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I have a Briggs & Stratton Lawn Mower 300E series engine 125CC.

I made sure the mower started before I replaced the oil sump due to leaking/cracked (from hitting something hard). So the motor was still good.

I had tipped the mower over which I assumed and I should have known better, I didn't remove any of the gas at the reservoir, so the gas appeared to drip down and seem to flood the cylinder. There was no oil at the time I tipped the mower in the pan since it all leaked out, thus the repair. I did put oil after replacing the sump.

Now, after replacing sump, I cannot start the mower, it seems like the spark plug keeps getting fouled no matter what. Checked the carb, float, all is clean, actually the motor isn't that old. I tested the spark plug outside the mower and its getting spark.

I tried to keep pulling the start cord to try to push out all the gas or whatever is in the cylinder but not much is coming out. Its been a week now and I still can't start the mower.

I put some drops of ATF to try to rebuild compression since I read that flooding would wash away everything. Didn't work.

I had tipped the mower now with spark plug up to try to get any fluids down past the piston and into the pan.


I'm curious, the mower is fairly new and had started before I replaced the sump and now it won't start at all after a week. Anyone have any tips? I already spent like 60 dollars on gasket, seal, and used sump, and the mower was like 150, but since it was still new I figured to save it, hope I dont have to spend more to the point it would have been a new mower.

Thanks in advance.

I am assuming this is one of the newer OHV push mower engines . If had the sump off it is possible that the extra slack create in the camshaft could of dislodged one of the push rods, May need to remove the valve cover and check your valve clearances.

You are right it's an ohv motor, what exactly would I be looking for and doing. I'm mechanically inclined but don't understand exactly the specifics of a mower compared to car. I know they're both 4 stroke but the way you handle, the part functions, etc. Can be quite different.

I appreciate your quick response.

Remove the 4 screws that secure the valve cover to the engine, and check the rocker arms, and also check the valve clearances. Should be around .004-.006 for both valves.

ILENGINE said:

Remove the 4 screws that secure the valve cover to the engine, and check the rocker arms, and also check the valve clearances. Should be around .004-.006 for both valves.

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just curious by removing sump, what exactly happened to these items above when I replaced the sump? its weird I've watched youtube videos and didn't see anything that would lead to this. if they are out of spec, is there a how to anywhere?

Sorry really just trying to get into the small engine repair scene.

When you remove the sump you allow slack to be created because non only one end of the camshaft is supported. Can lead to a change of position which can change the valve clearances. Either cause the push rods to get dislodged, or change the clearances.

Any valve adjustment video you find for a briggs OHV engine should give you the idea of what to do. The procedure is the same for all briggs engines.

It may not be a valve issue, but needs to be checked after opening the engine up.

i also forgot to mention there was no compression so it might be the valves yea?

You hinted at compression when you said you put a few drops of ATF in the cylinder to rebuild compression.

I set tdc and adjusted valves. Retested spark. Cleaned carburetor and floater. Pull cord feels better. Added starting fluid and it feels like it's going to start. I have a longer spin now. I tested compression now I'm getting a bit more but not enough yet. But I see some spitting of gas I think through the filter side cus I have the filter out right now. Am I flooding or flywheel key is needed or coil adjusting, not sure.

If you hit something hard enough to break the sump, I would bet the flywheel key is also sheared or at least offset.

Possible but I used the mower twice before I replaced the sump, I just kept refilling the oil. I also made sure by starting it before I replaced sump.

I'll double check the flywheel area and move on to the head and check. Would a flywheel key sheared cause low to no compression?

Thanks for being patient and very informative, it's actually pretty cool working on small engines compared to cars.

Replaced flywheel key it was sheared, put some starting fluid and now I'm getting some pressure pulling and it's now continuously popping like it's gonna start. I could see the exhaust firing at night. What could it be, flooding, floater and needle? stuck valve, burnt valve, compression​ leak down test? Sucks because the carb looks new with the float but it's still possible and it's weird since I started the mower before I replaced the sump. Leak down test?
Any other ideas to test?

I think I remember now where I screwed up. I forgot to set the cam timing.

When I do this do I need to set TD first or anything or I don't? Can I just check the valve/rocker clearance later?

I think all I need to do is line up the marks and that's it?

Not having the timing marks on the camshaft and crankshaft aligned can and will cause the problem you are having

Looks like it was timing. I had to use rtv instead cus my new gasket tore when I removed the sump. Should be ok.

Thank you for all your help, can never be more appreciative

Just be aware that the gasket thickness is what sets the crankshaft end play tolerance.

What would happen with rtv? I even see people use gasket plus rtv both sides with no issues.

I ran the mower for 45 minutes no issues. What do you suggest? Or recommend? Do I leave it alone or put the gasket instead? I used a thin layer of rtv not thick.

If it works at this point I wouldn't mess with it any more, and just use it.

Thank you very much, I now have a better appreciation for small engine repair in fact I'm fixing another one soon that I got for free