I've run out of ideas so I'm deferring to the SigForum small motor brain trust to see if you can suggest something I may have missed.

I have a Snapper Commercial self propelled walk behind mower. It has a B&S 7.5hp motor. When I pull it out of my shed to cut grass, it fires up on the second pull every time, and runs without any issues.

Here's the problem. While I'm cutting, if I stop and shut off the mower only long enough to empty the bag and then re-start, no issues. However, if I stop, shut off the mower, empty the bag, then do a few minutes of other stuff before returning to start the mower again, it will not start. No way, no how. If I let it cool completely down and try again, it fires right back up and runs without issues.

So what's getting so hot that its keeping the mower from running? I always make sure there's plenty of gas in it, so its not starving for fuel. Any thoughts or suggestions would be greatly appreciated.

Have you checked gas cap to make sure vent isn't clogged?

Sounds like possibly vapor lock. I'm not real familiar with your mower but engines usually get hotter right after they're turned off and then start cooling after a while. You can try insulating your fuel line and see if that helps.

Sounds like the carb needs to be rebuilt or it needs a valve adjustment.

difficult warm starts are caused by 2 things in my experience - tight valves & lean A/F mix.
Probably not tight valves (this was a thing on my 4-wheeler, the valve lash would get tighter over time & I could tell by how it started when warm).

Do you have to choke it to cold start?
Does choke help to warm start?
Could have some crud that is just leaning the mix enough to be problematic when warm starting - pilot jet partially plugged or something.

Normally 'rebuild/replace carb' is the answer to most small engine ailments.

quote:

Originally posted by snidera:
Do you have to choke it to cold start? No manual choke, just a primer ball.
Does choke help to warm start? No manual choke and addition prining via the primer ball doesn't do anything.
Could have some crud that is just leaning the mix enough to be problematic when warm starting - pilot jet partially plugged or something. I'd thought about that but figured a fouled carb would have the same effect on cold starts as on warm starts.

Normally 'rebuild/replace carb' is the answer to most small engine ailments. When it runs it runs so well that I'm struggling with this being a fouled carb issue.

I've wondered several times if this could be the coil becoming overheated while the mower is shut down. Just not sure if there's anyway to test that.

Wish I knew the answer... I have a toro that does (almost) the same thing. Always starts cold and runs fine but will not start when hot (no matter how short a period of time it is shut down). Have to let it cool 20 minutes or so to get it to restart.

I replaced the carb, fuel line, and plug this spring for not starting at all. Fixed that problem but dealing with the hot start issue every since.

If it's an OHV, then the exhaust valve is too tight. Adjust the valves.

When the engine gets hot, the exhaust valve expands and the gap between the rocker arm is reduced to the point that the valve is staying open enough to lose compression.

When the engine cools, the valve stem contracts and proper clearance is obtained, allowing for it not to lose compression.

Not my first rodeo with this problem.



If it is not OHV, then the fuel is possibly boiling out of the carburetor, leaving it dry.

Try choking it to express fill the bowl.

I had a craftsman 6 & 3/4 hp did the same thing.
I guess I just got used to it after a couple seasons. I never bagged just mulched & no breaks til I was done.

Vapor lock? When I had that problem with my old 5.5HP B&G Snapper, I took off the gas cap and it fired right up.

My Dad had a B&S motor on his tractor do that. It was a valve issue like Marzy said and he had to manually turn the engine shaft 90 degrees. There was no adjustment on his so he lived with it remainder of mowing season and rebuilt over the winter. He found some deal where the part was about $60.

Check the coil. Go on u tube. It will be explained by Terral Dactal. The coil can be checked with a multimeter.

I had a similar problem with my Toro and it was a burned exhaust valve.

quote:

Originally posted by Mars_Attacks:
If it's an OHV, then the exhaust valve is too tight. Adjust the valves.

When the engine gets hot, the exhaust valve expands and the gap between the rocker arm is reduced to the point that the valve is staying open enough to lose compression.

When the engine cools, the valve stem contracts and proper clearance is obtained, allowing for it not to lose compression.

Not my first rodeo with this problem.



If it is not OHV, then the fuel is possibly boiling out of the carburetor, leaving it dry.

Try choking it to express fill the bowl.

Once again, I believe you've scored the winning shot. I watched a couple YouTube videos tonight to see if I could find something to expand on your comments, and the mowers in both videos I watched exhibited starting problems similar to mine that were caused by out of spec valve adjustment. By the way, this is my mower (B&S 7hp OHV not 7.5hp as I'd noted previously).





What is your recommendation as to how to find top dead center and/or put the motor in the right position to adjust the valves? Haven't done a valve adjustment like this before so any advise or recommendations would be greatly appreciated.

If it's coil soak, drop the spark plug gap down from .030 to .025 to check. Often a weak coil will spark at .025 but not .030.

The auto choke on those engines is notorious for sucking. Plus the extreme lean factory jets that Briggs puts in them.... No joke a $15 eBay Chinese replacement carb often has larger (non EPA size) jets and run better than OEM ever did.

Try the plug first. Then mess with the auto choke, manually choking it by bypassing the bimetalic coil spring. Then try the valve adjustment. And then, throw an aftermarket replacement carb on it.

Gasohol is also susceptible to vapor lock.

If you are running E10 or higher amounts of corn, it might be another thing to investigate.

Cows love corn. Snappers, not so much.

quote:

Originally posted by bigdeal:

What is your recommendation as to how to find top dead center and/or put the motor in the right position to adjust the valves? Haven't done a valve adjustment like this before so any advise or recommendations would be greatly appreciated.

OK you DO have the OHV engine.

Remove the valve cover and spark plug.

Manually rotate the engine until the intake valve is all the way down. Grab the rocker and try to rattle it. It should have free play around .007" - .010". They probably set them at 0.007 at the factory and wear has seated it back. Err on the side of loose. Tight will get it burned. Set it at .010-012". It won't hurt it to be loose.

Do this for each valve.

quote:

Originally posted by thunderson:
Sounds like possibly vapor lock. I'm not real familiar with your mower but engines usually get hotter right after they're turned off and then start cooling after a while. You can try insulating your fuel line and see if that helps.

This
Had a craftsman that did that,I wound up with a piece of romex wire to keep the shut of break to keep it running while I emptied the bag into the yard cart.

quote:

Originally posted by Mars_Attacks:

quote:

Originally posted by bigdeal:

What is your recommendation as to how to find top dead center and/or put the motor in the right position to adjust the valves? Haven't done a valve adjustment like this before so any advise or recommendations would be greatly appreciated.

OK you DO have the OHV engine.

Remove the valve cover and spark plug.

Manually rotate the engine until the intake valve is all the way down. Grab the rocker and try to rattle it. It should have free play around .007" - .010". They probably set them at 0.007 at the factory and wear has seated it back. Err on the side of loose. Tight will get it burned. Set it at .010-012". It won't hurt it to be loose.

Do this for each valve.

Thanks for the input. One more question. I found the valve lash specs on B&S's website....Intake .004 - .006 and Exhaust .004 - .008. Obviously those are a whole lot tighter than what you recommended. Comments.

What I recommend is no chance of it burning a valve.

Always set to the loose, unless you are going to race it.

Is this a high performance racing lawn mower?