Motor only turns with spark plug removed

[Wayne mech]

I have a 2016 Troy Built Pony with Briggs and Straton 17.5 HP 31R707 0033_01. Built in 2016.
Motor tossed the cheap plastic cooling turbine. It overheated before I knew what was going on.
Tore down enough to figure out I needed pickup (ignition unit), the cooling turbine and a new spark plug.
Adjusted gap, Had lots of fun putting the plastic cowl back on (threads needed cleaning). It ran but sounded a bit loud.
About 2 hours of cutting and it died with a pop. The motor turns fine with the spark plug removed but with the plug in the motor stops on second compression stroke. I've done this with fully charged battery and starter appears fine.

Full disclosure, I'm engineer and licensed aircraft mechanic. The valves seem likely cause (compression release) but WHY now? Why would a one time overheat cause exhaust valve setting to change? I've seen many videos of people working on overheat issues with this piece of junk cooling turbine and most just have done what I did fix the issue replace damaged spark plug lead and off and running. What's killing me is I rebuilt the transmission last fall (wouldn't go into forward). I just want to know if there is any logic to why the valve adjustment is now off after overheat. PS also is this how bad quality on these riding mowers are now 5 years and you are out $1500? or nickle and dime you to death ($200 in parts plus my time so far).

I'll tear into the valve adjustment...of course I'll re-check ignition unit and oil as part of all this but tweaking valves (and a gasket) is it worth it vs $500 for a new motor.

 

[Scrubcadet10]

Common problem on those is the compression relief failing on the cam, and sometimes taking out the governor/oil slinger gear.
Now, as far as overheating, you may have had a valve guide move in the head.
Intake is .003-.005 and exhaust is .005" to .008"
Cam shaft issues were supposedly fixed a new part number was given ( https://www.partstree.com/parts/briggs-stratton-84005207 ) comes without new lifters, so i would buy those too. If the cam is the culprit

 

[slomo]

These are air cooled engines. They get fully HOT in roughly 60-90 seconds. They over heat fast as you found. Over heating can lead to exhaust valve seats coming loose and several other issues. Valve guides moving away from home base causing valves to not open or something foul like that.

What is your high speed RPM's set to? As Scrubcadet10 was getting at, what are the valve clearances at?

slomo

 

[StarTech]

Common problem on those is the compression relief failing on the cam, and sometimes taking out the governor/oil slinger gear.

Cam shaft issues were supposedly fixed a new part number was given ( https://www.partstree.com/parts/briggs-stratton-84005207 ) comes without new lifters, so i would buy those too. If the cam is the culprit

Well they are not an improved part as a customer had one to fail less than a year after I install the 84005207 camshaft. And another customer just two the 793880 camshaft ACRs this last week.

I am currently installing the after market version which looks to exact copies of the OEM camshafts plus the kit includes seals, gaskets, and the tappets. The seals themselves appear to be OEM seals as they have the OEM part number on them.

Anyway I am ordering three kits today. I just can't keep paying $59 for a camshaft only if they are going to continue breaking the ACR when I can get the after market kit for $30.

 

[Scrubcadet10]

Well they are not an improved part as a customer had one to fail less than a year after I install the 84005207 camshaft. And another customer just two the 793880 camshaft ACRs this last week.

I am currently installing the after market version which looks to exact copies of the OEM camshafts plus the kit includes seals, gaskets, and the tappets. The seals themselves appear to be OEM seals as they have the OEM part number on them.

Anyway I am ordering three kits today. I just can't keep paying $59 for a camshaft only if they are going to continue breaking the ACR when I can get the after market kit for $30.

Oh okay, thanks for correcting me, it was my understanding they "fixed" it.

 

[StarTech]

Oh okay, thanks for correcting me, it was my understanding they "fixed" it.

Boy do I wished they did but the new PN looks identical to the older version just no tappets.

I'm already on my 5th camshaft for year..

 

[JustJoan]

I have a 2016 Troy Built Pony with Briggs and Straton 17.5 HP 31R707 0033_01. Built in 2016.
Motor tossed the cheap plastic cooling turbine. It overheated before I knew what was going on.
Tore down enough to figure out I needed pickup (ignition unit), the cooling turbine and a new spark plug.
Adjusted gap, Had lots of fun putting the plastic cowl back on (threads needed cleaning). It ran but sounded a bit loud.
About 2 hours of cutting and it died with a pop. The motor turns fine with the spark plug removed but with the plug in the motor stops on second compression stroke. I've done this with fully charged battery and starter appears fine.

Full disclosure, I'm engineer and licensed aircraft mechanic. The valves seem likely cause (compression release) but WHY now? Why would a one time overheat cause exhaust valve setting to change? I've seen many videos of people working on overheat issues with this piece of junk cooling turbine and most just have done what I did fix the issue replace damaged spark plug lead and off and running. What's killing me is I rebuilt the transmission last fall (wouldn't go into forward). I just want to know if there is any logic to why the valve adjustment is now off after overheat. PS also is this how bad quality on these riding mowers are now 5 years and you are out $1500? or nickle and dime you to death ($200 in parts plus my time so far).

I'll tear into the valve adjustment...of course I'll re-check ignition unit and oil as part of all this but tweaking valves (and a gasket) is it worth it vs $500 for a new motor.

I have this same mower, apparently I have thrown a rod. I was wondering where you found a replacement engine, because with these numbers, I am coming up with nothing.

 

[bertsmobile1]

I have this same mower, apparently I have thrown a rod. I was wondering where you found a replacement engine, because with these numbers, I am coming up with nothing.

Any B & S engine with the same output shaft size will fit
No need to get an exact identical replacement
Go to places like The Surplus Center
Small Engine Warehouse
Small Engine Supplies
Find a cheap B & S engine around the same size as yours.
Spin on oil filter is good
Nothing else matters as alternators & flywheels, muffler & controls will directly swap from your old engine to your new engine.