Why does my B&S engine pulsate?

[spetho2001@msn.com]

I have had two WeedEater lawn mowers that I have bought from Walmart. They have both had the 500 series 158cc 5.00ft.lbs. Briggs and Stratton engine. Both of these lawn mowers in a very short period of time have started to run really lousy. The RPMs pulsate up and down while you are cutting the grass to the point where you think it is going to stall on the down RPM. This last lawn mower did stop running and would not start. I changed the plug, which was black with soot but the gap OK, and it started OK. The lawn mower had started using more gas for the same lawn. It would run out while I was mowing. When it was just about to run out the RPMs would go up and it would run fine (like it was getting more air and less gas). It seemed to me the lawn mower has been running to rich. A friend mentioned the diaphragm under the carburetor had gone bad. He said it was pulsating because it was getting air and then gas. Another friend mentioned the gas cap. He said it should have vent holes. I took the cap off, which has three vent holes. Inside I fine a cork gasket, round and taking up the whole inside of the cap. It had four vent holes which did not line up with the holes in the cap. It didn稚 make any difference if the holes lined up as the cork was loose in the cap and would just spin and misalign anyway. The cork had been sucked outward and it seems like it is sealing off the holes in the cap. I looked at my brother in laws exact lawn mower which he brings to a lawn mower shop to have serviced and there is no cork inside his cap. I don稚 know if he took it out or it did not come in the mower or the shop took it out at service time. Has anyone else had this trouble?
Thank you,
TomS

 

[Rivets]

Seen this many times. What you are going to have to do is give both carbs a very good bath with carb cleaner and replace the float needle and seat. The part number is 398188. Here is a procedure to follow if you feel you can do it.


Ok, let's solve this one. 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.

With the seat out clean the passage way with carb cleaner. Now you must find a drive 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. 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.

 

[briggs]

I have had two WeedEater lawn mowers that I have bought from Walmart. They have both had the 500 series 158cc 5.00ft.lbs. Briggs and Stratton engine. Both of these lawn mowers in a very short period of time have started to run really lousy. The RPMs pulsate up and down while you are cutting the grass to the point where you think it is going to stall on the down RPM. This last lawn mower did stop running and would not start. I changed the plug, which was black with soot but the gap OK, and it started OK. The lawn mower had started using more gas for the same lawn. It would run out while I was mowing. When it was just about to run out the RPMs would go up and it would run fine (like it was getting more air and less gas). It seemed to me the lawn mower has been running to rich. A friend mentioned the diaphragm under the carburetor had gone bad. He said it was pulsating because it was getting air and then gas. Another friend mentioned the gas cap. He said it should have vent holes. I took the cap off, which has three vent holes. Inside I fine a cork gasket, round and taking up the whole inside of the cap. It had four vent holes which did not line up with the holes in the cap. It didn稚 make any difference if the holes lined up as the cork was loose in the cap and would just spin and misalign anyway. The cork had been sucked outward and it seems like it is sealing off the holes in the cap. I looked at my brother in laws exact lawn mower which he brings to a lawn mower shop to have serviced and there is no cork inside his cap. I don稚 know if he took it out or it did not come in the mower or the shop took it out at service time. Has anyone else had this trouble?
Thank you,
TomS

its a pulse carb replace the diaphram gaskets its a 2 peace gasket ....then both your mowers will run great

 

[Rivets]

I assumed that by pulsing he meant surging. Haven't seen lawn mowers with pulsa-jet carbs. These carbs were used on horizontal crankshaft engines.

 

[natenkiki2004]

I assumed that by pulsing he meant surging. Haven't seen lawn mowers with pulsa-jet carbs. These carbs were used on horizontal crankshaft engines.

Aren't these the same things as the old pulsa-jet on horizontal shaft engines?
http://www.barrettsmallengine.com/image/mtdbriggsclassic.jpg
http://outdoorpowerinfo.com/repairs/images/briggs_carb_790206/_diaphragm.JPG

There's a diaphragm and spring. No regular carb bowl, just a small pocket of fuel with a pickup tube built into the tank.

 

[spetho2001@msn.com]

I assumed that by pulsing he meant surging. Haven't seen lawn mowers with pulsa-jet carbs. These carbs were used on horizontal crankshaft engines.

That is right. I mean surging.

 

[spetho2001@msn.com]

Aren't these the same things as the old pulsa-jet on horizontal shaft engines?
http://www.barrettsmallengine.com/image/mtdbriggsclassic.jpg
http://outdoorpowerinfo.com/repairs/images/briggs_carb_790206/_diaphragm.JPG

There's a diaphragm and spring. No regular carb bowl, just a small pocket of fuel with a pickup tube built into the tank.

This is right. My mower looks like these pictures. How about the cork in the gas cap that I mentioned?

 

[Rivets]

No, those are vacu-jet carbs. To repair the problem on that type of carb you will have to clean the carb and replace the diaphragm and gasket. Make sure that you watch how you take the gasket off And install gasket against carb and diaphragm again tank. Also, make sure that the tank is not warped in the area where the carb is attached. I doubt you have a gas cap problem.

 

[ThaxtexGM]

Have a look on YouTube you will find a vidio

 

[ThaxtexGM]

Have a look on YouTube you will fine a video how to replace the diagram & gasket and clean the carb and tank.