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.
Well after successfully mowing 3 times over three weeks, it wouldn't restart after I shut it down.
I noticed when I fired it up first off before mowing, black smoke blew out the exhaust for about 10 seconds which it has done on and off for a long time.
Then I would reckon it's run extremely rich as most of my fuel was used up, when it usually uses 1/2 tank at most (it's winter here and almost no grass growth)
It had cooled down a lot when I tried to restart it.
I haven't taken the spark plug out but I'm almost certain it's fouled with fuel.
I have a B & S magneto on order and it was supposed to arrive nearly 2 weeks ago.
Hi Geesey What I would try is lock the choke wide open and put a squirt of Start Ya Bastard in the air filter so you can get it started and see how it runs. If you don't have any starter fluid get it started with the choke closed and then lock it open somehow and see how that goes. I'm afraid you have wasted your money on a new coil because that is not the cause
I've tried the starter squirt and the choke was fully closed when I tried to start it...no go. These mowers seem to last 4-5 years before they need extensive repairs...well that's my experience.
But that was not what I said, I said wedge the choke open and then give it a squirt of starter fluid. Yes they are not a good motor, you might be better off finding a good sidevalve motor on a rusted out body and fit that to your body.
Do not waste anymore money on this Briggs Poo look around and see if you can come across a Honda GXV 160 but before you pick one up let us have a look here because the later GXV160 have the brake assembly underneath and it is a problem you are not going to be able to sort out
People also didn't like using Champion plugs because they had cut threads that they said weren't suitable in alloy heads so they used NGK or Bosch that had stronger rolled threads.
Only problem with cheaper plugs is to make sure they are genuine ..
Often wondered what is actually happening when a spark plug goes "bad".
I have plugs which are are 50 + years old still working perfectly.
Last week I was using a 20 year old brushcutter with it's original NGK plug and it refused to start after a fuel refill. I took out the plug, put the body on a good earth and couldn't get a spark across the gap, nuthing, yet when the body was held 1/4 inch away from an earth an excellent spark jumped from the body to earth.
Cleaned the insulation around the central electrode, brightened up the end of electrode and the bridge metal bit with a thin file but couldn't get a spark across the gap.
Seemed that the plug's insulation had broken down.
Seemed that the plug's insulation had broken down.
Why ?
Detonation subjects the combustion chamber to adverse mechanical shock pressures. The effect of this mechanical shock can damage spark plug electrodes or crack the insulator core. Detonation in an engine will be audible . When an engine has experienced light to medium detonation, there can be little or no additional evidence. Prolonged heavy detonation which can lead to spark plug damage becomes apparent upon cylinder removal or via borescope inspection. Detonation damage usually shows in the edges of pistons and on the cylinder head between the spark plug ports and valves. The cylinder head and piston should be inspected for a “sand-blasted” look. A lack of deposits or a clean head and piston face can also indicate that detonation has occurred,
carbon buildup within the cylinder itself or improperly functioning spark plugs can cause pre ignition.
In spark-ignition internal combustion engines, knocking (also knock, detonation, spark knock, pinging ,pre ignition ) occurs when combustion of some of the air/fuel mixture in the cylinder does not result from propagation of the flame front ignited by the spark plug, but when one or more pockets of air/fuel mixture explode ...
You can even drop a spark plug and damage the insulator but you can't see the damage.
Thanks for your reply Max but apologies - I didn't word my question properly.
"Seemed that the plug's insulation had broken down."
What I meant was the insulation had stopped insulating. There was no sign of any of the ceramic insulation material being damaged, but obviously there was high voltage going through the insulation material. The material around the central electrode exposed to combustion looked 100 % intact with normal deposits after years of use and I cleaned that carefully. The material outside the cylinder head also looked perfect, no cracks, clean and shiny.
When testing the plug by putting the body on bare metal there was no spark across the electrode gap and when testing by separating the plug body a few mm away from the metal earth there was a very obvious spark jumping across to earth.
The most common cause of plug failure is carbon fouling.
You may need to sand blast the plug and check it for spark.
Carbon deposits are conductive, and as they accumulate along the insulator nose they reduce the insulation resistance of the spark plug.
You may not always see Insulator Damage: Heat can cause the insulator (the ceramic part of the spark plug) to crack or degrade. This can compromise the spark plug's ability to maintain the necessary electrical isolation and can lead to misfires and breakdown.
There is no real way to see if the insulator is bad without disassembling the plug. Yes deep inside the metal the plug ceramic insulator can be cracked. I have done this just to prove to myself what I was thinking was happening on some engine where the plug would fire outside the compression zone but not when under compression load.
Of course this will destroy the plug. Just easier to test with a known good plug. Even new plug can be bad right of the box. And just accept the results especially under shop conditions where time wasted is not good for the bottom line. When shop rates are around $65 or higher a $3 plug can cost a lot more.
But during quite times learning what was the problem does help in future troubleshooting. Here the 10mm NGK CMR6 plug has such a high failure rate that I replace them with Bosch plugs which rarely see a failure with.
I only ever use NGK or Bosch plugs and yes that was kind of part of the problem.
Another guy tested the magneto and said it was on they way out...I have to trust him on that.
The real problem was the throttle cable which had become frayed, stretched and nearly broken inside the outer protective layer.
It must have just been hanging on just enough, to cause intermittent trouble, and stretching so in the end it broke and was not engaging the choke or the throttle control at all.
I couldn't get an original cable, and had to settle for another generic one, (it's control arm works opposite to the original Masport one) which was quite a fiddly fit and I had to cut quite a lot off the cable and inner wire to fit my mower.
I remembered a tip which was to first remove the inner wire and cut the outside cable cover before fitting the inner wire back on and shortening to suit.
The z bend tool I also bought with the cable came in real handy but it took me a few goes to perfect the z bend technique.
After installing the new magneto, spark plug and throttle cable the mower still wouldn't start UNTIL I realised:
1. the new plug had no visible gap straight out of the box (never happened before and I didn't notice when I installed it) ..maybe I bumped it or something. 2. the plug connection, as part of the new B&S magneto, had a much longer boot and required pushing much deeper onto the plug cap.
After that it fired up and after a few start stops, kept firing first pull.
Maybe I could have got away with only a new throttle cable
Thanks to everyone for their input and hopefully I get a few more trouble free years out of it now, rather than posting again with more troubles
It's more a learning journey for me rather than paying a hundred dollars an hour at a mower shop
