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[Haggis]

My 53 Matchless G3LS seems to tolerate unleaded pretty well.
Doesn't do huge miles but valves and seats have been fine over the last few years and 10k miles.

[Jakub]

Development of today. Have taken head to the workshop, they looked at the valve steams, tolerances, valve heads and seats and decided there is nothing seriously wrong with them. As mentioned yesterday I have filled exhaust duct with kerosene, there was not a drop on the other side.
So I have installed the head back, no exhaust, and run the engine. On idle it’s not bad but the very second revs are up the exhaust duct becomes wet. Can’t see where it’s coming from but it’s coming. Checked compression again it’s solid 7 bar on both cylinders.
Mystery continues.

[Haggis]

Could be worn rings or piston?
Have you checked the compression wet or dry?
Sloppy ring grooves will give the symptoms you describe.

[Jakub]

Could be worn rings or piston?
Have you checked the compression wet or dry?
Sloppy ring grooves will give the symptoms you describe.

I am seriously considering it. But there is one clue missing. Smoke.

Also, the compression is identical on both cylinders but the other runs as it should.

I did experience some broken piston with some other motorbike, it was worse than two stroke. Lots of blue smoke. Then again, now when I am thinking about it the engine was running, didn’t feel anything unusual with kick-start so compression must have been not so bad with seriously damaged piston.

Anyway, back to the garage, will inspect piston and the rings.

[Jakub]

Piston and cylinder out, the clearance on the rings on top of the cylinder is 0,8-1,2mm which is close to the maximum limit set by manual of 1/16in. Don’t know what to make of that. Seems pretty big.

[Wheaters]

Sounds like the top of the engine is pressurising. Maybe the easiest escape route for the excess gas is down the exhaust valve guide and it's pushing some oil with it. What breather arrangements does the engine have and are they definitely free of any restriction?

[Jakub]

Sounds like the top of the engine is pressurising. Maybe the easiest escape route for the excess gas is down the exhaust valve guide and it's pushing some oil with it. What breather arrangements does the engine have and are they definitely free of any restriction?

It seems previous owner did have some own ideas about breathers, as it is now I have three separate lines with free outlets to atmosphere, from below left cylinder, from top of oil tank (I understand originally those two are connected) and from oil filler.
All checked with compressed air, all do work.

[Daiwiskers]

Possibly a long shot but oil getting past the barrel / head studs then working in past the head gasket ? Could it be getting past the base gasket then up the stud tubes in the barrel or down from the head

I take it compression test is showing over 90 psi as that's where I feel it's rebuild time

Was that advertised on here perhaps previous owner could point us in the right direction

[Meteorstorm]

The engine may be suffering from over pressurization,in my experience that breather will only be an air pump, I've found the best breather for the twins is the outlet under the left cylinder needs changing to the 10mm outlet type and run that up through a Crank Vent or Reed valve to a Indian Bullet breather box fitted out of sight and the breather return can go back to the oil tank,it's the same oil that came from the tank,just recirculating.Sounds like that setup has 2 outlets from the oil tank.
Still it is a bit weird,as you would have noticed oil in the cylinder if that was the cause,maybe it's a crack in the rocker box?

[Jakub]

Summary for now and some time to come.
Once I have taken both barrels out it became possible PO was working with the problem, there were different head gaskets. Also there is 0,13 and 0,17mm clearance between piston and cylinder. Workshop suggested new pistons and rebore. So that’s the plan for now.

The motorbike was purchased in Germany from the estate of the elderly gentleman. I have received some old invoices and notes, no more knowledge available.

Breathers – I went through numerous pictures over the internet, there are different setups but nowhere I have seen anything similar to mine. I guess previous owner or owners did experience some excessive overpressure and tried to find a remedy.

Finally, I have an question to the twin owners, is it possible to open oil cap with engine running? I did try and there was oil splash all over.
As it happens I own two completely different bikes both also dry sump ones. Opening oil cap when running makes no problem whatsoever.