55 Matchless G9 primary chaincase

[Oggers]

Gents

Right, I think I have bottomed out the root cause of this problem! Chaincase still leaks - only drips - but annoying nevertherless. What seems to be happening is that the chaincase is filling up with oil, reaching too high a level, and leeching through either the clutch cover gasket or the filler hole gasket.

So where is the oil coming from?  I can think of only two places.

1 - Wet sumping leeching from engine/crankcase when laid up. However, as a precaution against this, I remove the sump plug after a run, let it drain, then replace the oil before a run. Thus I presume it is not possible for the oil to build up in the sump and overflow into the chaincase.

2 - Breather valve u/s. I have pulled the outer chaincase cover and it seems fine. The Maltese cross plate thing looks fine, but I guess it may have been in a permanently open position allowing oil/condensate into the primary chaincase and thus filling it up. Is this feasible? Spring holding the plate looks fine.

Once again, your thoughts are very welcome.

[mini-me]

oil in the case comes via the crankshaft;
either you breather is not working,
your pumps are not scavanging properly,so too much oil inthe cases
or your bores/rings are knackered allowing gas past.

I'd plump for the usual suspect,bores.

[Oggers]

Mini- thanks for that. Engine newly rebuilt, no smoke, rings renewed, seems tight. I am leaning to one of the other two.

[chaterlea25]

Hi
With a newly rebuilt engine the rings may not yet be bedded in ?
Recently I came across a similar issue with a G12, it turned out the bores were not true/square with the cylinder base
 
The owner has had the bike since the early sixties and had not had it rebored in his ownership ?
I believe the previous rebore was done using the head face as reference rather than the cylinder base

John

[iansoady]

I can't quite see why bore wear should lead to oil in the primary chaincase unless the breather isn't working. Admittedly it will increase crankcase pressure but there won't be any more oil in there than normal.

How much oil comes out when you drain the crankcase, and have you tried doing it again after the bike has stood for a while to check it's not wet sumping?

[Oggers]

Chaps

It is wet sumping when laid up. To stop oil leeching into the chaincase when it is laid up, I remove the sump plug. So, it seems to be oil build up in the crankcase when running and thus leeching over to the chaincase.

Either breather is stuck open, pump scavenge problem,blocked foam filter, scavenge line blocked?

[iansoady]

Do you see a decent return to the tank when it's running? I don't know the AMC twins but my G80 return was very sluggish.

[mini-me]

Oil pumps then
You'd be best off on the AMC forum where they have extensive experience of these pumps and their many foibles, and how many arms and legs you'll have to sell to get recon ones

[john.k]

I suggest you get a Norton single,or a twin.......then you will never complain about any other brand wet sumping.A Norton will often dump the entire contents of the oil tank overnight.....Back leakage thru the feed section of the oil pump.The common cure is a tap,but a much better one is a special demand valve fitted to a Yamaha outboard motor......unfortunately Ive forgotten the part no,but no doubt someone here will know it.......but last I heard Yamaha was back ordered on this valve due to unprecedented demand.........which they were at a loss to understand..........until they discovered the valves were for british bikes...........