Engine Starts Easy, & Runs, but dies after 1 to 10 minutes, or Under Load.

[Timothy White]

I have a Craftsman lawnmower with a Briggs & Stratton 6.75 engine (model #126T02) with automatic choke, which was given to me used and broken. After cleaning the varnish and trash out of the carburetor, I got it to run, but then it ran intermittently. It ran strong for a few hours, once, but since then it runs for 10 seconds, 10 minutes, until you put the engine under load by cutting grass, and then it dies. I can't get any clear understanding of what causes it to die, or why it was able to run for hours just a couple days ago.

I've watched a youtube video about the automatic choke, and I understand how that and the "governor" work. I've adjusted the metal plate to cause the engine to run quite fast and it still dies. I've disassembled and cleaned the carburetor at least 3 times, and each time I do this I find specks of crud in it. The bowl gasket is in terrible shape. The bottom is "fringy" and deteriorated and the gasket has expanded so that it's actually larger in diameter than the bowl. I have to use a small screwdriver to push the gasket into place all around the circumference of the carburetor bowl, to prevent it from getting pinched when the bowl is tightened.

So I know the carburetor bowl gasket is bad, and I've ordered one and it will arrive in a few days. I'm posting this thread in case there's another problem that causing this. I don't want to replace the bowl gasket and find out something else is the cause of the problem. So I have several questions.

1) The Youtube video I watched show a professional mechanic and he showed something he called the "float alignment" which was that when you turned the carburetor upside down the bowl was supposed to be level with the carburetor, more or less. He said if it's not level, it's bad. My carburetor is not level. When I turn it upside down, the float is pushed upwards, as if the needle is too long, and not by just a little bit either. It sits at least 1/8", maybe 3/16" inch higher than what I would call "level". The mechanic on Youtube talked about how the needle wouldn't "properly seat", which I assumed meant the gas would never shut-off to the bowl. In my situation it seems to me that the float would never stay open long enough to completely fill the bowl, since it would close the valve off too early. Half-filled bowl.

I just want to know if this could actually happen (is my analysis correct), and could this explain my situation. Has anyone ever seen this before? The engine had been worked on, and I wonder if the wrong bowl, needle valve, or even the whole carburetor is the wrong part.

2) Is there some kind of temperature protection system that is either working, and my engine is overheating, or it's not working, and killing the engine when it shouldn't.

3) Could there be a problem with the kill switch? There's a handle that kills the engine (somehow). It starts REAL EASY, as long as I give it a couple squirts of gas with a spray bottle. Which makes me ask:

4) I have the whole air filter assembly removed, since this engine does not have a primer on it. Does the engine need the restriction of the air filter in order to run properly?


Thanks in advance.

 

[primerbulb120]

The float's un-levelness is caused by a swollen needle seat. The needle seat is made of rubber, and is located under the float needle. It could indeed be causing your issues if it's restricting the fuel flow enough to starve the engine for fuel. It needs to be replaced regardless. To replace it, thread a screw into the old one and pull it out, being careful not to scratch the needle bore in the process. Then press the new old in with the groove facing DOWN (away from the needle and towards the top of the carb.

There is no temperature protection system on this engine.

Yes, there could be a problem with the kill switch, but it's not likely. When the engine stalls, spray gas or starter fluid in the air intake and see if it kicks over. If it fires but won't stay running, the problem is not the kill switch.

No, the engine does not need the restriction of the air filter to run properly.

And lastly, set your engine speed back where it was before you adjusted it. You could blow the engine running it over about 3800-4000 RPM. Anything over 3600 is not healthy. A faster engine speed will simply make it stall sooner because it's using more fuel.

 

[primerbulb120]

Oh, and you should probably replace the fuel line and add a filter as well. The black particles could be coming from a disintegrating fuel line , or from the gas tank itself.

 

[Timothy White]

The float's un-levelness is caused by a swollen needle seat. The needle seat is made of rubber, and is located under the float needle. It could indeed be causing your issues if it's restricting the fuel flow enough to starve the engine for fuel. It needs to be replaced regardless. To replace it, thread a screw into the old one and pull it out, being careful not to scratch the needle bore in the process. Then press the new old in with the groove facing DOWN (away from the needle and towards the top of the carb.

Thanks, you've confirmed for me what I suspected. I don't like blindly believing what I see on Youtube videos; I need a human being to tell me what's what.

The video shows the needle and seat replacement process, except he recommended using a "picking tool" instead of a screw. I get your point about making certain to not scratch the interior of the "bore".

Rather than messing with the needle & seat, what do you think about replacing the whole carburetor? They sell them for less than $10.00 on eBay:

https://www.ebay.com/itm/New-Carburetor-For-B-S-125K05-125K07-125K09-126K02-126T02-126T05-126T07-126T12/292583150920?hash=item441f509548%3Ag%3Aka0AAOSwQodZbcWX&LH_FR=1&LH_RPA=1&frcectupt=true

Obviously cheap, and low-quality, but is there a compelling reason to NOT get them. I haven't priced the official, "OEM" parts yet, but I'm guessing they're going to be at least twice what a new (cheap) carburetor will cost.

Thanks also for ruling-out all my other crackpot theories.

 

[Timothy White]

Oh, and you should probably replace the fuel line and add a filter as well. The black particles could be coming from a disintegrating fuel line , or from the gas tank itself.

For as often as I'm taking this thing apart and putting it back together, I should get a fuel cut-off switch also. Thanks for the advice. I hadn't considered that the fuel line might be deteriorating.

(rant begins) I have literally put this thing together EVERY. SINGLE. WRONG. WAY. THERE. IS. Everything has to go back together in exactly the right way and in exactly the right order. Also, NO, that little cup underneath the pull starter is NOT a "convenient place to put these little parts". It's the place where you find them after you take the whole thing apart again, while looking for them. Also: "NO. It's NOT a "good idea" to use a leaf blower to blow all the grass and debris off the disassembled lawnmower. You'll lose the tiny little auto-choke spring if you do that." i just needed to say that to some people that have been there. (/end rant).

On the up-side, if the marine corps ever needs a guy that can disassemble and reassemble this engine while blindfolded, I'm their man.

 

[bertsmobile1]

Obviously cheap, and low-quality, but is there a compelling reason to NOT get them. I haven't priced the official, "OEM" parts yet, but I'm guessing they're going to be at least twice what a new (cheap) carburetor will cost.

The after market parts companies like Prime Line , Stens , Oregon & Rotary all sell carbs from the same factories.
Theirs are a bit dearer because they guarantee the one you buy will work.
If you believe that you are going to win the lottery in time to pay all your bills then go ahead & get the $ 10 carb off ebay.

All of the scrap parts that Briggs , Stens, Oregon, Prime Line & Rotary reject as being sub standard end up on Ebay & Amazon
Over production also ends up being sold on the same platforms.
How lucky do you feel ?

Dad had a wonderful little line
"He who buys cheap pays twice"
With buying from the web I modified it to
"He who buys cheap pays thrice"

Want ti guess how many cheapskates come through the gate with a box full of scrap metal they bought cheap off the web ?
Carb kits with the wrong parts, complete carbs with no hole in the main jet , complete carbs with no main jet, complete carbs with the emulsion tube not drilled, complete carbs with the bowl vent not drilled ( that was really hard to find @ $ 60/hr ).
Complete carbs with the wrong floats, complete carbs with the wrong float needle. complete carbs that the idle outlet was not drilled so they just screwed the idle jet in till it made a hole in the side of the carb throat.
Now this is a biased list, because naturally the people who got good ones never brought them to me because their new carb would not work.

Just because it is OEM or from an OEM factory does not mean it is OEM quality, or for that matter will even work.
Just because it "fits" a particular engine does not mean it will work.
Find a supplier who puts the words "Guaranteed to work properly" and you have a better than average chance of getting a good one, even if you have to send 3 bad ones back .

 

[bertsmobile1]

And so you can understand what is happening.
At any setting that you make, without the engine being under load, the engine is idling.
At idle most if not all of the fuel that goes into the engine goes through the idle circuit.
Once the engine gets a load on it the governor relaxes and the throttle butterfly opens up fully.
In this condition nearly all of the fuel passes through the main jet, or in your case, does not pass through the main jet, so the engine stalls.

Not sure which model Briggs you have but a lot of the carbs can only be cleaned with a bath in a ultrasonic cleaner after all of the plugs & plates have been removed.
Modern fuel dries out to a very sticky very hard to remove gum that will not redissolve in fuel so in some cases they just can not be cleaned.
Most carbs now days require you to remove all or most of the blanking off plates or plugs because if you don't you end up with a plug of material that floats between the inlet & outlet holes so some times it passes fuel/air & some times it does not.
So from the point of a technician, if you have not removed them and put a new kit through the carb, you have not fully cleaned it no matter how may time you take it off , what you poke through it and what you dip it into.

 

[primerbulb120]

And so you can understand what is happening.
At any setting that you make, without the engine being under load, the engine is idling.
At idle most if not all of the fuel that goes into the engine goes through the idle circuit.
Once the engine gets a load on it the governor relaxes and the throttle butterfly opens up fully.
In this condition nearly all of the fuel passes through the main jet, or in your case, does not pass through the main jet, so the engine stalls.

Not sure which model Briggs you have but a lot of the carbs can only be cleaned with a bath in a ultrasonic cleaner after all of the plugs & plates have been removed.
Modern fuel dries out to a very sticky very hard to remove gum that will not redissolve in fuel so in some cases they just can not be cleaned.
Most carbs now days require you to remove all or most of the blanking off plates or plugs because if you don't you end up with a plug of material that floats between the inlet & outlet holes so some times it passes fuel/air & some times it does not.
So from the point of a technician, if you have not removed them and put a new kit through the carb, you have not fully cleaned it no matter how may time you take it off , what you poke through it and what you dip it into.

This is just a fancy autochoke Briggs Quantum. There's no idle circuit on this carb, just the main jet. Also no welch plugs. Simple and easy to clean, one of my favorites :thumbsup:

 

[primerbulb120]

For as often as I'm taking this thing apart and putting it back together, I should get a fuel cut-off switch also. Thanks for the advice. I hadn't considered that the fuel line might be deteriorating.

(rant begins) I have literally put this thing together EVERY. SINGLE. WRONG. WAY. THERE. IS. Everything has to go back together in exactly the right way and in exactly the right order. Also, NO, that little cup underneath the pull starter is NOT a "convenient place to put these little parts". It's the place where you find them after you take the whole thing apart again, while looking for them. Also: "NO. It's NOT a "good idea" to use a leaf blower to blow all the grass and debris off the disassembled lawnmower. You'll lose the tiny little auto-choke spring if you do that." i just needed to say that to some people that have been there. (/end rant).

On the up-side, if the marine corps ever needs a guy that can disassemble and reassemble this engine while blindfolded, I'm their man.

Oh, and you should also ensure that the holes in the float bowl screw are COMPLETELY clean. I had a Craftsman walk behind weedeater with the same issue you're experiencing, and it was because the holes in the screw were partially clogged.

 

[primerbulb120]

Thanks, you've confirmed for me what I suspected. I don't like blindly believing what I see on Youtube videos; I need a human being to tell me what's what.

The video shows the needle and seat replacement process, except he recommended using a "picking tool" instead of a screw. I get your point about making certain to not scratch the interior of the "bore".

Rather than messing with the needle & seat, what do you think about replacing the whole carburetor? They sell them for less than $10.00 on eBay:

https://www.ebay.com/itm/New-Carburetor-For-B-S-125K05-125K07-125K09-126K02-126T02-126T05-126T07-126T12/292583150920?hash=item441f509548%3Ag%3Aka0AAOSwQodZbcWX&LH_FR=1&LH_RPA=1&frcectupt=true

Obviously cheap, and low-quality, but is there a compelling reason to NOT get them. I haven't priced the official, "OEM" parts yet, but I'm guessing they're going to be at least twice what a new (cheap) carburetor will cost.

Thanks also for ruling-out all my other crackpot theories.

Chinese parts are hit or miss. I can replace a needle and seat in under a minute (yes, experienced, but it's not hard.) I don't replace these carburetors because they are so easy to clean.

If you really want to replace it, get OEM. $48 on Ebay.


The parts I replace when rebuilding one of these: Both float bowl gaskets, float, needle seat, fuel line and sometimes I'll add a fuel filter. You can get all the internal carburetor parts you'll need by ordering Walbro part #s "K11-LMR" and "75-570-1". This carburetor is a copy of the Walbro LMR, so all the Walbro parts fit perfectly, and it's a lot cheaper than buying Briggs parts.