AC720 limited power and backfiring

Hello all!

I tried searching the forum and didn't see exactly what I'm experiencing, although perhaps I don't know the correct lingo.

Reba is a '78 720.  Relatively new transmission.  Working great and then in the middle of a job lost *almost* all power to the wheels.  Front and rear PTO working fine.  If I jostled the gear selector I could get her to move a little.  We limped back to the shelter, BARELY making it up a little hill.  Same problem after I let her rest for a few days.  

Now, when ever she is shut down she shudders then backfires.

Any thoughts?

Points and timing is a good place to start but before you do that, see if your getting adequate fuel through the clear fuel filter if you have one. Fuel pump may be going bad and if enough fuel isn’t getting to the carb it will cause the same issue. Mine did it last weekend and I am now deciding if I’m going to rebuild the pump or put an electric pump on it. I like staying factory but the fuel pumps are problematic. Have also had a bad fuel cap do the same thing, but if it sits for a half hour it would run again just fine for a few minutes until it began creating a vacuum and start sucking the tank in from the sides and die. The vent goes bad in the cap, no cleaning I did resolved it. So I replaced both of mine with a cap that has a fuel gauge which makes it nice to get a quick visual of the level of fuel in the tank.

ONANs are known for having valve seats come loose: check compression.

Update?

I know of 3 things which cause backfiring
. 
1) A timing problem
2) Sticking valves that aren't closing
3) excess fuel detonating in a hot exhaust.

Check the valves to make sure they are opening and fully closing

I don't know what type of ignition system you have but if points, check they are in good condition and gapped correctly.  If you have an ignition module it may need to be replaced

If you have a carb. then check the fuel air mixture setting to make sure you aren't running rich

I just combatted this, this morning, on a 4041 I brought home last week.  A minor points tweak solved the backfire and a carb adjustment put the fuel mixture where it needed to be for full power.  Brought this thing home, after dragging it out of the spot it sat for over 30 years.  The last oil change date was '89.  Runs like a top!