Oil filling crankcase

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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Oil filling crankcase

Postby Dale » Mon Oct 28, 2019 10:32 am

I have just completed a year-long full restoration of my 1961 AJS 31CSR. When I turned it over with the kickstart, oil was returning to the oil tank so I decided to start it for the first time since rebuilding the engine. It started fine, but then began to smoke very badly and the oil returning to the tank was full of bubbles. Also, the oil level in the tank was dropping at an alarming rate. I drained the sump of the oil that had been lost from the tank and tried again to start it. Unfortunately, the same thing happened.
I have now removed the oil pumps for inspection, but can find nothing wrong. All the oil ways in the engine were checked for obstructions when I rebuilt the engine and again now I can find no foreign matter in them. The engine has had a rebore, new pistons, valves and springs and the seats recut.

Can anyone help as I am at my wits end!

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Re: Oil filling crankcase

Postby JEAN-NOEL » Mon Oct 28, 2019 10:50 am

It looks like the pumps have been reversed : looking at the engine, timing side, the narrower (pressure pump) must be on the right, the wider (return pump) must be on the letf.

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Re: Oil filling crankcase

Postby Dale » Mon Oct 28, 2019 12:28 pm

Many thanks, but that is the first thing I checked. I have previously rebuilt two twin engines with no problems.

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Re: Oil filling crankcase

Postby Groily » Mon Oct 28, 2019 1:21 pm

Is it possible the scavenge pump isn't priming until the timing cover is full of oil, the oil tank is half empty, and the smoke out the back is impenetrable? Could be worth looking at the oil pump plate to make sure the tiny bleed from feed- to scavenge- side is clear? Just a thought.

That said, I've had this problem with a set of the improved pumps with the brass prv assembly on top of the pump plate. No matter what I did, the thing wouldn't scavenge properly and behaved like yours is, every time, although older standard pumps worked fine on the 2 engines I tried them on. Bleed passage clear, so not that. Both pumps really nice and near-new, not them. Was it, could it have been, a combination of unsuitable pressure relief valve bits (given the different versions, captive, non-captive, non-existent) . . . that was my final unthought-through notion before giving up and leaving them to one side! (At the moment, the pumps are with one of the well-known restorers listed in the Jampot for any additional ideas.)

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Re: Oil filling crankcase

Postby Dale » Mon Oct 28, 2019 2:35 pm

Thank you for taking the time to reply. I've just taken the pump plate apart and checked all the oil ways including the ball and spring relief valve and the tiny bleed holes. Everything is clear, as I knew it would be because I took great pains to assemble this unit correctly when I did the engine. I know the feed pressure is right because after removing the rocker covers, they were all nicely oiled. The oil is getting past the rings into the combustion chamber and blowing out of the exhaust port which is causing the smoke. With new pistons and a rebore, it needs to be run in to bed down the rings, but I've never had this problem before. Could it be a faulty breather valve on the engine sprocket nut? Oil is not filling the primary chaincase, but if it is not letting the pressure out, could it, in theory, cause the problem?

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Re: Oil filling crankcase

Postby Groily » Mon Oct 28, 2019 5:06 pm

You're well ahead of me here Dale I think!

Others will be better able to comment on the flapper valve side of it. Should be possible to feel the valve working with the engine running I think? -but I suppose you could take it out and see whether the smoke stopped (and the floor got oily)? And whether the scavenge operated correctly. Or you could take a rocker box lid or two off maybe, to provide more venting? Doesn't quite stack up for me though . . . There's meant to be what?, about 8 fl oz in the crankcase after a run I think, when it's drained down from wherever. If there's loads more in no time flat, tis not right. Would poor breathing cause the excess oil in the cases?? Maybe it could, though I can't see quite why - but I'd expect the excess pressure to cause oil leaks as well as blowing oil past the rings, in weak areas like barrel base joint, timing cover, maybe crankcase halves even. And regardless of any breather problem, wouldn't the return pump pick up the excess same as usual, perhaps with extra gusto as excessive pressure down below added its own force to the suction?

When I was struggling with my Mod 31, the oil tank would half-empty fast as you like after start-up (a quart gone say), no return at all, and smoke would start very soon as the pistons slung it up and out. The oil level in the timing case would then get above the level of the return pump and it kicked in. It would scavenge like billyo for a few minutes, no bubbles, just a massive return flow, the oil tank would refill nicely, and the smoke would clear completely. Then it would do it all over again - oil tank draining in front of my eyes, lots of smoke, then scavenge, tank refilled at a rate of knots, no smoke . . . Repeat ad nauseam. Fitting other pumps fixed it immediately, although like you I could see absolutely nothing wrong with the pumps on there, or the oilways, etc etc.

I still scratch my head thinking about it, because I then tried the pumps on my 500 just to see what happened - and exactly the same thing all over again. I didn't understand it then, and I don't understand it to this day. But both engines have done thousands and thousands of miles since those experiments, with nary a problem.

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Re: Oil filling crankcase

Postby Bird dog » Tue Oct 29, 2019 8:00 pm

Hi After reading all of the posts above which all have sound reasoning to them, my thoughts are focussed around the possibility of debris covering the return line from inside of the crankcases.
My thoughts are to try and prove if the return line is blocked from the inside of the crankcase.
Firstly remove the scavenge pump body from the aluminium carrier plate to uncover the return line orifice, or better still the alloy body complete to inspect the oilways beforehand, you may then need the methodology below.
Devise a method to pull a good suction up and along the return line such as a syringe with a flexible tube or rig up a Hoover with small diameter piping perhaps or an air line across the face of the outlet as if it were a venturi, avoiding blowing air down into the oilway.
If no foreign matter is forthcoming I would then dribble oil down the return line to gauge the length of time it took to drain away, with this knowledge one could reasonably assess if the return was indeed restricted.
After assessing that the return is indeed blocked I would find something to poke into the oilway ensuring that you go all the way past the entry hole within the cases, then try and fill it with fresh oil, this should prove your position if it drains as you are filling it.
If you now realise that you have a major problem with sh1t in the lubrication system you can try flushing the cases out using paraffin poured down the return line after you have removed the drain plug and placed a catch bowl beneath the engine.
I do hope that your luck is in and you do flush the muck out by using this method.
Regards.

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Re: Oil filling crankcase

Postby Taid » Tue Oct 29, 2019 8:38 pm

I don't claim to know anything about the internals, and this may have been covered in other words in earlier posts, but the first post says the return is full of bubbles, so it would appear that the scavenge is sucking air easier than oil, so is there a leak on the input to the scavenge pump?

My apologies if I'm talking rubbish ..
Nostalgia taking over ..

Currently ..1957 AJS 16MS .. Restored/registered 1987

Previously ..
1951 Sun Villiers 125 (1951-53)
1955 AJS 16MS (1955-56)
1952 Vincent Comet (1958 - 84 .. Not ridden after 62)

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Re: Oil filling crankcase

Postby Dale » Wed Oct 30, 2019 10:29 am

Many thanks for all your suggestions. However, when I rebuilt the engine I checked all the oil ways meticulously and everything was clear on reassembly.
Yesterday I fed a length of outer boden cable down the return oil passage and found it to be clean. I haven't tried to start it again yet as I am waiting for more oil to come.
I've also fitted new gaskets to the oil pump body. If the problem persists, I shall have to rebuild the engine again.
Regards,
Dale

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Re: Oil filling crankcase

Postby dave16mct » Wed Oct 30, 2019 10:41 am

Have you got another oilpump that you can substitute to try and pinpoint the problem?
Dave.

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