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?
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.
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.
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
Re: Briggs and Stratton problems 550EX O9P702-0127-H5
Well this is proving a mighty challenge.
I had already cleaned the tank and lines, replaced the spark plug and oil, replaced the carby and air filter, both with genuine B & S one, cleaned all around the engine and sourced some fuel from another petrol station.
I have only ever used genuine Briggs and Stratton parts as well as quality SAE 30 (recommended) oil.
I had been advised to install a small fuel filter in between the tank and the carby but have read where that in itself can cause flow problems.
Immediately on starting as I've indicated, the mower started first pull and I allowed it to warm up, restarted it and then garaged the mower
The next morning the mower started but immediately misfired and wouldn't rev and started blowing black smoke.
This isn't the first time this has happened but to a much lesser degree.
I then joined this forum.
I hadn't done anything else to the mower and was astonished it again started first pull yesterday and ran perfectly for an hour allowing me to mow my son's yard.
During the mow I stopped and started the mower several times without any problem and always started first pull.
I observed the auto choke was behaving properly and presumably so was the governor mechanism.
There's only one thing left, the magneto, which I have purchased (genuine Briggs and Stratton) and am awaiting it's arrival to swap on the mower.
I will also check the valve clearances, the auto choke operation and anything else I can think off
Hope you haven't spent too much on parts Geesey as last summer I sold two newish serviced mowers for $100. each ,one was 3 years old and I think it got thrown out because it needed blades and the 4 year old Victa had a rusted out base that I replaced.
People around here mostly don't pay for repairs and I think just buy new mowers from Bunnings.
The biggest mower / machinery shop here that started in the sixties closed down this year.
I spoke to another mower shop owner who closed a long time ago and he said sales went down hill after Bunnings started selling mowers.
I wouldn't be surprised if the coil needs replacing , I've had new coils out of the box not work from eBay, also had a lot of newish Chonda coils that needed replacing.
I try to not pickup free mowers these days but it just seems such a waste for them to end up as scrap metal when all they need are blades or a service ,especially if the mowers are only a few years old.
