4-mix troubles...
I've been working on a friends FS90 for the last few weeks and it is becoming my bane...
Two years old, he claims to have only used stihl HP Super, although I told him a number of times to go with synthetic.
That aside, it was acting like the carb was out of adjustment or it had an air leak. Compression was down to about 90. I bought some decarbonizer and valve gasket and guage kit. Did the decarbonization process twice and then adjusted the valves. Had the compression up to 120 and it was running fine. I then put a new spark plug in it and it wouldn't run well at all. checked the compression and it was at 40!!!!
I just bought a harbor freight leak down tester (spark plug adapters dont fit!!!!) So my next step was to tear down the engine.
The inside of the crankcase was littered with chunks of carbon, the valves were still coated although the exhaust valve had what looked like clay on it (burnt carbon) I pulled the exhaust valve out and the seat was dashed with spots of black carbon, obviously the problem. I should have taken a picture before I cleaned the valve, what a mess!
Anyone have any comments? Whats the best way to clean the carbon out of the cylinder? More decarbonizer? Thanks for any help, this is my first 4-mix teardown!