B & S 12.5hp starting issue

[Digit]

Hi all, I have been lurking on this forum for a while and have now join in the hope someone can help with my issue. I have a ride-on mower with a B &S 12.5hp engine. Tried starting it a couple of weeks ago but it continually backfires. Decided to give it a new plug, air and fuel filter and had to replace the carb breather pipe as it had a hole in it. Checked the carb, all clean. First start, it ran for 10-15 seconds without backfire (good result), then stopped. Now when trying to start, the flywheel turns 1/2 turn then stops and won’t restart. Where do I go from here please?
Cheers

 

[bertsmobile1]

Welcome
!2,5 HP is insufficient information to give specific advice as we need the model numers off the rocker cover ( OV engines ) or blower housing near the spark plug ( SV engines )
IF you have never adjusted the valve lash, now is a good time to at least check it

 

[Digit]

Thanks for the reply Bert, sorry for the slow return,
I’ve got 219807 0421 B1060 10ZA, Took a while to get the valve cover clean enough to read.
What do you mean about the valve lash? I can do the very basics but my mechanical knowledge
is not what it should be. Time for me to learn.
cheers

 

[bertsmobile1]

When the valves get hot they expand so the shaft becomes longer
If thee is no gap betweenthe end of the valve and the rocker ( the bit that pushes it down ) then when it gets hot it will not properly easl agains the valve seat so some of the burning fuel will escape past the valve rather than pushing the piston down
This create excessive wear in the valve face
On the carb side it can cause a sarb fire on the exhaust side it causes the exhaust to glow red hot, some times backfires through the exhaust .
The gap between the end of the valve and the rocker is called valve lash if you are in the USA
On your engine, wear causes this to get way too big over time so it needs to be adjusted
Lots of videos that show it beter than I can describe it
Just remember on Inteks you set the gap with the big nut and you lock the nut with the little grub screw in the middle of it .
This is backwards to automative practice & confuses a lot of people

 

[Digit]

Thanks Bert, will check the valves the valves later if I get time. So what would cause the flywheel to stop after a 1/2 turn? Although it backfired before , at least the engine still cranked.
cheers

 

[bertsmobile1]

The starter is not strong enough to turn the mower over .
This is deliberate so in a hydro lock the starter stalls rather than blowing out the spark plug, blowing the head gasket or bending a conrod
To overcome compression pressure the inlet valve opensa tiny amout to let some of the pressure escape.
The movement is tiny, just a couple of thou
So if the valve lash is too big, it defeats the decompressor and the flywheel stops turning .
If you put your palm on the finger guard and rotate the engine just past where the pressure is you will see the engine do 1 1/2 turns till the piston gets to TDC on the compression stroke
In fact when the mechanism fails you can some times get the engine to start by doing just that as the inertia from the turn & a half can be enough to overcome the compression.

Very common to have a customer come in with that problem after replacing the battery, key, switch , starter motor, solenoid & battery cables and all that is needed is a valve lash adjustment

 

[Digit]

Bert, thanks for the info. Inlet valve was loose and the exhaust not much better.
Both adjusted and now works nice and smooth.
Thanks for you’re advice
cheers

 

[slomo]

Clean the cooling fins.

 

[bertsmobile1]

You are more than welcome
HElping people & countering the rubbish on social media is what we are here to do
Thanks for coming back to close the thread, happy mowing .