OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines briggs and stratton help

Hello I own a rover lawn mower with a briggs and stratton quantum XM 50 engine,Model number (12F802) Engine Type (1975-B1)ive have tried almost everything i can think of to start it, first i flushed the fuel tank and put new fuel in still didnt start then i cleaned the coil for surface rust and set the gap again and cleaned the flywheel/magneto from surface rust still didnt start then i cleaned out carby and bowl and bowl screw with carby cleaner and cmpressed air in all the holes still didn start then i took out the valves and cleaned all the seats and the valves and the top of the piston and any other excesse carbon there was a lot of carbon still didnt start then i put new spark plug and air filters in still didnt start i also tried starter fluid and still didnt not start. lol the only thing i can think of now is piston rings? i cant think of anything else . can anyone help me !! ???? lol

what does it look like?

do u mean gasket?

Hi Blown Vl, and a big warm welcome to the forum. We're glad to have you aboard.
With your problem, do you have any spark when you ground the spark plug and pull the rope? Be careful here, you don't want to start a fire. Make sure the throttle lever is in the cold start position; ( because of ignition cut-out).
Has the engine compression?
If you have good spark, good compression, and it still won't run, it has to be fuel....carby diaphram, dirt in the main jet etc. Best of luck with it,

Simply put the problem will be in one of three categories: spark, fuel, or compression (including valve timing). If it was running OK when it was last shut down, it is unlikely to be compression, and you can normally feel it when you pull the rope anyway. You can check spark by removing spark plug, attaching plug lead, grounding plug body, advancing the speed control to maximum, and looking at the plug gap while pulling the rope. If it doesn't spark, you've found the problem. If it does spark, it's likely to be fuel not reaching the combustion chamber. You check that by putting a modest amount of fuel in through the spark plug hole, then trying to start the mower. If it starts and runs briefly then stops, your problem is fuel supply: tank, hose or carburetor.

Sorry Darryl, we posted simultaneously.

No probs Grumpy, your answer is more straight forward and good advice.

hey and thank you. it has spark and fresh fuel to the carby and i dont think it has a diaphram as a hose goes from fuel tank to carby, i have tried everything except for piston rings as i said in my first question.

it started once with starter fluid then never ever started again.

and when i take out spark plug and put my finger over hole and try start it i feel it suckin my finger in and tryin to push it out.

Check the flywheel key if you have not already.

Compression isnt really an issue I have seen a briggs run with virtually none.

ok ill check it now, how do i get fllywheel off>? without it moving?

the exhuast valve was a little burnt out but i cleaned it up and the seats and i also did intake valve too while i was there.

it can be a bit of a bugger to get the the nut off, once the nut is off you can check the key is not broken.


If you have a rattle gun (some of us like myself dont have the liberty of one of these) it will come off very easily. If not I usually soak the nut in wd40 and wrestle with either the blade disc or the flywheel to hold it stopped, being very careful to not break fins on the flywheel.

The recommended way to check compression with a B&S engine is first and most important, disconnect the sparkplug, then reach under the mower base and turn the cutter plate backwards until you feel compression, then see if you can bounce it off the compression pressure: flip it backwards and see if it rebounds. If it does, compression is OK. (This is considered better than just feeling the compression on the start rope because of B&S's remarkable compression-ruining camshaft design, but with a small amount of practice you can feel it on the start rope anyway).

Because you have apparently partly dismantled and reassembled the engine, the number of things that might be wrong with it is now much larger than it was before you did that.

im a automotive mechanic so i have some exprience

just looked at flywheel keys they are fine, the coil wire i notice has some cuts in it. and i just check for spark again and it has none its weird because the other day it had spark and it started only with stater fluid and ran for an hour-2 hours ( did front back lawn) then the next day no spark. and the magento and coil are nice and silver no rust and the gap was set with a bussiness card. could the coil be gone?

More than likely then I have been hearing of a lot of quantum coils going bad lately. I have also had bad plugs which don't spark under compression but will happily spark when tested.

If the kill wire that goes back down to the throttle cable area is insulated well and you still have no spark then.

My best bet would be the coil....

They can be found on the online store here or someone may have a second hand one here. (I do but I would have to look around the wrecked machines for one.)

Sorry I guessed you might not have known what you were doing, you've cleared that up. The way I like to check spark is with a surplus plug that I cleaned then opened up the gap to about 3 mm. If you open it too far it won't spark across the gap even though it will work in the engine.

In my experience it is not uncommon for sick coils to work intermittently for a while before they die altogether.

The key point to check before buying a coil, is that the kill wire on the old one is not somehow becoming grounded when it shouldn't. That can happen due to problems in the sliding tin bits of the governor where it grounds the kill wire when the speed control is at minimum. It can also happen upstream of there along the wire as Joe said. The diagnostic history does seem to be coming together, though, if it would start on Aerostart when the spark was working, and you know that now the spark is not working.

Blown Vl.

Welcome to the forum and I am hoping you are learning from all the people here as I do.

I am going to put my two cents worth in here and ask a question.

Did you check your valve clearance when you refaced the exhaust valve and did you lap the valves in?

I have read all the post and this is the only thing that has not been asked so far.

hey everyone and thanks for the help!!!!! ill start from top lol. about 3 days ago my mower wasnt starting so i changed my fuel with fresh stuff and it didnt start so i got a new spark plug cause it looked all old and needed to be changed and she still didnt start then i remembered the old fuel i flushed out had some chunks in it so i thought maybe carby is blocked so i took the bowl off( had chunks in it) and the bowl screw and the carby and then i used a compressor to blow out the bowl bolt holes and the carbys holes as well still didnt start but i saw on a website to clean the magneto and coil cause they get rusted after a while and give no spark so i cleaned them up because they were rusted pretty bad from mower sittin in the shed for ages and i also read on some other website to take off the black cut off cord so i did then it didnt start !!! so i brought a can of starter fluid puts heaps of that in then gave it a couple of pulls then she fired up with huge black cloud. so i left it running and put black cord on so i could turn it off when i was finshed. so i did front lawn turned it off then went to do back lawn and then it started stright away i was scared it wasnt going to start again lol but it started so i did back lawn put her back in shed came back 2-3 days later just to see if it would start and it wouldnt so thats when i did all the valves and stuff cause i thought maybe no/low compression so i cleaned up both valves and seats and the piston top and they were pretty gunked up (6 year old mower) and then it still wouldnt start so just out of curiosity i wanted to check spark again and it had none i was liek WTF! so i cleaned up the already cleaned magneto and coil and did the gap again with a bussiness card still no spark.

should i try take off the cut offf cord again?

i think it only started again on taht day because it was already warmed up. and that day it had a nice blue spark.


Yes bruce i checked valves with a feeler gauge after i cleaned them all up and the seats.


yeah joe and grummpy i think yous are right the black cord

Blown Vl,

Did you take the sump cover of when you were checking the valves and remove the lifters?

Not that i think it is valve clearences but how did you check the valve clearences and what were they?? where was the piston when you made these checks. The next question is have you checked the venturi and main jet are clear. Can you please tell me do you prime the engine with fuel or is it equiped with a choke? If it has a choke does it close properly when you place the throttle in the start posistion? When you cleaned the piston and valve were they covered in thick black carbon (big pieces of dry black stuff)?

i did not remove lifters i just took valve springs out and plates and cleaned valves thats way

i used a feeler gauge to chekc valve clerances i think i did 6thou on intake valve and 8thou on exhuast. the commpression is fine though i checked it today and its blwowing my finger right off the block and sucking in very hard and when u reverse the crank it has a stopping point. yeah the valves and piston were covered in big pieces of dry black stuff (carbon?) and exhuast valve was a bit dirty and pretty burnt and intake valve i could bearly see it was that covered and i could see sunlight througth the valves lol so iclaned the seats and took valve out and cleaned them and then checked clearnces with a feeler gauge.

I am confident that your problem has nothing to do with compression, but while the subject is being discussed, if your engine has the usual B&S "Easy Spin" camshaft design, it is important to check valve clearance with the engine on top dead center. Easy Spin holds the inlet valve .010" open for something like the first half of the compression stroke, to make the engine easy to rope-start. B&S claim that it has virtually no effect at running speed because there isn't time for the gas to leak through with the valve lifted such a small amount. If you try to set valve clearance with the engine in the first part of the compression stroke, you'll end up with an enormous intake valve clearance on TDC. Some of the tappet adjusting techniques that work for car engines, don't work for B&S engines because of Easy Spin.

With regard to spark, I think you always need to check for spark with the black cord disconnected before you condemn the coil. Make sure the cord isn't touching anything anywhere along its length, too. It's probably a dead coil, but its very irritating (to me at least) to buy parts I don't need.

guess what happend today!!! i took black cord off and primed it 3 times she almost started started backfiring and blowing out black puffs then i got the starter fluid out and put some into it and took about 4 -5 pulls then it sputter then it started right up with huge black cloud and runs way bettter then before.

The black wire was the culprit!!!! something so small and all the work i did for nothing lol! ah well it was a good learning exprience. thanks everyone for all the help.

Hi Blown VI, as I have said in many posts: I only had 2 problems with my original Scotty engine.....One was called Briggs and the other....
Viva La Honda!!

Hi all,
It is so easy to overlook that tiny pathetic black wire! 1/2 the time it is either that or the points (if installed). Briggs seems to make a fairly reliable coil, but on the other hand they are pretty troublesome at times.
Regards Jay

Hi Jay, you are so spot on there mate, that little black wire can be the difference between go or no go!!

Now that you know the kill wire (black cord) is the problem, you still need to fix it. Most likely the problem is where the grounding contact on the governor base grounds the kill wire when the speed control is set to minimum - in other words the engine stop control. It is a pretty tin-pot piece of gear (in my opinion) and also is affected by the adjustment of the speed control cable. When you look at the contact and slide the speed control between max and min, you'll probably see what is wrong. (Having said all that, I've never even seen a Quantum 50 - I'm describing the smaller vertical crankshaft engines).

If your engine was a bit hard to start and then blew black smoke, it was almost certainly too rich, probably due to the amount of priming you did (pumping the primer bulb) and the use of starter fluid - it seems to have been too rich before you used the fluid and really messed it up. If you close it down properly, let it cool completely, and then use the B&S starting procedure, it should start either first or second pull. Don't prime it unless you are cold-starting it, and don't ever use starter fluid unless you are pretty sure it's broken and isn't going to start otherwise.