troy bilt mower wont startwhen warm

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My troybilt 21 in. Lawn mower. Brigs and stratton eng. 6.75 hp.mod# 126T02-0675-B2. 10072757. Auto choke and auto prime. Starts on first pull every cold start. If I dare let it turn off, such as to empty bag. It will not restart till engine is cool. Maybe 1 hour. New spark plug, new air filter, I cleaned the brass orfice nut on carburetor. Noticed the choke flapper stays open when warm. If I have someone cover carb. Hole or close flapper manually it fires right up. Could the bimetal auto choke be bad? All springs are connected.
 
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I would check valve clearances. Compression begins to suffer and won't pull in the fuel.
 
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B&S flatheads valve clearances seldom goes bad (and to adjust them is a pita) 1st thing first: check your flywheel key is not sheared (to ensure the timing is correct). Your auto-choke carb is troublesome. take the fuel bowl down and clean out the debris and get compressed air to blow clear all the orifices. also: ensure that your paper air filter is new and not clogged. Q.
 
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Originally Posted By: dwsmith43
My troybilt 21 in. Lawn mower. Brigs and stratton eng. 6.75 hp.mod# 126T02-0675-B2. 10072757. Auto choke and auto prime. Starts on first pull every cold start. If I dare let it turn off, such as to empty bag. It will not restart till engine is cool. Maybe 1 hour. New spark plug, new air filter, I cleaned the brass orfice nut on carburetor. Noticed the choke flapper stays open when warm. If I have someone cover carb. Hole or close flapper manually it fires right up. Could the bimetal auto choke be bad? All springs are connected.
Check your fuel line(s). We have an older mower that did something similar before. It ended up being an internally collapsed fuel line that only showed its bad side when it got warm from the motor.
 
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Check the flywheel brake/bale control cable. It may be the ignition failing but start with the cable. Replace it if the cable binds up, requires effort to pull or is kinked. the number of Briggs flatheads I see, I would estimate 1% actually have a failed ignition, 75% need the carburetor cleaned and 24% need a new cable.
 
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I doubt that partially sheared flywheel keys would only act up when warm.Engine timing would be affected and that engine would run/start poorly all the time. Same with the cable mentioned. If he manually chokes it ,the engine fires right up. That tells me it is not pulling in fuel when warm to start the engine.I would bet that the compression is borderline when warm making the engine hard to start.
 
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Kestas

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I once had the exact same problem - no warm restart, but starts cold and runs fine. The problem was a partially sheared key.
 
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Yup! +1 to what Kestas said. I have once serviced a 126T series Craftsman 21" B&S autochoke, same symptom. It was due to partially sheared key. Also just a little reminder: these B&S sidedraft engines run very lean (when compared to the older pulsajet or even vacujet type, which I consider them to be quite "dirty" running due to rich A/F ratio). Because it runs so close to the stoichiometric ratio, any slight shift of the ignition timing will have a detrimental effect to the engine especially when it's warmed up (autochoke completely opened, carb in normal running condition). Any slight timing "off" will cause serious hard starting when warm, and will pop occasionally when you repeatedly try to pull start the warm engine (choke off). Q.
 
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Mine has same problem, starts first pull when cold but warm/hot restarts are a no-go. I have to prop the auto-choke open manually...it then starts first pull. My auto-choke isn't sticking and moves freely to the touch. I'm practiced enough now that I can prop the plastic arm open w/ a stick.
 
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PS: you can test this by dropping the air filter and propping the choke open with a screwdriver (big enough not to get swallowed when the engine starts).
 
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With the auto choke, if it's operating normally, you must have a clean carb with good fuel in it to get it to start after the engine has been run for awhile. If you try to restart it, and it draws water and yucky fuel into the intake on the first pull, the next couple pulls will just flood it out and it won't start until things dry up. Auto choke demands a clean carb with good fuel.
 
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If you have to prop the choke open when the engine is warm/hot, there is a problem with the choke, as it should be open on its own, and not closed!
 
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The reason they start after you prop the choke open is that every pull after the first couple are not sucking large amounts of fuel out of the carburetor bowl through the nozzle and into the intake. If the choke swings free back and forth, the choke isn't the problem, though it exacerbates the problem by staying closed. I have seen debris behind the muffler keeping things from swinging free. Look out after you take the muffler bolts out, it could be confusing to someone who hasn't done it before…the parts just fall away from the engine. Where are you at in Nebraska? I'm in Bellevue.
 
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Originally Posted By: yeehaw1960
The reason they start after you prop the choke open is that every pull after the first couple are not sucking large amounts of fuel out of the carburetor bowl through the nozzle and into the intake. If the choke swings free back and forth, the choke isn't the problem, though it exacerbates the problem by staying closed. I have seen debris behind the muffler keeping things from swinging free. Look out after you take the muffler bolts out, it could be confusing to someone who hasn't done it before…the parts just fall away from the engine. Where are you at in Nebraska? I'm in Bellevue.
Clogged mufflers are often seen with 2 cycle engines.Never seen one with a 4 cycle.
 
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I've notice that eggs are most often brown or white. Sometimes they are many different colors, i.e., during the Easter holiday. I'm actually talking about the "heat riser" behind the muffler that causes the choke to swing into the open position after the engine warms up. I know, if my mother heard me smart off like that, she'd slap my face. No need for child protective services though.
 
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