Hello Mark Bruce, Eppa Wimbish and all others who followed my thread originally posted 4/25/23. Ihave just now finished working on my Coleman 550X, and it is now running and starting like new. It's problems were twofold, but I believe they all came from a bad air filter installation.
(1) We removed the MAF Sensor and it was very dirty -the opening was actually clogged. At the time, we also noticed a lot of crud in the Intake manifold. We cleaned MAF and the intake manifold, and we could then start it with using a small amount of pedal. It would run ok, but not perfect.
(2) Decided to check valve clearances and found absolutely no clearance on the intake, but the exhaust was a little greater than .005". We set the intake at .005 and tightened up the exhaust to just a little above .005. Then the engine started without any pedal and now runs like new.
Then I checked out all the air intake pipes between filter and manifold.- they were all filthy, so I cleaned thoroughly. I investigated the air filters and housing, and believe that sometime the housing lid was not closed properly. I do not like the housing design, because the lid has to be installed only one way and fastened down correctly in order to hold the filters securely in place. I believe it is virtually impossible to secure the clips on the lid with medium to large fingers, so I made a tool to do this. I believe this may have been the root of my problems. Unfortunately, now I have to worry about how much damage may have been done to the engine by it's breathing dirty air.