The exhaust is a series of heavy wall galvanized pipe elbows that have a Ĺ inch pipe welded as a water injector at the first elbow. A simple effective design that has no hot spots and lasts about 10 years. Removing the hose didnít show any abnormal carbon build-up.
I can only imagine your frustration.
But imagine this, once you figure it out there is going to be a reason.
It has to be fuel, compression or air.
The only choice is to keep going over it and simplifying if possible until the problem is found.
To me removing the hose doesn't rule out a clog further down the pipe. You said there are multiple elbows, maybe the blockage is further down.
I know each of my ideas take a long frustrating session of knocking knuckles, so far with no good result. But how about one more.
Disconnect the fuel input line and run it into a clean diesel container.
Disconnect the exhaust from the back of the motor.
Turn off the intake water to save the engine.
Make sure the shutoff is not engaged.
Throttle to full.
Double bank of batteries.
Don't forget that there is a fuel return line that pumps the extra fuel back to the tank. I don't know what will happen if this line is clogged.
I don't understand the lack of air filter thing. There should be a way to make sure that the engine is getting air. I would like to know what the point of all the little holes is.
I visualize a funnel with a honeycomb at the wide end all ending up to a hole that is somehow plugged where you can't see. What if you sucked in the sound insulation.
What happens when you spin the engine. Other than that one time does it even try to catch.
If that doesn't catch for you check compression and let's go from there.
Usually when we are stuck on something like this the answer is one of four things:
1. Something we didn't know about the setup. An engine I worked on last month threw all its oil out. Come to find out it had second dip stick that we didn't know about that lost its plug. In your case maybe some other cutoff, clogged return, Some kind of air cut off valve hiding on you.
2. Something we are doing just backward. Checking the fuel shutoff from the engine and pushing it off instead of on or something just silly but we are so tired and frustrated we can't see it.
3. A combination of things. Compression is a little low and there is some water in the fuel and cranking is a little slow.
4. Something extremely rare. A broken tooth on the flywheel. Impossible stuff happens every day. I chartered a boat in St. Vincent. Motored into an anchorage and shut off the engine. Two days later stated the engine and the shaft had snapped off. The shaft was good enough to get me to the mooring but snapped off by itself two days later. Some things are just hard to explain.
Sometimes you just need another set of eyes.