Diagnose this engine problem if you can

Seems I have no end to troubles with
engines. Today's issue is the 126 gas in
my 955 swather. Been having issues for a
while, it seems to get rich and eventually
flood out. Gets worse the longer it runs.
Been getting worse over 3 years time, but
it only gets maybe 10 hours a year of use.
Originally I thought since it gets worse
with use it could be the coil or
condenser, so both got replaced with a
complete tune up including copper wires,
cap, rotor and points. I got no change.
Took the carb off and found nothing wrong
there. I have good fuel flow and the
original mechanical pump works fine.
Bought a rebuilt carb and got no change.
Since it's running rich I tried shutting
off the fuel and letting it run dry
thinking that when it ran out it would
clear up just before it shut down, but it
just ran poor til it died. Switched out
the coil and condenser from my DC too
confirm they were good and got no change.
Put the coil and condenser from this
engine in the DC and it runs great. Put
another new cap, rotor and plugs in and
got no change. Battery shows 12.6v sitting
and 13.6v while running. Key switch was
replaced last year. I haven't done a
compression test yet, ordered a kit
today, but I don't think that's an issue.
This is a low hour engine and there is
little blowby. If it had gotten dusted
there should be a lot of blowby. I can see
the valves through the spark plug holes
and they look fine and are moving. This is
not a miss on a single cylinder or two
anyway. It runs but spits and sputters and
back fires out the exhaust when it loads
up.

I'm at my wits end here, where do I look
next?

Did you check with air cleaner hose off? Did you play with ignition timing?

Air cleaner is not in now and I did play with timing in both directions.

Until you said that you tried another carb on it, I was thinking a sinking float. Unless the rebuilt also had a bad float. Some other thoughts that come to mind are a worn out distributor shaft, a fuel pump leaking fuel into the crankcase, or is it too rich?
Too lean will also make it run poorly. Things that can make it run too lean are vacuum leaks like PCV, EGR (probably does not have EGR), or a ruptured diaphragm in the vacuum advance. Intake manifold leaks are also a possibility. Try spraying carb cleaner (NOT brake cleaner) around the intake where it joins the head. Look also for unused ports that may have been blocked at one time, but the plugs fell off.
That covers most of the things I would look for.
As a second thought, is it possible that there is something in the fuel tank that could be partially blocking the fuel line?

It sounds like you've been very thorough. Is the fuel clean and fresh?

SWAG - Hole in the float?

Air cleaner a real possible factor. A plugged up fuel cap can cause over pressure in the system as well as starvation. If the tank can get hot, pressure can reach 25 to 30 psi. I saw my 350U do that (it has horrible tank location) I unscrewed the cap and 8 gallons of fuel fountained out of the filler neck. I witnessed it on an Audi automobile with Denver 10% MTBE in the tank. the car was turned off, and the air compressor it flooded with 10 gallons of fuel was not running. I was not at the car, the shop owner was holding the gas cap. I ran to the breaker panel and shut off the mains on 3 panels all pilots were shut off as was the natural gas feed. We squeegied out the floor onto gravel and stirred it to evaporate. The fire department was called and a chief showed up and said we did what was needed. Then we went home for the remaining day with the rear doors open. The owner stayed till 12am then closed up shop till 8 the next day. Woof. Jim

Air cleaner hose collapse! Jim

Vacuum leak around Manifold?

When everything has been checked on the front end of the engine, look behind. May have a plugged muffler, especially if it is running rich. Happens more on 2 cycles but can happen on 4 strokes. Loosen muffler, see if it runs better.

When it runs poorly, is the spark still good or is it weaker?

Is the compression good? You have probably already checked compression pressures. A leak down test helps pinpoint where any compression leaks are going: rings, intake valve, exhaust valve, or head gasket.

Would too tight valve lash get worse as the block heats up or would it improve?

Air cleaner is not connected at the carb right now, but its clean and open. I haven't checked the tank cap, but since the issue starts right away I doubt that's it. But I will check it.

No, checked that first.

Hadn't thought of that. I did have that happen once before on the combine. It has a Chrysler 318 v8 with dual exhaust and a squirrel had filled one muffler with corn so tight that half the engine didn't run. That took some looking to find.

Checked that and found nothing

Let us know if that solves the problem,IV had it happen to me on a JD gator, it backfired a lot and destroyed the interior of the muffler and plugged it.

Does it have a fuel pump? May have too much pressure and lifting the needle off the seat.

Is the needle and seat not holding and flooding enough to make it rich? Is it making some black smoke. If not could the valves be tight and when it warms up loose comprrssion?

"Seems I have no end to troubles with
engines. Today's issue is the 126 gas in
my 955 swather. Been having issues for a
while, it seems to get rich and eventually
flood out. Gets worse the longer it runs.
Been getting worse over 3 years time, but
it only gets maybe 10 hours a year of use. "




Is there a chance that some old fuel is in the tank, maybe mixed with new fuel.
Old gas will deposit waxy crap on the intake valve stems resulting in sticking valves misfire, stalling,bent push rods,etc.
Gas that smells "turpid" will stick valves.

I think I would check valve clearances. If they are to tight ,can cause problem you describe.