Briggs 18.5 hp ohv backfiring out intake and exhaust

[GrantM]

I have a Craftsman riding mower with a Briggs 18.5hp Intek OHV. I mowed my lawn and then shut the mower off. I came back a couple minutes later, tried to start the mower, but it would not start and cranked very fast. I did a compression test, and it had 0psi of compression. Unsure of the problem, I did my research, and found out that the cam could be bad. Ordered new cam, removed engine, replaced cam, lined up timing marks on shaft and cam, and put it all back together. I then reset the valve lash to (in) .004 and (ex) .006. No difference. I removed the flywheel, inspected the flywheel key, which was NOT sheared, and put it all back together. I replaced the spark plug, no difference. I reset the magneto and flywheel gap, thinking maybe the spark was weak, no difference. I then inspected the valve again and found that the exhaust valve looked slightly open. I removed the head, and sure enough, the valve seat was ajar and the valve wasn’t closing fully, causing the no compression. I removed the valve, and carefully tapped the valve seat back into place, and it looked secure. I reinstalled the valve, which now sealed excellently, and put the head back on. Reset valve lash, and… it runs, but terribly. Runs only on half to full throttle, and shuts off after short time. When trying to start it still backfires through intake and exhaust if the throttle is in idle position. I did a compression test, and it has 130psi. I then cleaned the carb, removing the jets and running a guitar string through them, and spraying carb cleaner. Reinstalled carb, no difference. Ordered new carb, installed, no difference. It is still backfiring through the intake and exhaust. Sometimes there is ever flames through the intake. Throttle linkage working properly. I am utterly stumped, and ready to blow up this mower. Please help.

 

[bertsmobile1]

The head has over heated and gone soft so the guides are moving in the head buggering the valve timing
You could also have installed the cam incorrectly
And finally a single backfire can shear or crack the timing key in the flywheel so while it was good last week that does not mean it is good now .

 

[GrantM]

I was thinking the same thing about it overheating. Everything I have done I just did today. Today even after installed the new cam, thinking I could have installed it wrong, I removed the motor and rechecked all my work again, ensuring the timing dots were set right. Sure enough, they are. I watched several YouTube videos on the cam installation also to triple check I did it right. Would a new head solve my issue? I just thought it was fine because after tapping the seat back in, it has good compression. Throughout this whole ordeal, I have checked the flywheel key twice (today).

 

[bertsmobile1]

When you return the guides to their correct positions they need to be staked or even pinned because they will move again
The only ways that you can get a flame coming out the carb ( which can easily set the mower on fire ) are
the spark plug to fire while the valve is open = valve or ignition timing off
head severely over heated so a red hot spot ignites the incoming charge
inlet / exhaust overlap wrong or inlet slightly open on power or exhaust stroke
Check the valve lash every time you goto use the motor
If it is all over the shop then the guides are shifting

 

[slomo]

Take a picture of the cooling fins and block,post up here. Also pictures of the fan on top of the flywheel. Some have plastic blades some metal. Trying to see if it is over heating.

 

[StarTech]

You compression is too high for a properly adjusted engine with a good camshaft with a working ACR. It should be between 70 - 100 psi. I would not be surprised if you are not having cranking problems as Briggs starters don't like high compression.

Also on the valve seat did you stake it in place? I personally had an exhaust valve seat to come loose on my personal mower. It was cause by a bad exhaust gasket.

Plus whenever the intake manifold is removed a new o-ring need to installed; unless, it new. a leaking one can cause lean burn problems. I had one engine that would sneeze at idle but ran fine otherwise because of this.

And you may have a failing ignition coil or spark plug as after firing out of the exhaust indicates that the engine is not firing at times on the compression stroke. These engines uses wasted spark system that also fires on the exhaust stroke and any un-burnt fuel will ignite on the exhaust stroke creating the afterfire.

 

[GrantM]

Take a picture of the cooling fins and block,post up here. Also pictures of the fan on top of the flywheel. Some have plastic blades some metal. Trying to see if it is over heating.

It has plastic blades

 

[GrantM]

You compression is too high for a properly adjusted engine with a good camshaft with a working ACR. It should be between 70 - 100 psi. I would not be surprised if you are not having cranking problems as Briggs starters don't like high compression.

Also on the valve seat did you stake it in place? I personally had an exhaust valve seat to come loose on my personal mower. It was cause by a bad exhaust gasket.

Plus whenever the intake manifold is removed a new o-ring need to installed; unless, it new. a leaking one can cause lean burn problems. I had one engine that would sneeze at idle but ran fine otherwise because of this.

And you may have a failing ignition coil or spark plug as after firing out of the exhaust indicates that the engine is not firing at times on the compression stroke. These engines uses wasted spark system that also fires on the exhaust stroke and any un-burnt fuel will ignite on the exhaust stroke creating the afterfire.

Ignition coil seems fine to me, I have great spark? I can try replacing the intake manifold gasket but I’m not sure if that would cause this drastic of a problem. About cranking, I figured the battery was low, so I put a jumper to it, just 15 amps. That gave me enough juice to crank, and I was cranking it a lot I’m not surprised the battery seemed low. I did not stake the valve seat in place, but it seemed solid to me. Just tapped it back into place lightly with a hammer.

 

[bertsmobile1]

The guides are an interfearance fit
This means that when new the guide was bigger than the hole it went into.
The head is heated and the guide is chilled so it will go in under what engineers call a light press fit
Bronze is harder than aluminium so when the guide shifts it gouges the hole bigger
When cold it will seem quite rigid but when hot it will go up & down like panties in a brothel

 

[ragain's repair service]

I have a Craftsman riding mower with a Briggs 18.5hp Intek OHV. I mowed my lawn and then shut the mower off. I came back a couple minutes later, tried to start the mower, but it would not start and cranked very fast. I did a compression test, and it had 0psi of compression. Unsure of the problem, I did my research, and found out that the cam could be bad. Ordered new cam, removed engine, replaced cam, lined up timing marks on shaft and cam, and put it all back together. I then reset the valve lash to (in) .004 and (ex) .006. No difference. I removed the flywheel, inspected the flywheel key, which was NOT sheared, and put it all back together. I replaced the spark plug, no difference. I reset the magneto and flywheel gap, thinking maybe the spark was weak, no difference. I then inspected the valve again and found that the exhaust valve looked slightly open. I removed the head, and sure enough, the valve seat was ajar and the valve wasn’t closing fully, causing the no compression. I removed the valve, and carefully tapped the valve seat back into place, and it looked secure. I reinstalled the valve, which now sealed excellently, and put the head back on. Reset valve lash, and… it runs, but terribly. Runs only on half to full throttle, and shuts off after short time. When trying to start it still backfires through intake and exhaust if the throttle is in idle position. I did a compression test, and it has 130psi. I then cleaned the carb, removing the jets and running a guitar string through them, and spraying carb cleaner. Reinstalled carb, no difference. Ordered new carb, installed, no difference. It is still backfiring through the intake and exhaust. Sometimes there is ever flames through the intake. Throttle linkage working properly. I am utterly stumped, and ready to blow up this mower. Please help.

weak or broken valve spring or carbon around valve stems when the valve seat is loose you need to take centrt punch and peen aroun seat before putting back in place also use j b weld around valve seat and let it dry before setting valve lash