Hello,
I've recently bought a '69 A65 Firebird scrambler. Oil level was full. I did a 300 mile journey and it was only half down on dipstick. I left it for a week and it showed empty. I filled it up to full, after running it was way above full mark. Now after not using it for a couple of weeks it hardly shows oil at end of dipstick.
Is this the oil draining into crankcase? Should I fix something? e.g oil pump?
Any answers appreciated.

Oil is draining into the crankcase, so when you top it up, the oil returns from the sump and adds to what you've added!! First question is what oil are you using? then have you fitted or is there fitted a billet sump cover with oil drain screw?
Reason for oil type is that these engines were really designed for single grade oil ( say 40W or maybe 50W in hot climates ) and when multigrades became available BSA recommended 20W50 which is a 20W oil with additives that reduce its viscosity loss when hot. So a 20W oil will flow more freely under gravity than a 40 or 50W thus 'wet sumping' becomes an issue. But when these bikes were being ridden back in the day, they were used as daily riders so not much chance of excess oil in the sump with standing for long periods.
There should be a non-return ball and spring on the feed side of the oil pump which may need attention, or the pump is worn, or the sump scavenge pipe ball may need cleaning. If oil consumption is an issue, this could be due to excess crank pressure due to worn piston rings which will force oil through the breather (and potential leaks) or is it burning the oil due again to worn valve stems or guides or oil control rings?

Thanks for the reply.
I've finally got around to trying to sort things out. From a previous receipt it seems oil was changed with 15W- 40. I then topped it up with at least 1L with same grade. I drained oil at billet sump cover - loads came out, oil tank was empty.
The ball valve in scavenge pipe seems to work. At present I can't undo the 2 parts of non-return valve to check spring, I can see a shiny ball through gauze. I'll try and split the parts later with a bigger wrench.
The plan is to reassemble and fill up with 20W-50, see if oil still drains back to crankcase. If same problem, I'll look at non-return valve behind oil pump.

Not sure what you mean by [At present I can't undo the 2 parts of non-return valve to check spring, I can see a shiny ball through gauze.] The sump scavenge pipe ball should be easily pushed up with something like a small screwdriver then should drop easily back onto it's seat.  The non-return valve on these bikes is behind the oil pump. 15W-40 is too thin in my opinion.

Hi,

Good advice from Ian and I always try to remember to check my oil level after a ride when all the oil will be in the tank rather than after the bike has been standing with oil potentially seeping through to the sump.
If the tank has drained completely then the sump will have to be drained.  I would say two reasons for this are that start in with all that oil in the sump will be just about impossible and if it does start it will be trying to blow the oil out of the breather, past the rings and out of every joint it can find.  If the amount is not too much then. the pump will quite quickly return the oil to the tank as the the return side if the pump is about double the capacity of the feed hence a 'dry' sump can be maintained.

Dave.

Hello,
What I meant by "2 parts of valve" I should have called oil pressure relief valve below timing cover. This probably doesn't affect syphoning. I'm learning as I go.
I think I might have worked out why oil drained to crankcase. Bike's got a SRM sump cover. The gauze filter part had been assembled the wrong way, so that the scavenge pipe didn't go through hole in gauze but stretched  flat against  the pipe probably stopping ball fully closing.
I'll be filling it up with 20W - 50 and work it out from there.

Thanks for replies