OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines B&S LOSING POWER

Hi people, wondering if some one could help me out. I have a B&S 4 stroke MODEL TYPE CODE
98902 2163 B1 02042951
IT SEEMS TO BE LOSING POWER AT FULL REVS, I HAVE CLEANED FUEL TANK,CARBY, PUT IN NEW SPARK PLUG AS WELL AS CLEANED COIL AND FLYWHEEL MAGNENTS AND CHECKED VALVES JUST IN CASE. IT PURRS LIKE A KITTEN WHEN IDLING BUT WHEN TAKEN UP TO FULL REVS IT WILL HOLD THERE FOR APPROX 10 SECS THEN DIE DOWN AND "COUGH" AS IF IT IS MISSING. ANY HELP WILL BE APPRECIATED.

Hi Paul.Foot, full throttle usually means choke for starting and once it starts if you leave it on full it could be flooding , so once you start your engine if you back it off just a little it should run smoothly at high revs and your throttle should have choke/run and stop written on it or it may have worn off or replaced . Hope this is of some help ,

The engine was built on 29 April, 2002. The plant code (51) refers to one of the administrative centers at Briggs' head office in Wauwatosa, Wisconsin, not an engine plant. This seemed to start happening about the time Briggs' China plant opened, and I have never seen or heard of a Briggs engine that actually has the China plant's code (28) on it. I'm not accusing anyone of anything, but I'm not discounting that particular conspiracy theory either.

Here is the Illustrated Parts List:
http://bsintek.basco.com/BriggsDocumentDisplay/18howHXCnfBhU7y.pdf

Here is the Operator's Manual:
http://bsintek.basco.com/BriggsDocumentDisplay/flmrtKX-nfBhU7y.pdf

Note that this is a Pulsa-Prime carburetor, and there is no choke coupled to the speed control. The manual says always start it with the speed control on maximum.

If a Pulsa-Prime carburetor like yours will only run at full speed for 10 seconds, then show signs of running out of fuel, the chances are it took 10 seconds to empty the fuel reservoir in the top of the fuel tank. Since it will run properly at lower speeds, there is a good chance it is suffering from insufficient fuel pump output. I think the first thing to do is to inspect the fuel pump diaphragm.

Sorry Peter foot and Grumpy, I should read the question properly.

Thanks Roebuck.

Hi Grumpy, have had a look at the diaphragm, looks okay to me BUT should it have a slit in it as it looks as there are 2 pieces that have been press heated together which are coming apart. Will get some photos first thing in the morning show you what I mean.
Thanking you Paul.

Hi Paul.foot, your symptoms suggest a dodgy diaphragm. At about $9-$10 for a genuine one or $3-$5 for a non genuine one they are fairly cheap to replace and more often than not will solve your problem. A quick carby clean and replacement of the diaphragm and you should be good to go.

Thanks Bigted, I think you and Grumpy have sussed it out for me.
Hi Grumpy here are the photos of the diaphragm I said I would send along. In the third photo you can see a slit that has started (pointed out with a pencil) will this be the cause of my problems or do they just wear out over time.
Thanks Paul.

The diaphragm is shown as Item 394 in the Illustrated Parts List, Paul, and it is shown in two pieces. The one that goes against the carburetor acts as a gasket, and the one that goes against the top of the fuel tank acts as the diaphragm - it seems Briggs now only sell the two parts together rather than separately.

If you look at the diaphragm part, you will see the area that has to stretch up and down as it pumps (the diaphragm function), and two flaps of the rubbery material that have to open and close holes in the top of the fuel tank (the inlet and outlet valve functions). Diaphragms usually fail because the flaps no longer lie flat and seal the holes in the tank, or less often, the part that pumps by stretching up and down has gone a bit stiff or baggy. You may be able to see a fault in one of those areas by looking at it closely. Sometimes you can't see a fault, but the problem still goes away when you replace it.

Hi Grumpy, have had a good hard look at the parts you mentioned and can see what you are talking about. I will get a new one and let you know how I go, hopefully flat out again.
Thanks Paul.

Sounds good Paul. Just remember the assembly order: diaphragm against the tank, gasket against the carburetor:


If you aren't sure you had the diaphragm against the tank last time around, it might be worth assembling it again and giving it a last try before replacing it.

Hi Grumpy, got myself a new diaphragm yesterday after a long drive ( that's the only problem with living out in the sticks with no suppliers around, 200kay drive, lucky the missus wanted to do shopping as well or it would of had to wait another week ). Put new diaphragm on, got to excited and almost forgot the spring, had to take apart again and fit it, second pull kicked over and now purrs like a kitten idling and on full revs.
Thanks everyone for your help, much appreciated, Paul.

Thanks for reporting back Paul. You've contributed another good clear case to our archives, and it will probably help a lot of other people in the future.