Planenut
Indeed, the intake valve guide is cracked in three places. After cleaning the cylinder head, I noticed that the exhaust valve seat had moved 1 mm down the outer exhaust duct and destroyed the valve.
Bitsobrits
The engine was built by myself from a 117 cubic inch engine with about 500 km on it and a damaged crankshaft. I bought a new crankshaft, camshaft bearing, 618 cylinder, piston, cylinder head, large valves, lightweight rocker arms, throttle body, and everything needed to get the watts. Don't ask me who made the valve spring retainers and who installed the valve seats in the cylinder head foundry. All I can say is that the valves are of good quality, judging by the results. A few moments before, it was very quickly reaching 6500 rpm in fifth gear.
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