Morris Minor Owners Club

I'm trying to track down the source of a new engine noise..our Morris had recently been getting hot and running on after a bit of motorway use, although not overheating.
Resetting the ignition timing and some lead replacement additive has stopped this, but there is now what I would describe as a harsh chuffing or knocking noise when driving, in particular once the engine has warmed up and accelerating. The engine seems otherwise to run ok and has 57k miles on the clock.
My first thought was to check the exhaust manifold and system, but this appears leak free. It doesn't sound like main bearing rumble or a big end rattle...but I may be wrong.
however I have memories from the dim and distant past of hunting for worn bearings on another A series engine only to find that the head gasket had failed between the centre cylinders, would this be a likely culprit? Other than whipping the head off or buying a pressure gauge, is there an easy way of checking?
many thanks
Dave

Is the car down on power? A burnt exhaust valve would mean the engine is only running on 3 cylinders and therefore ~25% down on power.

Head gasket failure between 2 and 3 would mean only 2 out of the 4 cylinders have compression and the car would be so sluggish and sound so bad that you'd not want to drive it.

If it gets up to motorway speed, it could just be a burnt exhaust valve. Try removing the plug leads one at a time when the engine is running. The one that makes no difference is your culprit. Then turn the car over on the handle (ignition off!) if one compression feels easier than the others then that means, get the head off you have a burnt valve.

Cracked piston or block is possible but much less likely on an "a" series.

First check is - valve clearances! It may be one or more of the exhaust valves have 'recessed' slightly - simply re-setting the clearances (I use 15 thou on exhaust exactly because of this possibility) will cure the problem - but you may need to pop the head off and regrind the valves. Simple compression check is with the starting handle - ignition OFF - slowly turn the engine over and feel if all the compressions are much the same. Proper compression check is the best diagnostic - and these gauges don't cost that much - see ebay. As above - if gasket is blown the car is barely driveable.