OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines Briggs and Stratton problems 550EX O9P702-0127-H5

My mower was purchased in 2020 and has performed well up until early this year. It's been exhibiting intermittent problems like surging, misfiring, blowing smoke, etc. The latest problem is intermittent as sometimes it will start up and run perfectly and other times it will misfire and won't rev or operate properly. I have replaced the carburettor, spark plug, air filter, cleaned out the fuel tank and the fuel lines. I am now thinking it may be a faulty magneto or auto choke. What do others thin?

Hi Geesey,

I'd take the covers off and check the choke is operating correctly and clean out any grass trapped around the motor with compressed air.

then I would adjust the valve clearance , found this video below that shows the correct procedure ,lots of other videos were wrong
and were adjusting the clearances at top dead centre ,that doesn't work with a compression release motor.





Cheers
Max.

Thanks so much. I have been trying to find that valve timing video for 550EX motors for ages. The auto choke as you know runs from an inbuilt heat sensor on the exhaust, so if it's faulty it means replacing the whole exhaust @ $100 AUD. My choke linkage wire is badly bent and the sensor on the exhaust is also bent, which was probably done when I sent it into the repair shop to clean the carby a year or so ago. I may have to replace the choke wire linkage first. Anyway you've been a great help and I'll let the forum know how I go with the valve clearance settings.

If the choke linkage wire is badly bent and the sensor on the exhaust is also bent you should be able to bend it straighter so
the choke butterfly is fully closed when the engine is cold and when the engine is warm the butterfly should be fully open.


You could also check the coil is ok ,check the air gap ,make sure the kill switch wire isn't shorting intermittently ,sometimes
you run the motor with the kill switch wire disconnected to the coil and if the motor runs fine you know the problem
is in the kill switch side, another test I sometimes do is hook a timing light to the coil and watch the flashing light
when the motor misfires ,if the timing light stops flashing when the motor misfires you know it's a spark problem ,even
a cheap spark tester will work if you don't have a timing light.

https://www.briggsandstratton.com/e.../ignition-system-theory-and-testing.html

First question is where did you source replacement carb from? Genuine or aftermarket?

Have had issues of the choke binding up and/or jet block being drilled improperly.

I would do is change the plug again

I just fixed a victa that had an intermittent issue with similar symptoms- the barb on the tank was not moulded right and had a little thin whisp of plastic intermittently blocking flow. Somehow it managed to run fine for 5 years and only then start to show symptoms of surging and spluttering