Stihl 028WB Chain Saw Ė Starts but wonít run - ďNozzle Check Valveď?
Can you please help diagnose this problem?
First the problem history:
Been cutting fine for 2 weekends. Then, after 30 minutes throw the chain off. The engine revs high for just a second. I shut down, re-fuel, re-oil, put the chain on. (Same fuel and oil as Iíve been running for weeks.) Saw fires, but wonít run.
I pulled the air filter and itís a mess. I brush it off and reinstall - same performance. I try to run it without the filter Ė same performance. I splash fuel in the carburetor throat Ė runs a few second longer. Remove the plug and check spark Ė looks strong and blue.
I ordered a Tillotson HU40B carb kit. Replaced meter needle, pivot mechanism, diaphragms, gaskets. Removed/replaced Welch plug and filter screen after a thorough jet cleaning with carb cleaner, but not compressed air, oops. I re-set the H/L screws to factory 1 ľ settings (I hope I did not crash these into their seats.) I did not mess with brass plug.
I removed and inspected impulse tube and intake manifold. I can not find any defects, but I have not inspected with specialized leak finding techniques. No giant holes here.
Compression seems OK Ė same as ever, but I have not measured. Some old dried oil spun radially outward from crank bearings. Iím sure these need attention, but I donít want to go there if I can help it. Running a new OEM spark plug now.
Reassembled the saw except bar and chain and basically have the same performance. With lots of pulling, fires and runs very slow and rough for a few seconds @ WOT only. A fine mist of fuel sprays out from carburetor throat. Oil/Fuel is coming out of muffler. I tweak the L screw closed some, then open some, also close H screw completely and canít perceive a change in performance.
I think that covers most everything. Do I need to press-out the brass carburetor plug ďNozzle Check Valveď? Apparently that may influence idle.