G12 cs oil consumption

Information relating to the Matchless G12 or AJS Model 31 650cc twin
Random
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G12 cs oil consumption

Post by Random »

Hi all, I’m Barry.
I. Have a G12 cs. I’ve had it around 4 years and spent a vast amount of time and money rebuilding the engine.
The problem I have is with oil consumption creating a lot of smoke from the exhaust. Although it has a 2 into 1 pipe, the piston and combustion chamber shows greater oil burn on the left hand (near side) cylinder.
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Duncan
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Re: G12 cs oil consumption

Post by Duncan »

H Barry, welcome to the forum a quick couple of questions that might help others respond:
Has the oil consumption happened after working on the bike (i.e. a rebore) or has it always been like that, how many miles have you done since?
What work has been done (i,e, rebore, new pistons/rings, valves and guides. does the head have metering plugs, etc)
Does the engine wet sump?
g80csp11
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Re: G12 cs oil consumption

Post by g80csp11 »

what year is your bike , there were a number of oiling changes over the years on the G12 engine that can effect oil consumption. did you rebuild the engine yourself
SPRIDDLER
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Re: G12 cs oil consumption

Post by SPRIDDLER »

Random wrote: Thu Dec 02, 2021 2:06 pm ...........combustion chamber shows greater oil burn on the left hand (near side) cylinder.
Welcome, Barry.

Here's a link to an article (c/o 'Groily') detailing year by year changes on twin lubrication:

https://www.jampot.com/article_read.asp?id=576

Here's just an extract re metering jets from the comprehensive article in the link given above and also a ref to l/h cylinder over-oiling..............

Lubrication changes Twins 2.jpg

There are also details of all the year by year changes to Twins in the link below:

https://www.jampot.com/article_read.asp?id=20
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Random
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Re: G12 cs oil consumption

Post by Random »

Random
Thanks for reply’s and interest
Work done rebore.new pistons 8.5 to 1.valve seat inserts.valves.valve gides.valve springs.rocker spindles and bushes and
much more. bouth heads have meatering jets yes it wet sumps don’t they all. Engine sounds sweet and runs nice but for clouds smoke. Striped top end and can see nothing wrong lam at a loss. engine number 60/g12csx4038
Kind regards Barry
Groily
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Re: G12 cs oil consumption

Post by Groily »

If it IS a drive side only problem Barry, then could be the barrel spigot thing, but 'clouds of smoke' sounds a bit more than that might cause? Does it get so oiled up that the left hand side stops running after a bit?
Did it smoke before the rebuild, (if you know, that is)? How many miles has it actually done since you assembled it?

If we are truly talking one side only, and assuming you are starting with a drained-empty crankcase, not a half-full wet-sumped one, then it can only really be a cylinder / piston / rings / valve guide problem I'd have thought. A missing metering jet might make some difference, but not, I think 'clouds of smoke'. (I always put jets in all 4 holes, I think in some cases the book says three, and then there was the 'no jets' period which wasn't too clever.)

Are the rings a correct fit 'up and down' in the grooves in that left side piston? There are various wrong combinations possible. If they float in their grooves, they'll act as a pump and quickly cover the piston top in slime and there'll be smoke billowing out everywhere. Got the T shirt for that in the past . . .

On one engine, I had a terrible problem with smoke one side only and couldn't find the reason. It turned out after umpteen goes at it that there was a pinhole deep within the inlet port on one cylinder head, and oil was literally being sucked in via the adjacent pushrod tunnel. Highly unlikely and quite improbable etc etc - but that was what I thought too, until I found it. Another T shirt . . .

If the problem is actually a 'both sides' thing, then are you happy with the scavenging? After a run there shouldn't be more than about 8 fluid oz of oil in the engine when it's settled. Does the oil level in the tank behave consistently (before any wet-sumping happens when it's left standing for a while)? (Have to presume the oil pumps aren't reversed by mistake, too . . .) I also once had problems with a return pump (modified set-up, not factory original) not wanting to kick in correctly, owing to blockage of the narrow priming channel from feed to return pump in the mounting plate. The oil tank level went down and down until the timing chest was almost full of oil, when the return pump kicked in and the oil level in the tank was part-restored - until it repeated the cycle. Loads of smoke . . . Yet another T shirt.

These can be quite contrary motors sometimes, especially when we don't know to what extent the parts have always lived together in harmony, but when sorted they really can hang together well for a long, long time - so don't depair!
g80csp11
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Re: G12 cs oil consumption

Post by g80csp11 »

could you have piston ring fitted upside down on one side ? , usually they would be marked T or top or even a “pip” mark or dot fitted upwards depending on the manufacturer of piston and rings . they also need to be correct for the piston . for example often modern pistons have metric rings and original pistons imperial. were they supplied with the pistons
electrajohnt
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Re: G12 cs oil consumption

Post by electrajohnt »

I had this trouble, turned out I had broke the oil ring on the r/h piston when assembling. Finished up with new rings in both barrels. Problem solved.
JohnT
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Random
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Re: G12 cs oil consumption

Post by Random »

Random
Thanks again for thoughts on my problem
Clouds of smoke was an exaggeration but more than you want or expect.oil pump returns strongly.rings fitted correctly.have managed to do 250 miles since rebuild but not happy miles because of smoke .today received aluminium spigot thing (which l know is called an oil distributor) on inspecting old and new there was .005thou wear could this possibly be the problem
Kind regards barry
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Re: G12 cs oil consumption

Post by Groily »

Daren't rule anything out Barry, but think wear on the distributor bush is probably a stretch as the reason. Could change it and see though, now you have a new one. Daft not to!
I've had quite worn and grooved distributor bushes now and again, both the sorts with flats and with holes, but they didn't cause anything I could ever really grumble about. Certainly not like a knackered inlet valve guide or piston & ring trouble, anyway.

Just thinking at the keyboard now really -

but if you put the compression rings from the offending side squarely into the cylinder one at a time, and hold the whole thing up to a bright light, do they let any light past apart from at the gap?

Are you using copper, or compound head gaskets? Is there any sign of oil crossing from an oil way to the cylinder? Copper ones especially without O rings can weep/seep a bit (but then I imagine the mating surfaces were all checked out when the rebuild was done).

Or could one head gasket be slightly proud on the inside edge adjacent the other cylinder? Sometimes (often) they need a good trimming to prevent interference with their fit. But again, I imagine compression tests good . . . so it's a longish shot. But if we're still talking one side only . . . has to be something to do with that barrel, head, piston or valve, is what I keep coming back to thinking.

Clutching at straws here probably, I know . . .
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