the crankcase floods with oil, presumably involving some sort of problem with the return pump.
This only occurs after I have ridden very close to 22 miles (but again, does not occur in every such case), which would seem to indicate a possible mechanical distortion arising from thermal effects.
The problem appears to rectify itself when the machine is shut off and allowed to rest for approximately 5 minutes.
I have checked for obvious causes, such as air leaks in the feed and return lines, or any obstructions in the suction plug.
There are no identifiable oil leaks from the suction plug, the oil pump base, any of the feed or return lines, or any of the fittings associated with the oil tank itself. - Paul S. Greenberg 20/8/2002

First could you tell us what symptoms are indicating that the crankcase is flooding?
It could be that you are finding oil is flowing out of the drive-side shaft; or that the exhaust becomes very smoky, or simply that the oil level in the oil tank has dropped significantly....or any combination.
If it is simply that it starts producing copious blue oil smoke under the conditions you describe, then it is more likely to be a rocker-box drainage problem. The rocker-box might be gradually filling up with oil until it goes down the valve guides.
Oil pumps seldom give trouble unless an engine component has broken up, in which case it is the return side that gets damaged rather than the feed so it could indeed cause crankcase flooding - but consistently rather than occasionally as you are experiencing.
I know of are a few other potential causes of poor crankcase scavenging, but please can you confirm your symptoms first? - TW 20/8/2002

In terms of the symptoms that lead me to surmise that the crankcase is flooding, you listed the following possibilities:
"It could be that you are finding oil is flowing out of the drive-side shaft; or that the exhaust becomes very smoky, or simply that the oil level in the oil tank has dropped significantly....or any combination."
In fact, all three occur simultaneously. Although I have only recently become a Velocette owner, having reviewed all of the information available to me, I can only conclude that this combination could only arise via the accumulation of oil in the crankcase.
The actual cause, however, is turning out to be somewhat more elusive. As I noted, if you putter around and never push the revs, this situation is not observed. In his defense, the restorer never rode the bike very hard, so to his knowledge everything worked properly. This only seems to occur after a few minutes of fairly high revs. For example, while riding earlier this week, these symptoms recurred after riding down a lengthy hill at about 55 mph for a few minutes. After the bike was allowed to sit for a few minutes, the situation alleviated itself. Dave opined that a slightly tired pump would be consistent with the symptoms. Specifically, it would not have sufficient suction to reestablish prime after evacuating all of the oil in the return pump and the lines. In sitting for a few minutes, enough oil drains back into the crankcase and pump to reestablish prime.
While Dave's explanation appears sensible at face value (and he arguably knows infinitely more about all of this than me), there is a something in what he says that troubles me: if the return pump has sufficient strength to clear all of the oil out of the return path, why doesn't it have sufficient strength to reestablish prime? This promotes the notion that some other factor or combination of factors is occurring. I will also note from the responses I have received thus far, the pumps either work or they don't. Clearly, it does work, or at least it certainly seems to.  - Paul S. Greenberg 22/8/2002

With all those symptoms, I do agree that it is most likely that the crankcase is filling up with oil.
The most likely cause of this is indeed a worn out or damaged oil pump, or damage to the bore of the crankcase in which the pump needs to be a perfect shrink-fit.
The other possibilities are a suction filter with an air-leak (which might not actually leak oil), though I think this would more typically cause smoking after starting rather than after running for a while, or if your oil tank had a filter chamber (which it didn't originally for your engine number) then there are various ways in which it can restrict the return flow.
I think your first option is to have the oil pump and its housing checked by someone with appropriate experience. - TW 22/8/2002

With the exception of this flooding behavior that I have only observed at high rpms, everything appears to work properly. You could putter around town forever (i.e. staying below 40 or 45 mph) and never observe any difficulty whatsoever. Thus, if there is a problem with the return pump itself, it is perhaps a bit subtle.
While checking the oil level in the tank over the weekend, it occurred to me that the cap is not vented. The seal and the spring behind it seem like they are capable of sustaining a significant amount of pressure. I checked my Goodman Engineering parts list, and lo and behold they list two caps: one vented and one non-vented. I assume the vented version exists for a reason. It would be very easy to test this theory by temporarily replacing my non-vented cap with a rubber stopper with a hole in it. - PG 26/8/2002

Well spotted! I should have thought of that myself.
You do indeed need an oil filler cap with a breather hole, and I think the symptoms may well be as you describe if you don't use one. Alternatively, the return oil pipe might have blown off its union.
It is easy to convert your non-breather type; you just drill a small hole (about a 1/16") in the top, and a similar one in the brass base underneath - usually the one on top is half way between the centre and the periphery, and the one underneath is in the centre - TW 27/8/2002