Replacing engine valves

[crankedup]

My vintage car (1928) has had bad misfire issues, all ignition has had attention and is fine, as is carburettor functions. I believe the problem to be cracked or burnt valve(s). My question is
if only one or two valves are found to be faulty is it sensible to replace just these?
Or should the whole lot be replaced along with the springs?
The engine has overhead inlet valves (4) the exhaust area side valves (4)
Thanks for advise and comments.

[Boosted LS1]

I'd wait until you inspect them before deciding. That said, this may be an opportunity to fit hardened valve seats along with new valves. Peter Burgess (on PH) is well known for this sort of work.

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Boosted LS1 said:

I'd wait until you inspect them before deciding. That said, this may be an opportunity to fit hardened valve seats along with new valves. Peter Burgess (on PH) is well known for this sort of work.

Thank you, yes I will see what they are like when removed later this week. Just wondered if one has gone the rest may well have done thier time, so to speak Same as springs, I wonder if it's a job worth replacing them, they work but I have no idea how old they may be.

[Boosted LS1]

They sound pretty ancient and I'm sure there will be a modern improvement. What car is it?

[227bhp]

They sound even older than Mignon.

[JagerT]

Mm,1928 that sounds pretty old ,I think you've probably had your moneys worth ,best scrap it and buy a new one on PCP or whatever they call it ,very cheap by all accounts.

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Boosted LS1 said:

They sound pretty ancient and I'm sure there will be a modern improvement. What car is it?

It's a Humber 14/40 saloon, perhaps the innards are original as the car has only completed 81,000 miles since new. I'm the third owner.

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JagerT said:

Mm,1928 that sounds pretty old ,I think you've probably had your moneys worth ,best scrap it and buy a new one on PCP or whatever they call it ,very cheap by all accounts.

[Allan L]

As has been suggested take it to bits and look.
We who have a pile of s/h bits under the bench are still inclined to just replace what's damaged, but there is logic to replacing more valves than the one(?) giving trouble as you start with them all in new condition and you don't have to dismantle it again for a while. Probably find it's an exhaust valve in which case don't touch the inlet valves!
Do not bother with hardened valve seats as the research showed VSR problems were in the high power/high speed range and a vintage Humber engine is not high-revving and most engines of any sort do not have to deliver their highest power for long periods.

[Megaflow]

I'd start with a compression test first. If that comes back as ok, then you might be looking in the wrong place

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Thanks again for advise and comment, I am having a compression test on Wednesday, although I have convinced myself that it is a valve issue. If they do turn out to be fine I am well and truely stumped as to what else could be causing the problem. I will post back when diagnosed the problem.

[PaulKemp]

The other possible issue would be rings and you can test for that by putting a spoon full of oil down the spark plug hole and testing again, if compression better it's a ring if not could then be valve

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^^^^^
Thanks, the engine does not smoke or burn oil but it's a good tip worth knowing.

[Boosted LS1]

PaulKemp said:

The other possible issue would be rings and you can test for that by putting a spoon full of oil down the spark plug hole and testing again, if compression better it's a ring if not could then be valve

Dessert spoon or teaspoon? Would honey help?

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^^^^^^*
:cough cough cough:

Compression test shows that one cylinder is down at 30% or so from others. Now convinced it's an exhaust valve burnt out or cracked.

[Little Pete]

crankedup said:

^^^^^^*
:cough cough cough:

Compression test shows that one cylinder is down at 30% or so from others. Now convinced it's an exhaust valve burnt out or cracked.

A cylinder leak test could give you an idea before you strip it down.

[Megaflow]

crankedup said:

^^^^^^*
:cough cough cough:

Compression test shows that one cylinder is down at 30% or so from others. Now convinced it's an exhaust valve burnt out or cracked.

Bugger. Time to get busy with the spanners...

[Allan L]

Little Pete said:

A cylinder leak test could give you an idea before you strip it down.

Sorry, but what could that tell you that the compression test hasn't?
Whatever it was, the 'head still has to come off.

[227bhp]

Allan L said:

Sorry, but what could that tell you that the compression test hasn't?
Whatever it was, the 'head still has to come off.

It shows WHERE the compression is being lost, not just that it is being lost 'somewhere'. That said, you'll still end up taking the head off!

[Little Pete]

Allan L said:

Sorry, but what could that tell you that the compression test hasn't?
Whatever it was, the 'head still has to come off.

I was responding to the op's comment regarding the exhaust valve. As 227bhp said, it can tell you where the leak is.