Sigh, the valve seat in #3 is cracked. If you put in another exhaust valve, it will burn through again. The block and head , in the 948, both have design flaws, with too little iron reenforcing the exhaust seat an #3 cylinder, causing the seat to crack, and the cylinder to warp. The first can be fixed by cutting out the exhaust seat and installing an alloy seat. The second was somewhat fixed by installing a special piston that was made with an extra oil scraper ring at the bottom of the skirt. They are likely impossible to find today. Possibly resleeving it with a steel liner might work.
I haven't inspected the valve seat yet, but I think that head has hardened seats (there's an invoice in the paperwork). It's a 1098. It's happy enough with the spare head until I get the chance to deal with thus.
Old fashion advice,1 egg cup of petrol Redx down each spark plug hole leave over night.then make sure your your plugs are clean don’t put them in turn the engine over for a few minutes then put plugs in start up this will make a lot of white smoke loads of smoke rev up until clear and then a good run old fashion decarbonise, but it is only my opinion I use to do this on my old BMC engines from the 1950s and oil and filter change . Also the rule of 9 for the valves adjustment,and a egg cup of redx in the petrol tank will be a good idea now and then.
