Small Engines (Lawn Mowers, etc.): Vanguard V-Twin 23HP fuel problem, air fuel mixture, engine crankshafts

Question
Hello Ben, I live in Anchorage, Alaska and have an boat outboard engine equipped with two each 23HP Vanguard V-Twin OHV engines (a "Mud Buddy").  The engine crankshafts are connected in phase from front of one to back of the other.  The ignitions systems are wired together to both fire off the key from the front engine.  There are 42 hours on the units.  Anyway, it (they) have always been very hard, or impossible, to restart when hot (vapor lock?).  They start easilly when cold.  It also usually backfires a few times when shut off.  Now it shuts off after running for only a few minutes under load (runs fine in the driveway with no load!).  I have checked for spark and replaced the plugs, replaced fuel filters, oil, oil filters, fuel lines, and oil lines.  I drained the fuel system and tried using fresh gas.  I can hear what I assume are the anti-after fire solenoids clicking on both units when the ignition is switched on.  Why does the engine shut off under load?  Both engines are Model #380447, Type #0065-11, Code #03080711  Any help would be miraculous!

Answer
Andy
Ok first of all you can bring these units back to any B & S service dealer as they are still under warranty.
These are the common problems and some help

Problem: Engine backfires.
Possible causes:

Carburetor set too lean.
Magneto is not timed correctly.
Valves are sticking

Timing
For power to be developed, the ignition of the compressed air/fuel mixture must take place at exactly the correct instant - just before the piston reaches Top Dead Center (TDC) on the compression stroke. With automotive engines, there are mechanisms to advance the spark at higher revs but simple small engines do not have this complication, at least.
Timing is set on older mowers with point type ignition systems by adjusting the point gap and generally only changes due to wear. However, these changes are gradual and unless the points come loose for some reason, will not likely suddenly prevent the mower from starting. On newer electronic ignition systems, there is basically no adjustment as the position of the electronic ignition coil/module fully determines ignition timing and this is fixed.

However, timing can be grossly messed up if the flywheel key gets sheared and the flywheel then rotates a fraction of a turn on its mount on the crankshaft. The result may be a mower that does not start, backfires or runs erratically, lacks power, won't run and/or start when hot, etc. This is very likely to happen should the blade strike a rigid object causing the mower to stop instantly. In this case one or both of the blade lock key and flywheel key have sheared to (hopefully) protect the very expensive internal parts from damage.

There are likely not going to be any timing marks for that old timing light you have sitting gathering dust somewhere. The only test really is to inspect the flywheel keyway to determine if damage has occurred.