2000 MK2 misfire problem

[Oily-Al72]

Good evening, All..
I am a Triumph 2000 novice and would be very grateful for your thoughts on the engine on my car, a 2000 MK2, 1972. engine number ME60543HEBW
It has a misfire on possibly 2 cylinders and I'm hoping it's not going to be an engine out situation!
Because I want to drive the car to Le Mans for the Classic, it needs to be reliable, so the head is now off.
Could any members suggest why all the piston crowns are relatively smooth except number 4, which is like the surface of the moon? Number 4 is the cylinder that is down on compression, compared to all the others. The Engine is fitted with twin SU carbs

and has electronic ignition. Many thanks in advance.
Al Bentley.

[Alec]

hello Al,

the likely cause is some foreign body damage to the piston, but that won't really effect the running. If that cyclinder is low on compression and the valves are good it must be piston rings. That said how low is low?
Personally I woul take the valves out and have a close look at the seats, both valve and head. If you can eleiminate the valves as being the problem then it only leaves the rings. Another way is to turn the head upside down, with the plug in and fill the number 4 chamber with some thing like paraffin, it will soon be obvious if a valve is not sealing properly.

Alec

[Oily-Al72]

Many thanks, Alec.
The figures I was getting with the engine cold were around 150-160, apart from number 4, which was more like 90.
The unit is on +30's but all seems nice and tight and the bores are all undamaged apart from some very light vertical scratches.
It's all ready to dismantle now, but I'll try the paraffin test beforehand. I'm praying it's either a valve or head gasket, because removing the sump looks like an absolute B*****d of a job!
Regards, Al.

[Alec]

Hello Al,

if it was the head gasket it would, more often than not, affect two adjacent cylinders. It should be obvious from inspection as you have the head off.

I don't even remember if I have tried to drop the sump with the engine in place, I suspect it is possible although you may need to release the mountings and jack it up to remove the sump? (Alternative is to unbolt the cross memeber and see if it will drop sufficiently?)
Anyone else have any comments on this?

Alec

[Oily-Al72]

Hello Alec,
Thanks again. I think that with a cylinder head refurb and skim, we might be okay. I'd say that looking at the photos, it definitely looks a bit dodgy between 2 and 3 and 3 and 4.
I'll take it over to the engine specialist near me in Bicester and keep you posted.
Best wishes,
Al.

[Alec]

Hello Al,

the head face looks quite normal but the exhaut seat in particular do not look as if they are sealing well. I would just regrind the valves and do a leak test again.

Alec

[Oily-Al72]

Good Afternoon, Alec.
Thanks once again. I'll be in touch when all is sorted.
The inlet valves are very tired, so I've ordered a set from Canley Classics. The exhaust valves are in good nick, but as you say, the seats look very ropey.
Cheers, Al B.