lot of smoke after rebuild, Help Please

Information relating to the Matchless G12 or AJS Model 31 650cc twin
maurizio66
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Joined: Sat Mar 26, 2016 7:34 pm
Location: italy

lot of smoke after rebuild, Help Please

Post by maurizio66 »

After the rebulid of the top engine of my G12 CSR 1965 we turned on the bike with the new JP pistons +020 bought trough the club, the bike started immediately but we noticed a lot of smoke, so the bike has been dismantled again and found lot of oil on top of the pistons, checked the clearance again ( is 4,5 thou ), valves are ok do not leak, clean all the oil passages, clean the oil pump put new gasket, is the one with the pressure release valve on the pump body, the gaskets, etc.. then build again cylinders and heads, but still a lot of smoke. The oil retourn in the tank seems ok just occasionally some bubble of air.

Drained the sump with the bike hot and turned on again for about 10 minutes, at the beginning no smoke but as soon become more hot a lot of smoke, constantly under power, drained immedately again the sump and found 170 ml of oil.
If Ileave the bike for a night I found in the sump about 380 ml of oil probably too much. I put a new relief valve and the chain case have the correct level of oil.

With only mounted cylinders and without the heads you can notice that the movement of the piston trough the kick starter pulls upward oil and the oil is going up past the pistons.

Several mechanics and machine man have found that the rings are of low quality and could create this problem, they don't have any sense of installation as a top sign or a corner but this may be normal. Even T&L enginering confirm that could happen problem with this kind of rings and sometimes with pistons too.

Now I'm looking for a different kind of rings to use, but is not easy as probably they need some modification, But your valuable suggestions could help me to understand if other problems different from rings and pistons like faulty oil system of the bike may cause this problem. Please help me, I have no expert people in my area who know this engine.

Thank you, regards, Maurizio
JEAN-NOEL
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Re: lot of smoke after rebuild, Help Please

Post by JEAN-NOEL »

Hi Maurizio,
I have experienced that with JP Pistons 20 years ago. The piston rings are taper, and one of them was fitted wrong way. There was a lot of crude oïl on the top of one piston. This problem was solved immediately with rings in good place.
What kind of oïl are you using ?
Oil in the top of the engine is one thing, but wet sumping is another problem.
Cheers.
maurizio66
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Joined: Sat Mar 26, 2016 7:34 pm
Location: italy

Re: lot of smoke after rebuild, Help Please

Post by maurizio66 »

Ciao Jean Noel,

the rings that come with this new set of JP pistons have no sense of fitting, we check also with a strong magnifier, no taper, no "up" signs, all 3 rings are absolutely the same in both side, this has been confirmed also from a couple of machine man before mounting, They said they are older type...I use 20/50 mineral oil. As you said I have more than a problem, but it will be important to find the way to fix first the excess of oil coming up the pistons...

Happy New Year at all of you,
Mick D
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Re: lot of smoke after rebuild, Help Please

Post by Mick D »

Hi

Are you sure the crank case breather is working correctly?

Regards Mick
maurizio66
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Re: lot of smoke after rebuild, Help Please

Post by maurizio66 »

I used a new relief valve, the one like a bolt on the alternator i have checked a couple of time the inspection plug of the chaincase and the oil rise the lower part of the chain, there is something else to chek to be sure the breather work correctly in the crank case and the chain case? As i have not understood very clear this concept, I can't find in this engine any hole in the case with a large rubber tube for venting that go to the filter box or along the mudguards like the triumph or other bike,
Thank you
petert120r
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Re: lot of smoke after rebuild, Help Please

Post by petert120r »

It is possible you have the wrong crankcase breather (the bolt you refer to). I had similar problems on a recent G11 rebuild. On examining the breather it had a ball and spring instead of the small flap, preventing the crankcase from breathing into the chaincase. If you blow gently from the threaded end of the bolt there should be virtually no resistance where if you blow from the opposite end the flap should close.
Groily
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Re: lot of smoke after rebuild, Help Please

Post by Groily »

The only breather (usually) is the one in the end of the crankshaft which vents into the primary chaincase.
Extra breathers can and have been fitted in various places by people - I have a big one on the timing cover of my 650. But it shouldn't be necessary.
It sounds as if your problem is probably the piston rings. If they are a slack 'up and down' fit in the grooves in the pistons, they will pump oil up, visibly, when you turn the engine even on the kickstart. (Could equally be a general quality problem though, I suppose.)
However, I am running JP +20 8.5:1 pistons in my own machine, supplied by T&L Engineering when they rebored the cylinders, and haven't had a problem I have to say. I'm very pleased with them in fact.

Wet sumping though - that's another thing! Not going there as there is a lot of information on this forum, along with debates on the use of taps in the oil line and loads of discussion of oil pumps, oil grades, metering of the oil to the cylinder heads, and so on. I use metering jets (in all 4 holes) on my twins.
The use of oil taps can get people a bit overheated sometimes - but I use them on three bikes, with ignition cut-outs on the magnetos, and haven't had a memory lapse or problem with the switches - yet!

A final thought relates to the 'priming' of the oil return pump, which is supposed to be done by a small drilling that goes through the pump mounting plate from feed to return. If the hole is blocked, there can be problems priming until the return pump is under the oil level, when it will kick in. The oil level in the tank will go up and down dramatically in a cycle as the pump returns and then doesn't, and there would usually be a lot of smoke when the level in the crankcase is too high (ie when the pump isn't returning the oil properly). Oil could then be forced up past the rings - but I don't think that is your problem as it would take a lot of kick-starting to see anything!

I have one set of pumps, modified with O rings and with an after-market brass pillar of a pressure release valve on the top of the plate, which has NEVER worked properly whatever I have tried to do to it. The return pump simply refuses to scavenge until half the oil is in the engine, despite nothing being blocked and all apparently working properly in the remainder of the lubrication system. When swapped for factory-original pumps, the problem disappears . . . . I have tried those modified pumps on three different engines now, and the same thing happens every single time. Bizarre - or I'm blind to something that should be obvious!
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robcurrie
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Re: lot of smoke after rebuild, Help Please

Post by robcurrie »

One more thing to check concerning oil build up in the sump - are the feed pipe and return pipe fitted to the correct pump and are the pumps fitted in the correct positions. The smaller pump is the feed and should be in the front and the larger pump is mounted at the rear and connects to the oil tank return.

There is a vent hole in the inner primary cover and is close to the hole where the alternator wires go through.

Rob C
maurizio66
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Re: lot of smoke after rebuild, Help Please

Post by maurizio66 »

I checked all parts as you suggested and it seems that everything is right, I think my problem is probably the piston rings, I'm looking for some 72.5 set that can replace them, in case any of you know a different kind it would be great.

The only doubt I have is about the amount of oil that I find in the sump, today I have drained again the sump than tourned on the bike for 5/10 minutes, the oil in the chain case was alwais at the same level, I have removed the filler plug of the chain case and noticed a good ventilation, drained again the sump and found 170 ml of oil about 0.29 pint, is a normal quantity or too much ? This amount could help the oil past the rings?

thank you, Maurizio
Groily
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Re: lot of smoke after rebuild, Help Please

Post by Groily »

If my memory is working, FW Neill's book refers somewhere to 8 fluid ounces - which is 0.4 UK pints, or just under 1/4 litre.
So 170ml after a few minutes running seems OK. I'd be happy with that anyway. There will actually be a bit more oil in the engine than that, but the captive anti-drain ball valve and spring in the big screw-in cap for the oil filter chamber (left side of the crankcase above the front of the primary chaincase) will stop most of it from draining back into the bottom of the engine, except over a much longer period.
If the chaincase oil level is staying constant, and the oil is coming UP from the pistons not DOWN from the cylinder heads, then I think everything does point to the piston rings, as you say.
I'm afraid I don't know where to get a good fresh set of rings, but the Club may have them, or someone like T&L Engineering - I bet someone here will know for sure.
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