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#113823 20/01/22 02:45 PM
Joined: Jan 2022
Posts: 8
Novice
Hi people, my father has a 10.5hp briggs engine in his ride on mower. When you first start it it runs fine, but after it has a chance to get hot it begins to run rough. The revs drop and it pops and backfires etc. You can stop mowing and just let the engine run and it MAY return to normal and you can continue to mow, Then 5, 15, or 30 minutes later it'll do it again. There doesnt seem to be any rhyme or reason to it, it just plays up whenever it feels like it. Initially i thought if may have been a fuel blockage but ive been right through the fuel system and its clean. Then we thought it may be the coil breaking down when hot, so we replaced the coil, but unfortunately the problem is still there. If anyone has any ideas of where to look next i'd be grateful for any suggestions.

Thanks.

Joined: Sep 2015
Posts: 2,070
Likes: 148
SENIOR TECHNICIAN & HISTORIAN
Hi dexter

Check the head bolts aren't loose and the head gasket is still sealing.

I would check the valve clearances ,possibly when the motor is hot the exhaust stem has expanded and clearance is lost
causing the valve to be open.

https://outdoorpowerinfo.com/engine_specs/briggs_valve_gaps.asp



Cheers
Max.

1 member likes this: speedy
Joined: Jan 2022
Posts: 8
Novice
Hello Max, Thank you for your input. Since i wrote the post i have spoken with a mechanic at our local mower dealership, and he has suggested there may be an issue with the valve seats coming loose. In all my time playing with engines i have never heard of this happening. Have you ever heard of a valve seat letting go in this manner?.......at any rate it looks like i'm going to have to pull the engine down and have a good look through it. That job will have to wait as i need to fix my own mower as per the problem described in my 1st post.

Joined: Sep 2015
Posts: 2,070
Likes: 148
SENIOR TECHNICIAN & HISTORIAN
G'day Dexter

Sounds like this motor will need a few things checked for serviceability , make sure you have a good spark plug
first as I've had a few plugs that break down when hot and cause a misfire.

Yes I've seen a few motors that the valve seat has come loose ,when the seat drops completely out you end up with
no compression.

It's a possibility that the seat is starting to move when hot, you would need to remove the valves to check.

If you fast forward to 7 min and 54 seconds in ,he discuses centre punching the alloy to hold the seat in.



If you fast forward to 10 min and 48 seconds the video is showing the loose seat.





Cheers
Max


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