Anyone have any more suggestions?
Let's brainstorm....
What are the chances that ECU is bad? (then it wouldn't work at hig revs also...?!)
Too tight valve clearence? At the moment they are in spec (closer to the min. value) but maybe loosen them a little bit?
Crankshaft/connecting rod bearing f***d again? Symtoms are similar before i replaced the crankshaft/cylinder/piston but started suddenly and appear only when hot.
Decompressor spring too weak, so it starts to open exhaust valves at low revs? What are the chances of that happening? Decopressor looks working when moving it with finger but MAYBE somehow at low revs engages. How to check that?
Anyting else....?
