Sabo lawnmower with Briggs & Stratton engine won't start when warm

She is old.
She is all carboned up. I preach on here to de-carbon much as I can. Says so in EVERY engine manual out there.
She has valves that have never been checked for clearance.
She probably has valves that leak from excess carbon, again.
She probably has a scratched up bore from all the carbon now burning oil.
 
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I got back to working on it the day before last; I hadn't managed to find the time before then.

I unscrewed the cylinder head and the side cover so the valves at the rear end are visible.

The valves aren't sealing properly; they probably need to be lapped. Brake cleaner seeps through slightly when I spray it into the rear chamber.
I’ve cleaned the chamber as best I could.
Here’s another shot of the chamber. Is it normal for the valves to protrude into the housing at the back?
Where exactly should I measure the clearance here?
In my opinion, the cylinder looks fine; at least, I can't see any scratches.
 

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LOVE the pics.

So all this clean'ish look was AFTER you cleaned everything up? THIS is all part of the de-carbon process. Yes valves will need lapped. HEAD and block area where the head sits as well.

Still see a lot of carbon around the valve seat slash sealing area. Can't see where it really counts just under that the face and margin. All of this needs to be carbon free before lapping.

Raiden said:
Is it normal for the valves to protrude into the housing at the back?
Click to expand...
From what I can see they are not.
Raiden said:
Where exactly should I measure the clearance here?
Click to expand...
From the end of the valve or tip to the lifter that directly butts up to the tip. Do ALL of your cleaning and lapping THEN check for clearance.

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Do NOT use a high speed grinder to grind off the valve tip. It will remove too much material to fast. Then your clearance is too fat. Run it with your HAND on some 150 grit wet/dry paper. Reinstall the valve and recheck your clearance. Might take 5-10 tries. Do NOT get in any hurry doing this on an L-Head engine. Slowly creep up on the proper gap.
 
Also with the head off, if you can rotate the valves in their closed positions with your thumb only, they are net sealing proper or lapped in.

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Slomo is giving you good advice. You need to remove the valve and spring in order to lap it well. However, Before lapping I would clean up any carbon build up on the valve and seat so you know it is seating as well as it can and check the valve lash. I use WD40 to soften the carbon and a small wire wheel on a dremel tool. Be-careful on the aluminum. If the valve lash is too tight you can not lap it. Therefore, check the lash clearances next. they should be .005 +/- .001 on the intake and .007 +/- .001 on the exhaust. This is likely going to be the biggest problem. The intake is usually too tight. Often from running with low oil. After you get the lash right you can check the sealing of the valves. Any hand lapping will only take off tenths. If you need to take off well over .001 then you need a machine for valve grinding. I doubt that is your problem. You also need to check the valve guides and make sure they have not become egg shaped. Running with a tight lash can cause that. If the valve guides are very sloppy you may be out of luck. Oversize valves are no longer available. I have put machines with slight valve guide play back in to service out of practicality and they have done OK. I am saying you can use the machine like that but do not expect it to perform great for years to come. Start looking for a donor machine for an engine replacement. In the meantime you can get by. It would be anyones guess for how long.
 
I’ve cleaned everything up and lapped the valves now. The exhaust valve had absolutely no clearance left.
In my opinion, the valve guides are still in good shape. The exhaust valve has slightly more play.
I’ve noticed now that the exhaust valve opens just a tiny bit every now and then. Is this normal?
See the video - link below "Valve".

Won't I just run into the same problems with a donor engine if it already has some mileage on it?
How much does a replacement engine cost? Though it probably wouldn't be worth it.


Valve
 
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