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

[Rivets]

You might want to have your torque wrench tested and checked.

 

[StarTech]

The problem partially was the design of the flywheel and crankshaft keyway. It leads to the key being sheared easily which is why Briggs went to a steel key and the new torque amount.

And yes check the flywheel for a cracks. Here is just an example of what one customer here had happen to his 310000 series and this happen with an aluminum key when the flywheel was torqued to 100 ft/lbs. I would have thought it possible with the aluminum key but here is proof that it can happen. Now of course he also had a blown head gasket. The picture is after it been sitting in my scape pile for a year.

 

[Novo]

The problem partially was the design of the flywheel and crankshaft keyway. It leads to the key being sheared easily which is why Briggs went to a steel key and the new torque amount.

And yes check the flywheel for a cracks. Here is just an example of what one customer here had happen to his 310000 series and this happen with an aluminum key when the flywheel was torqued to 100 ft/lbs. I would have thought it possible with the aluminum key but here is proof that it can happen. Now of course he also had a blown head gasket. The picture is after it been sitting in my scape pile for a year.
View attachment 56838

StarTech
I thought I would post some pic's so everyone could kinda see what I see. My camera sucks but it should help.
I'm not posting the pics to prove anything, just trying to help with info. I know just how hard it is to diagnose anything from words and lack of info/pics.
I've done nothing yet except inspect the the flywheel again. As you can kinda see from the pics, the mating tapers are shiny and some what polished except for scaring.
If the general opinion is to lap the two surfaces, I'm happy to do so. Happen to have some lapping paste.
I know its hard to tell from the pics, but unless there's a hair line crack, I can't find any cracks.
I'm not say there are none, I just can't see any. When I spray cleaning fluid in the tapered hole I hoped to see a tiny line show up from the crack picking up the fluid but no.
"Lawn Addict" Suggested magnaflux, but I'm not sure where I would take it for that.
"Lawn Royalty" Suggested lapping of Crank/Flywheel which I can do if more friction is needed.

Looks like the general consensuses is a cracked Flywheel or lack of friction to help the key-way do it's job and the backfiring is due to key-way shearing immediately.
My next move is to lap the flywheel/crank, buy several new key-ways and try again. The only loss is time and $2.25 key-way.
I Check this post between breaks, so looking for any other thought's.
Thank you very much Guys! Y'all have kept me from letting this sit for a year out of frustration.

 

[slomo]

Let's brake it down. Your FW is moving shearing keys. Bertsmobile1 had a super idea to lap it in. Do it. There's no other explanation on how or why the FW would move. You got the paste. I would finish up with toothpaste (fine abrasive) or whatever Bertsmobile1 recommends. Follow his words to the T.

Still a good idea to take it to an engine machine shop and have it Magnafluxed. My thought was under 110ft lbs of torque, the ol' girl might spread her legs shearing keys.

As you said the crank has some good scratches. Take some medium then fine grit emery paper and clean her up. Use the back side to polish. Front abrasive side to well, clean her up. Then lap her in as Bert said.

slomo

 

[Novo]

Let's brake it down. Your FW is moving shearing keys. Bertsmobile1 had a super idea to lap it in. Do it. There's no other explanation on how or why the FW would move. You got the paste. I would finish up with toothpaste (fine abrasive) or whatever Bertsmobile1 recommends. Follow his words to the T.

Still a good idea to take it to an engine machine shop and have it Magnafluxed. My thought was under 110ft lbs of torque, the ol' girl might spread her legs shearing keys.

As you said the crank has some good scratches. Take some medium then fine grit emery paper and clean her up. Use the back side to polish. Front abrasive side to well, clean her up. Then lap her in as Bert said.

slomo

Thank you slomo.
Just finished doing all you suggested above except the magnaflux. Filed the journal edge next to key-way on the crank back to surface level. "Was standing a bit proud" Sanded the crank journal and flywheel with 220 paper by hand. Then used lapping compound and spun the flywheel by hand. Wasn't perfect but looked allot better. "more surface touching" Then cleaned up, walked across the street to borrow a different torque wrench and reassembled. Torqued to 110 ft lbs!
Hooked up the gas line and cranked it. Eureka!!!! She STARTED!!!
Now I have a new question! lol

The motor is running away. "governor is only thing controlling speed" No idle at all.
I shut it down and restarted controlling the butterfly by hand. Idles great and speeds up like you would want but as soon as I let go of butterfly, it runs away.
When I pulled the carb off previously, all the linkage came apart. I'm pretty sure I have everything back where it was but it acts like I'm missing a spring that pulls the butterfly closed when you set throttle to idle.
I'm searching now for any diagram showing the throttle operation, linkage or parts list.
Thank you again!

 

[slomo]

Bertsmobile1 to the rescue again. Awesome tip on the flywheel cleanup.

The gov-na' closes the throttle plate or tries to. You are right, there is a small choke return spring and a throttle spring that need to be fitted proper.

Throttle linkage connects the throttle plate on one end and the other goes to the gov-na'.

Idle speed is quoted around 1750rpm. Max revs are safe at 3200rpm with the blades turning. Not a big fan of idling down a splash lube engine. Lack of lube.....

Didn't mention a file. Removes too much material or a good sharp one can. All you needed was med and fine sanding cloth/paper and a lap dance. But hey it's WORKING!!!


Does yours have the NIKKI Six carbutrator or the Ruixing?

slomo

 

[Novo]

Slomo
Couldn't tell which carb I have, don't know the difference. Took pic looking down on linkage and one showing connection to the governor arm.
The choke works like it should. When you move the throttle to full, the governor arm gets stiff and the throttle butterfly arm is held open very strong.
When you set the throttle to idle, the governor arm is very easy to move and so is the throttle butterfly arm but it never moves the butterfly arm to idle.
The spring "by the way" on the governor arm is still in place.
Is it normal for the butterfly to remain full open with throttle set to idle and engine off?
Should I adjust the governor arm?
Thanks again!

 

[Novo]

Couldn't let it go!
I pulled the carb to make sure I had the linkage correct and it appears I did. I also found out I had not reinstalled the crank case breather tube.
Then I attempted to adjust the governor arm but it was already set correctly "no adjustment was necessary"
I reassembled it all, then started it and bingo she idled.
I then proceeded to install all the engine covers and started it again. Seemed to still idle fine.
Drove it for a little bit and started cutting some grass. Everything seemed fine except for a little surging while cutting grass but when I disengaged the blades, the engine went back to very high RPM and very little RPM change when I cut the throttle back to idle.
If I could get it to idle consistanly, I figure I've got a pretty good mower now.
Thank you to everyone for the help!
Any suggestions on the idle issue are welcome.
Thanks again all!!!

 

[bertsmobile1]

Slomo
Couldn't tell which carb I have, don't know the difference. Took pic looking down on linkage and one showing connection to the governor arm.
The choke works like it should. When you move the throttle to full, the governor arm gets stiff and the throttle butterfly arm is held open very strong.
When you set the throttle to idle, the governor arm is very easy to move and so is the throttle butterfly arm but it never moves the butterfly arm to idle.
The spring "by the way" on the governor arm is still in place.
Is it normal for the butterfly to remain full open with throttle set to idle and engine off?
Should I adjust the governor arm?
Thanks again!

YEs that is normal
Engine starts on full throttle then the governor will slow it down to whatever you have set the throttle to.
It is a balance between the spring tension and the force generated by the governor bobweights.
Stationary the bob weights are of course fully in so exert no force at all .

 

[StarTech]

First the carburetor looks to be a Nikki carburetor. Should have an aluminum fuel bowl on the carburetor.

As the surging try doing a static governor adjustment as there maybe a slight ply in the linkages from wear. If continues after the adjustment then the carburetor needs cleaning and probably a new main jet as the o-ring on the jet is probably gotten loose. Just to note some these Briggs engines do surge when cold as Briggs run them a little too lean. Usually adding a little choke to smooth it out indicates a lean burn condition.

Also another thing I have over the years is if the idle speed screw is set too slow the engines will surge too as the governor closes the throttle plate too much. I usually set the idle speed by holding the throttle plate closed and setting the idle speed using a tach.