16 HP B&S model 326431
* Installed Nova II module tractor ran very good no problems at all. Electronic components can fail intermittently, despite manufacturer's claims!
* I never removed the flywheel. There is a good possibility the key is partially sheared. ....Any indents in the key are enough to affect timing - it does not have to be sheared completely.
* When I installed the coil I put three layers of painters tape between the coil & Magnet because of no business card handy. The air gap should be .010"-.014" for this engine. ...The air gap will affect ignition timing.
* First time start up last week, hard to start, back fired a few times but started several times & ran well. Backfiring could have caused the flywheel key to shear.
* Tried a few days later but wouldn't start just backfired through muffler a few times. Backfiring through the exhaust is caused by late ignition timing (Nova module, magneto air gap, or flywheel key), or exhaust valve not seating (burnt valve, loose valve seat, carbon build-up, or improper valve lash). ....The intake valve lash should be .007"-.009". ...The exhaust valve clearance should be .017"-.019".
* Rebuilt carb still won't start. While turning over I open & close fuel/air mixture valve, when I get around 2 1/2 turns out it starts back firing again. With the mixture valve closed, no fuel mixture is getting into the cylinder. ....The large opening is allowing fuel to get into the combustion chamber, allowing the engine to backfire.
When I turn it to 1 1/2 turns out no back fire. But it still doesn't run! .....This setting may not be open enough to overcome a vacuum leak.
* I have hot blue spark Always? .....While the plug is installed? ....At the proper time?
* Full tank of gas (990 owners know about having no less than 2/3rds tank) Clean, fresh gas? ....No water in gas?
* Float level was adjusted to level when turned upside down. That's correct.
* Gas in carb.
* Plenty of compression. How did you determine that? ...Does it forcefully blow your thumb off the sparkplug hole during the compression stroke?
* I can see the intake valve going up & down as normal. ....That does not confirm proper seating of the valve, correct valve lash, or correct valve timing.
* Pulled the exhaust pipe off & could see the exhaust valve stem going up & down as normal. ....That does not confirm proper seating of the valve, correct valve lash, or correct valve timing.
* When I pulled the spark plug it looked dry not flooded as expected concerning the amount of gas in carb. ....A vacuum leak or insufficient compression will prevent fuel mixture from being drawn into the cylinder, as well as a carb fault.
* I put a few drops of gas in the intake then put the plug back on & it back fired a few times then nothing still wouldn't start. ....Backfiring, again indicates ignition does fire, but is it constant and at the correct time? ....Does it always backfire though the exhaust, or does it sometimes backfire through the intake? ....If only through the exhaust, see my comment above naming possible causes. .....If it backfires through the intake sometimes, and exhaust sometimes, I would be 99% sure your problem is in the ignition.
* I blocked the drive shaft & tried to turn the engine at the pulley in the front to see if a pin had sheared. ...This will not detect a bad flywheel key.
* I have two concerns about what I done:
1. When I installed the carb there was no gasket between the cylinder & metal plate. The only gasket is the thick one between the carb & manifold pipe. There needs to be a gasket on both sides of the metal plate, as well as a gasket between the manifold and carb.
2. Could the three layers of painters tape be a problem instead of the typical gauge of a business card? ...The correct air gap spec is named above. ....Too much gap is as bad as not enough gap!
My thoughts are what would keep a large amount of gas to not be in the cylinder head area when I removed the plug. ...There should never be large amounts of liquid gas in the combustion chamber! ....Too much liquid will not ignite, and is known as "flooding" an engine.
I made sure I cleaned everything & air blasted all openings when I rebuilt the carb. Maybe I'll take the carb off & clean again to make sure I have no blockage. ...If you're sure you did it correctly the 1st time, and your fuel is clean with no water, what would have changed? ...I would be more suspect of the missing intake gasket.
If I haven't sheared a key & didn't remove the flywheel is there something else that could cause the timing to be off. ...Yes, as described in above comments.
At this time, my only other suggestion is to disconnect the wire that grounds out the ignition, at the engine. ....Sometimes safety interlock or key-switch problems can cause ignition problems to occur. ....If the engine starts and runs without backfiring, most likely the problem is in this area.