Lawnmower Won't Start When Engine Hot

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I have a push lawn mower with a B&S engine 123K02-0185-E1. It is 13 years old but I maintain it very well. Starts right up when cold. And it runs fine. However I have an awful time starting it again when it's hot. Sometimes takes 5 or 10 minutes before I can restart it. Has new plug, new gas, new air filter and gas cap replaced also. Like I say...starts right up on first pull when cold. Thought it was the cracked fuel supply line but I replaced that and it did not make a difference. Haven't checked for spark when it is hot. How about the valves? Doesn't look like they are easily adjustable. Any ideas please? Thanks.
 
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Float level?
I once got an issue similar to yours.
Gas got inside the plastic float somehow, resulting in an "always open" float needle valve.
 
My mower did this. Started fine when cold. But once hot, it would not start. I was sure it was the coil, so I replaced it. That was not it.

Turned out to be a sheared flywheel key.
 
Originally Posted By: Atesz792
Float level?
I once got an issue similar to yours.
Gas got inside the plastic float somehow, resulting in an "always open" float needle valve.


Bingo. That was my first thought. What kinda gas do you use in it? Small machines prefer high test gas but let's be honest, not everybody does it. I bet the floater is the culprit. You can take the carb apart and give it a soak or try running it and doing a seafoam tune-up. That might do the trick. I usually throw a cleaner in my gas tanks when I fill them up anyways. Never had that issue.
 
Originally Posted By: jkon
Haven't checked for spark when it is hot.


That's the key right there; it might have an ignition coil on its way out.

In the meantime, I got the parts list and the service manual. PM me your email address.
 
Originally Posted By: drtyler
My mower did this. Started fine when cold. But once hot, it would not start. I was sure it was the coil, so I replaced it. That was not it.

Turned out to be a sheared flywheel key.

I had the same experience. No hot start, which was traced to a sheared flywheel key. Started fine when cold.
 
I see two have replied back with sheared flywheel key. I quite do not see what that would have to do with not starting when engine hot. Please explain. Thanks!
 
I have always had that problem with B&S engines. I just bought a Poulan lawn mower (not riding) with Koehler motor. Starte up very first time. Ran it for an hour and a half in the Texas heat and it still started on the first pull. Home Depot Online only. All the ones in the store were B&S. $199 and free shipping. My neighbor has had a top of the line Sears riding mower w/pto and Koehler V twin. Still running like brand new after 15 years.

I am ready for the first oil change. Does full synthetic do well at the extreme heat that air cooled engines produce? If not which holds up better to extreme heat 5w -30 or 30w?
 
I as well would like to know what did the trick cause I have a 2year old 6hp aprox Briggs that is now doing that. I will say when it happens, all I do is pour in fresh gas and it starts right up. So I wonder if there could be some debris in our tanks. I need to check that.
 
You need to do some troubleshooting first, otherwise anything else is a guess. Your engine needs 4 things to start-fuel, air, spark and timing.

Start with the simplest first-when the issue happens does it have a bright, snappy spark? If not, it's an electrical issue.
Open and close the fuel tank cap. Does it start now? Then it's a partially plugged vent. Pull the breather and check the choke-is it choking itself out with a faulty choke (you may not need choke when the engine is hot)? Linkages working correctly?
If spark and choke are fine, then it's probably fuel or air. Will it quickly start when hot with a shot of ether? Yes, then it's likely fuel. If not then you can look at timing.
 
Rivit- very good call. These engines are not complex.
Wait till it fails. check the basics.
A volt meter can be your friend.
Do a resistance test on the coil, the plug wire , and the cdi.
You can find the values online of working units.
On a CDI motor its usually the coil , not the CDI.
I am fixing a newer Solex moped right now.
The coil failed when hot , now it will not fire at all.
Checked specks on new units and found the coil has too high resistance.
Part is on order. Problem solved.
 
Originally Posted By: Bgallagher
Originally Posted By: Atesz792
Float level?
I once got an issue similar to yours.
Gas got inside the plastic float somehow, resulting in an "always open" float needle valve.


Bingo. That was my first thought. What kinda gas do you use in it? Small machines prefer high test gas but let's be honest, not everybody does it. I bet the floater is the culprit. You can take the carb apart and give it a soak or try running it and doing a seafoam tune-up. That might do the trick. I usually throw a cleaner in my gas tanks when I fill them up anyways. Never had that issue.
Not a single piece of OPE in my possession has an owner's manual which says to use "high test" gas. Not one.
 
So it's a flat-head, side valve B&S so the valves can be adjusted for lash but they need to be removed to file down the stems. The lash can be measured while the valves are still in the block.

One possibility is that the lash on the exhaust valve is too tight and when it gets to operating temp the valve won't fully close. While it's running that's not a problem but when you go to restart it you cannot get enough compression to fire off the A/F mixture.
 
The magneto is getting heat stressed. Same thing happening to my 1963 Zinsco 30 Amp breaker.
grin2.gif
 
Originally Posted By: HerrStig
Originally Posted By: Bgallagher
Originally Posted By: Atesz792
Float level?
I once got an issue similar to yours.
Gas got inside the plastic float somehow, resulting in an "always open" float needle valve.


Bingo. That was my first thought. What kinda gas do you use in it? Small machines prefer high test gas but let's be honest, not everybody does it. I bet the floater is the culprit. You can take the carb apart and give it a soak or try running it and doing a seafoam tune-up. That might do the trick. I usually throw a cleaner in my gas tanks when I fill them up anyways. Never had that issue.
Not a single piece of OPE in my possession has an owner's manual which says to use "high test" gas. Not one.

I'm on my 2nd gas jug of 93 octane. I'm trying it. I bought 3 gallons and mixed a 1 gallon of 32:1 The rest went in the 4 stroke OPE. The Toro has been starting better because I fixed it. The 11 HP B+S flathead starts great because I bought a new battery. I store gas in the shed in shade. My stuff works good on either but I was hoping the high test would keep better.
grin2.gif
 
Originally Posted By: jkon
I see two have replied back with sheared flywheel key. I quite do not see what that would have to do with not starting when engine hot. Please explain. Thanks!


It's the same as your car. On a cold start the timing is generally retarded and it advances as the engine warms up, so if the flywheel key is sheered to the point where it's advancing the timing more than it should be then the engine is probably running hotter than necessary, thus not wanting to restart. The timing of an engine refers to when the fuel is ignited in relation to the position of the piston during the combustion cycle. There is an actual fire going on inside an engine it's not just an explosion and it takes time for that fire to light up and burn, it doesn't happen instantly.
 
Originally Posted By: Bgallagher
Originally Posted By: Atesz792
Float level?
I once got an issue similar to yours.
Gas got inside the plastic float somehow, resulting in an "always open" float needle valve.


Bingo. That was my first thought. What kinda gas do you use in it? Small machines prefer high test gas but let's be honest, not everybody does it. I bet the floater is the culprit. You can take the carb apart and give it a soak or try running it and doing a seafoam tune-up. That might do the trick. I usually throw a cleaner in my gas tanks when I fill them up anyways. Never had that issue.


Bingo? How does the type of gas and your additives prevent fuel from entering the float? That would be a bad float.
 
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