Low compression - Head off

[EddieDay]

Moving on from my previous post about low compression on my 1958 M18S, I bought a compression tester and tested it myself. The compression was still low so I decided to have a look. The bore doesn't look too bad. Some light marking but nothing dramatic. (See pics) I will lift the barrel off later in the week to have a look at the rings and piston skirt. I will also measure the bore and the lip at the top of the barrel. What does not look good is the colour of the exhaust valve. As shown in the other photo attached it is very brown. Does this look normal? Anyway I will take the valves out later in the week and look at the seats. Any thoughts welcome.

[1608]

Looks to be burning oil, probably due to worn rings and or bore. May just get away with a hone and new rings. Usually the brown valve head is the i/l valve. Low compression is either due to the rings and or bore, or the ex valve is carboned up and not seating properly.

[SPRIDDLER]

Hi Eddie. Good to meet you at the Jampot rally.

In addition to/in agreement with John's comments above ........

As you lift the barrel stuff the c/case mouth with rag before the piston exits the cylinder. You don't want a broken piston ring or errant circlip dropping into the depths.
Certainly it's burning oil rather than running over-rich. Is the oil metering screw for the inlet valve open too much? A fifth of a turn open is about right.
Before dismantling the cyl head I would check whether the valves have been sealing by filling the combustion chamber with petrol and observe if it seeps out past a valve.

Tip:
Rather than starting a new thread add new comments regarding progress to your original post. In that way he whole story is easily visible without having to scroll through to find out the timeline of the symptoms and what you've previously found and done.

LATER........
Browsing your previous posts I see that the bike is new to you and you've ridden it only about 100 miles. It's necessary to be circumspect about the condition of previously 'unknown' bikes....e.g. the zorst valve may be a recent and pre-used replacement from the P.O's spares box under the bench! The contamination of the cyl head isn't a recent thing.

[Harry44]

IMHO That exhaust valve is not sealing and the exhaust gasses going past it are causing it to overheat and produces that colour.

[EddieDay]

Thanks guys. I will definitely try filling the combustion chamber with petrol as suggested by Neville. Also I'll check the oil metering screw setting. But I do suspect that the exhaust valve is over heating as suggested by Harry.
I'll provide an update on this post.

[Expat]

Hi Eddie,
Had similar symptoms as you on my G3 which suddenly lost power and compression. Traced it to the ‘zorst valve not seating properly due to a build up of burnt oil on the stem or so it seemed. Found bore was undamaged but slightly worn, wire wound piston undamaged but very worn so was obviously burning oil, though didn’t seem to smoke overmuch. Am in process of fitting rebored cylinder, new piston and rings and new ‘zorst valve which was also worn on stem. Just completed thorough decoke, valve lapping to follow.
Hope this helps and you find and cure the issue with your bike. Good luck.
Steve

[EddieDay]

Thanks to Harry44 and Expat for your comments. Made some good progress today.
Head:
I followed Spriddler's advice and filled the combustion chamber with petrol. Although I couldn't see petrol in the ports I could smell it in the exhaust port and not the inlet port. The level of fuel went down 1/8" over 2 hours but some of that must have been evaporation. So I took the valves out.
(As an aside hair pin valves are new for me. All engines I have worked on before have had conventional co-axial coils springs. In the pub last night Duncan said he removes them easily by hand. So I had a go and - hey presto- they just popped out. The technique shown on this You Tube video helped. ).
The exhaust valve seat and sealing face looked dirty compared to the inlet valve so I am fairly sure it is letting by. (See photos). Also both valves are stamped "INL.VL" so it looks like someone has stuck an inlet valve in the exhaust port. I understand that they are the same dimensions but different materials. So I need a new ex valve. The only other issue with the valve gear is that the upper inlet valve spring is twisted so I need another of those too.

Cylinder and piston:
As mentioned in my earlier post the bore looks OK. So does the piston and rings. A bit of light scuffing on the inlet side of the piston. Duncan gave me another tip last night (I owe him a pint). I put the top ring in the bore about 2 inches down and measured the gap. It is 0.019". I can't find much info on this but I saw a post where someone mentioned 0.003". I assume that is for a newly rebored barrel and oversize pistons. My simple (but maybe wrong) thinking is that a 19thou gap is equivalent to 19/pi on the diameter, about 0.006". And that is for both cylinder and ring wear. I am thinking that a rebore is not necessary but advice welcome on this.
There is no discernible radial movement of the big end bearing but I could rock the con rod slightly on its bearing. Should there be any rocking motion at all? I am talking about a movement of a mm at the small end.

I can't see how to load pictures in this post so will add them on the next.

[EddieDay]

Ah. You have to select "Post reply" and not "Quick reply" to add pictures. Here they are.

[EddieDay]

And the twisted valve spring

[Expat]

The new ring gaps in my rebored and honed cylinder were between 0.007” and 0.010” as far as I could tell. The spec sheet I have says acceptable at 0.012, condemned at 0.030”.