Yes, pinning the valves wide open will make it run like s**t down below about 8 grand.

You need to pull them out and have a look to see which part is worn and then try to replace with either new or good used bits.

The shaft was available separately on the MC18 (14211-KV3-000, R2J & R4J, 14211-KV3-770, R5K) but I don't know if it's still possible to buy it.

We don't have new shafts, and I don't have any used 18 valve/shaft assemblies that I can think of.

Maybe some other blokes out there can help.