# B&S Engine Won't Start When Hot



## InstantLegend (Jun 5, 2005)

Hi,
My lawn tractor has a B&S 404437 twin cyl engine. It starts right up when cold and runs great. It will run for hours. Once I turn it off it won't start again untilt he next day when it is cold. Here's what I have tried so far:

Rebuilt the carburator
Rebuilt the fuel pump
Replaced spark plugs
Replaced air filter and element
Replace the fule line and fuel filter
Cleaned the gas tank
Replaced pulse hose to fuel pump

When it was hot and wouldn't start I tried this:
Checked spark and it has good spark
I checked the compression and it is good (92 in both cyls)
When it won't start, both spark plugs are completely dry

Coud the problem be a vacuum leak? I don't have a vacuum tester.

One other thing I noticed si that gas spurts up in the air when trying to start it with the air filter off. I am not sure if this part of the problem or not.

Please Help!


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## 30yearTech (Sep 14, 2006)

I think you might need to check the valve lash, it sounds like to me at least that there is insufficient clearance on the intake valves, particularly when hot.


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## InstantLegend (Jun 5, 2005)

If it was a valve clearance issue, wouldn't I have a low compression reading?


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## 30yearTech (Sep 14, 2006)

InstantLegend said:


> If it was a valve clearance issue, wouldn't I have a low compression reading?


Not necessarily, are you checking the compression when Hot? 

The valves may seat good when cold, but not as well when hot, causing the blow by you described. L head engine loose valve clearance over time. They can also have issues with the valve seats getting loose in the block. 

I am not saying that this is the problem, but it's where I would start looking, based on the information you gave.


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## InstantLegend (Jun 5, 2005)

I found out the o ring on my compression tester was bad. I fixed the tester and did more testing.

Tested cold showed 100 on one cylinder and 105 on the other.
I mowed 2 hours and did the test again and got the same readings.

I haven't had time to check the valve clearances.
Does anyone know what the valve clearance specs are for this engine?


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## Ross369 (Apr 16, 2012)

The valves may seat good when cold, but not as well when hot, causing the blow by you described. L head engine loose valve clearance over time. They can also have issues with the valve seats getting loose in the block.


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## 30yearTech (Sep 14, 2006)

InstantLegend said:


> Does anyone know what the valve clearance specs are for this engine?


Intake = .004 - .006"

Exhaust - .007 - .009"


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## InstantLegend (Jun 5, 2005)

Thanks for the valve specs!


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## InstantLegend (Jun 5, 2005)

I noticed one cylinder pops and back fires a little when hot. Is that another indication of poor valve clearance? I need to find time to check the clearances.


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## rtgirard (Jun 7, 2012)

Regarding the hot restart problem... is this a manual or automatic choke? I ask because I'm having a warm restart issue on a B&S 190cc on my Snapper which appears to be choke related. Even when warm, this engine wants a bit of choke and the thermostat that controls the choke plate isn't closing the choke at all until the engine is nearly stone cold. If i manually move the choke slightly closed then it fires up right away when warm.


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