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I have 2 B&S lawn mower engines that are behaving basically the same way. On the 1st one, I could put gas in the air inlet and it would fire right up but died as soon as the gas was exhausted. I soaked the carburetor in a cleaner then reinstalled. It now cranks without putting gas in the air inlet but dies as soon as it revs up. Repeats same as many times as I want to pull on it as long as I wait a bit between pulls.

2nd engine runs a bit after priming but then dies after revving up. Keeps doing the same thing over and over. Pulled carburetor off and now soaking.

Any suggestions?

Seen this problem hundreds of times in the shop and on this forum and my answer is still the same. Clean the carb and replace the float needle and seat. Briggs part number 398188, here is the procedure I recommend.

Needle and seat replacement

Remove the carb, and then remove the float bowl. Check the float bowl jet (which is the bowl screw) and make sure the jets both horizontal and vertical are clean and open. Tip the carb upside down and remove the float pin and float with needle attached. Look in the float needle passage and you should see the red float seat at the bottom of the passage. This is where a #5 crotchet hook would come in handy as you need to remove this seat. If you have no hook, but compressed air, you can blow through the fuel inlet and try to pop the seat out. Put your thumb over the passage to prevent the seat from flying who knows where. No air or hook try bending a stiff paper clip to dig the seat out.

I would either give the carb a good 24 hour soaking or have it ultrasonically cleaned at this time.

With the seat out clean the passage way with carb cleaner. Now you must find a drill bit slightly smaller than the passage way, to be used to press in the new seat. Apply a very, very small amount of a very light lube to the new seat. 3-1 oil or lighter, to help seat it better. Carefully insert the new seat in the passage way with the rings on the seat down toward the carb body. Slowly and carefully force the seat down with the back end of the drill bit. Once it is seated, check to see that it did not flip and the rings are up. *Next check to make sure that the float does not have any liquid in it. *If it does, replace. *If everything looks correct, attach the new needle to the float and install with the float pin centered. It everything is correct, the float should seat level to the carb body, when looking at it upside down. If everything looks good reattach the float bowl, making sure that both the bowl gasket and the nut gasket seal properly. Reinstall on the engine and test unit. Remember to have patience and take your time. Good luck, but I don't think you'll need it.

PS: *On the side of some Tecumseh carbs you will find a plastic cover. *Under this cover will be an idle jet. *Remove it and check to see that the jet is open both horizontally and vertically. *You should be able to push the old float needle wire through the vertical opening.

de04nise,

You didn't identify the B&S engines you're having a problem with but since the carb design for the 3hp-4.75hp engines hasn't changed for years; if you have an older mower available that you know ran when it last was pulled out, you might try swapping out carbs.
3 years ago on a Weedeater 500 168cc B&S engine that was purchased the previous end-of-season Fall closeout, I had the same issue when I pulled the mower out the first time to use it the next Spring.

Turns out, I believe there was a defect in the carbs manufactured for that model as there were all kinds of similar posts on this and other forums indicating the same problem. One fella said he pulled several out of the Walmart trash bin and refitted them with new carbs and they ran fine - excellent profit as well.
I was still under warranty and when I contacted B&S directly they sent me a new carb. On examination I found that there was a carb design change on the pickup tube - where the piece on the end of the tube previously was 'plastic' was now a 'stainless steel' cap fastened on the pickup tube.

BTW, I'm still using the old carb on the Weedeater 500 I scavenged off of a 15yr old Murray B&S 3.5hp whose deck frame had cracked to the point of no-repair but whose engine was still good.

Good luck,
greynold99

de04nise said:

I have 2 B&S lawn mower engines that are behaving basically the same way. On the 1st one, I could put gas in the air inlet and it would fire right up but died as soon as the gas was exhausted. I soaked the carburetor in a cleaner then reinstalled. It now cranks without putting gas in the air inlet but dies as soon as it revs up. Repeats same as many times as I want to pull on it as long as I wait a bit between pulls.

2nd engine runs a bit after priming but then dies after revving up. Keeps doing the same thing over and over. Pulled carburetor off and now soaking.

Any suggestions?

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Not knowing what you have for a B&S engine its hard to know what style carburetor you have. I was reading the two post and both post are talking about two different style carburetors. Just what are these B&S engines in other then a lawn mower because there are many.

Check your gas for contamination/water and debris. Two engines same problem

reynoldston said:

Not knowing what you have for a B&S engine its hard to know what style carburetor you have. I was reading the two post and both post are talking about two different style carburetors. Just what are these B&S engines in other then a lawn mower because there are many.

Click to expand...

It is a Quantum 5.0/21" mower. Model 124802; Type 3126-01.