This engine has a hard starting problem caused by a mixture of normalI have fuel, air, spark and compression, so the only thing left that I can think of is timing.
The flywheel is keyed, so it's almost impossible to install that incorrectly
vacuum leaks and choke plates allowed too much manifold vacuum to
escape (Yamaha Service Bulletin).
I would bet there is a fuel primer installed and the pop-off lowered.
Possibly someones internet fix too.
A pressure test does a poor job of determining engine condition unless thegot a good pressure test first try. I got the engine back in and buttoned up yesterday.
inside spring (on the seal) has been damaged or fallen off. Hard starting and
other carb related problems are discovered by performing a vacuum test.
Did you test the pop-off? The carbs could be leaking fuel.
Be careful, a backfire can cause the waterbox to explode!!But, here's where I need help. The engine spins freely on the starter, but it doesn't even try to fire.
I'm priming through the flame arrester ports with fresh premix so I know I've got fuel.
I also have spark at all three plugs. I had originally put the old plugs in since
I did get a backfire out the exhaust one time when I probably primed too much, but otherwise no fire at all.
Do not ever use starting fluid.
It is very possible you have too much fuel. Ignition occurs within
a very narrow range of rich to lean.
Try this if the needle and seats are not leaking.
New spark plugs. Turn off the fuel supply (or pinch it off), hold the
throttle wide open until it lights off, then feather the throttle and
turn on the fuel.
After the engine starts, it is still possible that there will be a hard
starting problem after it sits for a week.
A completely worn out main seal is not enough to seize an engine.No smoking gun, but the front seal was a bit chunked up. I suspect it was drawing air causing the front cylinder to go lean.
Everything went together smoothly, and I got a good pressure test first try. I got the engine back in and buttoned up yesterday.
Was there rust damage inside the mag cover?
Obviously it was running on only 2 cylinders. A common mistake onBut at the end of last year, the one my son rides took a dump.
It would run, but it took some finessing to get it over ~30 mph.
Your engine seized several times 5 to 10 times or more.