I'm embarrassed to say I went down the wrong diag road! It was friggin hot, was being attacked by wasps, and figured it was something simple. This is a little 5000w with the 389 engine. We use it to run a water pump, but very frequently. Tons of hours. Everything up and running good and then that engine started to stumble really bad. Sounded like running out of fuel. I've had issues with water in the fuel lately so figured I would drain the bowl and we'd get going again. Nope.... I ended up pulling the plug to see it was bone dry....leading me to a fueling issue. It has the shut off solenoid on the carb so I am doing my testing and with switches on, no power going to that solenoid. So I chase that around for hours with no schematic, just a lot of electrical experience. Got all the way down to a little rectifier that certainly was not pushing power. Again no parts desc at all so I broke open the module so I could learn what it was. Indeed, just a bridge rectifier, no regulator, etc. It's used only for that fuel solenoid. After seeing the circuit, I could then meter test the module and it was actually fine. I was pissed because I had more hunting to do. Then I realized that solenoid was NOT to get power at start up, but actually during shut down! It uses power on coast down to hold that thing shut down to a very low rpm I would say. In any case, chased my damn tail! Then and only then do I get back to basics, verified the puny little sparky, but also realizing no fuel on that plug at all, yet it had compression, no noises, etc. I pulled the air filter and bam.....friggin pumping air right out the 'in hole'. The mechanic in me just wants to know, but I know I should not waste more time on it. I am thinking broken valve spring, slipped cam time somehow, etc. Very odd that it has compression. Maybe I will regroup and at least pull the valve cover. Anyone seen such a thing? Only times I'v run into this was on OHC engines when the chain slacked out. Common on our high output race engines. But that would tag all the valves and dump compression. So I guess.....the compression part is baffling me. Like I can slowly pull the rope right up to compression and feel it is good. So odd....but that breathing out of the carb is a guaranteed event.
If a push rod or rocker breaks, you will get a perfectly good compression reading but no run. V-twins do this, exhaust breaks on one cylinder, puffs air back through the carb, I think it makes it too rich traveling through the carb 3 times, will not run on one cylinder. If you lose spark on a v-twin it will run almost normally, except it won't pull a load.
I have found a few rocker bolts backed off that causes either no air in at all or blowing air back out.
Ha, another plot twist.... So I couldn't leave it alone and realized the valve cover was as simple as it gets....one bolt. Problem is I saw nothing out of place. Mint.... So I dug deeper on the intake too confirm what I felt and realized the crankcase breather tube dumps right next to the carb mouth and it wasn't air blowing out of the carb, but out of the breather tube, and it high qty. Pondering on the list of possibility, I'd put 90% on a blown head gasket. Broken rings just wouldn't come without other symptoms. I find it odd that a gasket would blow out of the blue like that so maybe the head cracked, but it's a lot of air so a barn door opened and quickly. Might have to put that one on the list for later as I sort of want to pull the head and see what 10k hrs looks like in a China engine.
We are patiently waiting for you to tell us what it is
Well.....I debated if I should embarrass myself and share the conclusion..... Friggin fuel contamination! I guess I was right, it wasn't getting fuel....ha So my sniffer has never been right after covid so gas smells different to me. But after finding virtually nothing wrong, then and only then did I decide to bump the intake with carb cleaner. Fired right off. Then I knew I was indeed on the hunt for a fuel issue. Having nothing to go on with the fuel solenoid, I chopped it, and to my surprise, it still wouldn't run. Went a bit on instinct to drain the carb and put known good fuel in that is running fine on something else. Ran like a top with what was in the carb, even took the hit from the pump without issue. So I could a 1qt sample out of the tank and I have all sorts of crap there. bubbles of water, milky, you name it. Water won't mix with pure gasoline, but it sure will in Ethanol.....which we are forced into. So after all that.....friggin $10 in fuel...... That fuel was fresh from pump to tank. I want to send them a bill......for my stupidity.
That would be my luck also. The easiest thing to check done after a long search for everything else our mind says it must be...