Briggs & Stratten 20HP won't start, backfiring through exhaust and carb.

[Novo]

Okay, I have to eat some "crow" here.
"Raider820" was correct to check the Key on fly wheel again!
As you can see from the pics below the flywheel was out of align again. I pulled the fly wheel and you can see the sheared key on left of third pic with new key on the right.
Thanks for suggesting to look again Raider820.

My question is:
1. is there requirement to push the key down beyond the end of the crank shaft?
2. Is backfiring capable of shearing the key again?
3. Is the key hardened at all? "looks like its not hard at all from sheared one"
Before it's asked "I would" Yes I tightened the flywheel bolt to spec's the last time.
Any guesses what sheared it? Going to reinstall new key tommarow and try to start again.

"StarTech" Your pic is exactly what mine looked like when I reinstalled it. Dots are what I lined up.
"Tinker200" What I should have said was "I'm certain I installed it correctly as far as which side faces the (BOTTOM) of the sump."

Thanks all!

 

[Scrubcadet10]

You did torque the flywheel bolt as well, correct?
Improperly torqued flywheel bolt/nut will cause a key to shear as well.
I'm near certain the torque spec is 100ft lbs.

 

[StarTech]

Need to know engine date code. It determines which steel key to use also flywheel torque was increased to 110 ft/lbs. Briggs change the crankshaft and flywheel back on July 01, 2012.

 

[Novo]

You did torque the flywheel bolt as well, correct?
Improperly torqued flywheel bolt/nut will cause a key to shear as well.
I'm near certain the torque spec is 100ft lbs.

Yes, Torqued to 110 ft lbs actually, which is spec I found.

 

[Novo]

Need to know engine date code. It determines which steel key to use also flywheel torque was increased to 110 ft/lbs. Briggs change the crankshaft and flywheel back on July 01, 2012.

Not sure how to read the date StarTech.

 

[StarTech]

2011 Oct 18. (YYMMDD(ZD = assembly plant))

The APSI # 82 Dated Feb 2012 says to use the 593935 steel key and torque to 110 ft/lbs.

Just to note I have found a couple of these flywheels busted out in the center. And even found one just cracked. None these even had the steel key yet.

 

[slomo]

The key is a soft sacrificial metal not hardened at all. Look for a flywheel crack like StarTech said.

110lbs should lock that thing down tight. Crank still have a good taper? Flywheel not tapered to match crank end?

Did someone install or is yours supposed to have a washer below the flywheel? Washer no washer? Checks parts list.

slomo

 

[Novo]

2011 Oct 18. (YYMMDD(ZD = assembly plant))

The APSI # 82 Dated Feb 2012 says to use the 593935 steel key and torque to 110 ft/lbs.

Just to note I have found a couple of these flywheels busted out in the center. And even found one just cracked. None these even had the steel key yet.

Okay, so new update "Thanks for all the responses by the way"

There is no washer on Flywheel now and none are called for in the parts list I've looked at for this engine.
I went ahead and cleaned/inspected the fly wheel for cracks and couldn't find any. Just a little scaring from previous key shears.
I assume both tapers match well because they are pretty clean and no sign of missmatch.
I installed the new key and tightened the fly wheel bolt to 110 ft lbs.
Instead of hooking up the gas line, I sprayed starter fluid in the breather tube and tried to start it. It started and ran for a few seconds and died as you would expect. "first time it's actually run since rebuild".
I figured that was the timing issue and proceeded to hook up the gas line. Tried to start it again and immediately it went back to backfiring!
After a few more try's and backfires. I stopped and pulled the flywheel bolt. The Key Is Sheared again!

I can't say whether it sheared after it ran or because it was backfiring on gas. I assume the backfiring reverse momentum is shearing it.
All the gas in the gas tank is new, I drained it a week ago and put fresh in. "no water"

I'm at a loss why it would shear so easy and why starting it from carb would make it backfire.
Any new thoughts?
Thanks again!

 

[slomo]

Amazing story. That flywheel HAS to be cracked. I bet once under torque it might show its arse. At 110ft lbs, should be down for the count.

Take it to an engine machine shop. Have them magnaflux it for cracks. Shouldn't cost a few bucks.

I've never heard of backfiring causing this. Maybe Bertsmobile1, StarTech or ILENGINE (just to name a few superstars) on here can chime in with some real wisdom.

The key is soft yes. Plausible yes. ?

slomo

 

[bertsmobile1]

The backfiring causes the key to shear
Get some fine valve grinding paste and lap the flywheel taper to the flywheel, exactly the same as you would lap in a valve.
Make sure you clean all of the lapping paste off the top seal before refitting the flywheel.
When the taper is perfect the flywheel should seal agains the crankshaft with nothing more than thumb pressure then should be near impossible to pull back off again.
You can get really anal and go from valve grinding paste to lapping pastes getting progressively finer right down to 0.5-1.0 microns .
When we are doing drag bikes that run a sort of standard boxes or scramblers where the HP is doubled or better this is what we have to do to keep the clutch on the main shaft .