Good day, I have a Briggs & Stratton 'Champion 35' petrol mower. I bought it second h and, and it has always been hard to start. Now it's impossible to start! I downloaded the B&S owner's operating & Maintenance manual hoping to find so me detailed instructions but there was little in there of any help.
What are some things I can check? I tried to remove the carb but it doesn't want to come completely free even with the screws removed. It seems to be catching on something. I was trying to get to the jets to see if they are b locked. I noticed that the rubber right-angled sleeve that joins two metal pipes at the carb is not very airtight, and nor is the joint between the ca rb and the engine. I can see the rubber O-ring in intact, but the joint sti ll felt a bit loose when I jiggled it.
Any suggestions as to what I can try to get it to start? It's full of petro l and the air cleaner is A-OK.
Fixing the carb securely to the engine seems the obvious first port of call. Any air leak here will weaken the mixture which will make cold starting very hard.
Lots of other things in *could* be, but you're going to have to fix this anyway so get it sorted.
Replace the spark plug and the diaphragm in the carburettor. Take the top cover off and check that it's not full of debris which would prevent the governor vane from working properly.
Years ago, I appropriated my dad's old B&S-engined mower on which the engin e would not start. I eventually discovered that the exhaust valve seat inse rt had detached itself from the head and was preventing the exhaust valve s ealing properly. I imagine that I discovered this with a compression test, but I might just have stripped the thing down for the fun of it and then ha ppened upon the problem (it was a while ago).
A compression test is easy and will allow you to rule in/out the problem I had.
I simply fixed the valve seat insert back in place with exhaust paste and t he application of a blow torch, to cure the paste. It ran fine when put bac k together.
My old B&S-powered mower never starts. Not, that is, unless I spray a couple of bursts of "Sure Start" into the carb. Then it starts first time, and always starts after that, once it's hot. (Another regular mentioned that spraying gas from a blow torch (i.e. not lit, obviously!) into the carb works just as well. It's a bit of a fiddle taking the air filter off then screwing it back on before pulling the cord -- but I know it works, so that's what I do.
I never have the time, nor the inclination, to take it apart and try and diagnose the problem further: my aim is to cut grass, and this is the quickest route to that goal.
If the mower were newer I might be more concerned to get the problem fixed.
Mowers seem to be an assembly of hard-rock components that will take piles of punishment .... combined with little itsy-bitsy delicate things like springs that have to be *just so*: nightmare.
I had a B & S like this,. I got the valves reground and it was if anything worse.
Then I noticed the choke wasn't coming on - the throttle cable that pulled the choke on past 'maximum rabbit' had stretched.. A simple two screws, slide the cable up a bit and re-tighten had it as new.
"Sure Start" in the air filter works, too. I sort of point it in an accessible air hole in the air filter cover, give it a spray which presumably ends up that foam thing, and done.
Our 20 year old 3.5hp B&S lawnmower engine "stopped" last year due to lack of oil. A few squirts of 3inOne down the plug hole and a bit of jiggling freed off the seized piston. As a special treat I put some new oil in the sump. ;-)
I had expected to have to replace the mower soon after but it's been a year now and it's running perfectly with no smoke. Started on the second pull with old fuel after its winter hibernation.
This great tip got me our of trouble - excellent tip. Luckily I had a so-ca lled 'weed wand' a.k.a. blow lamp with long neck for burning weeds. It was fairly easy to prop this up against the underside of the air cleaner while holding the throttle open and pulling the rope (without needing three arms) .
I still wish I could cure the starting problem though. I bought a brand new carburettor, complete with diaphragm, gasket and o-rings etc, but it didn' t make a ha'p'orth of difference. So I now suspect the ignition system. I c leaned up the spark plug but that didn't help. I pinched the plug gripper t ogether slightly inside the suppressor cap, but that didn't help. I guess I could try buying a new spark plug and see if that helps, otherwise, I don' t know what to try... A new coil perhaps? I don't really understand how the spark is generated on these motors. Clearly there is no contact breaker (u nless it's hidden somewhere). Clearly the spark must be occurring, or it wo uldn't run at all. I tried shortening the throttle cable to that it really does open to the max when starting. The carb doesn't have a separate choke or limiter cable. The motor does pulsate a bit rather than running dead eve nly. I also notice a little bit of smoke coming from around the silencer-th ingie after I shut the engine off.
So, seemingly not a fuelling issue (assuming no unwanted air leaks past the sealing rings or gaskets where the carb is bolted onto the inlet manifold or anywhere else between the inlet port and the carb body).
Ignition problems can be the result of several possible deficiencies or defects which can be aggravated by fuelling issues. In two stroke engines, plugs can become fouled up with a carbonised oil film on the insulator nose which provides a leakage path which can dissipate enough of the spark energy to prevent the inductive back emf mechanism in the traditional CB/spark quench capacitor magneto ignition system from generating a high enough voltage to jump the spark plug gap (and carbonised whiskers on the central electrode which shortens the effective spark length and hence its efficacy as an ignition source).
In four stroke engines, spark plugs can also suffer similar electrical leakage from extended running with an over-rich mixture (sooting up). And, finally, plugs can develop faults which mimic all of the above so investing in a spare plug is highly recommended.
Also, the cylinder pressure at the top of the compression stroke which depends on throttle opening, will vary the breakdown voltage across the plug gap, the higher the pressure (larger throttle opening), the higher the voltage required to create the spark which is why it's not a good idea to have the throttle wide open at cranking speeds (also, wide open throttle compromises the quality of the fuel/air mixture ratio at cranking speeds with simple carburettors where the slide is directly controlled from a throttle lever rather than indirectly via a vacuum controlled diaphragm).
In the classic flywheel housed magneto system, it's hidden under/behind the flywheel itself. The flywheel in this case contains magnets embedded into the outer rim of the flywheel which pass within a millimetre or less from an inner laminated ferrous stator with both low and high tension windings with a contact breaker in the LT windings, timed to interrupt the induced current at just before top dead centre (in a simple single cylinder engine, on both the end of the compression and exhaust strokes), the flywheel is keyed to synchronise the ac waveform generated in the magneto windings to achieve maximum current at the point when the contact breaker opens to generate the spark.
The nice thing about magneto ignition is its almost zero maintenance requirements in regard of 'points adjustment' compared to the battery powered Kettering system and a spark energy output proportional to demand (rather than inversely proportional as is the case with the traditional battery coil ignition system).
The battery powered Kettering ignition system has to be designed to provide enough spark energy at the minimum engine rpm limit and consequently is over-specified for startup and tick-over where, despite the CB points spark quenching capacitor, most of the surplus to requirements energy lands up counter-productively eroding the CB points in rather short order necessitating frequent filing and burnishing of the contacts themselves, along with a gap adjustment to recalibrate the ignition timing[1].
Incidentally, it's worth noting that the HT overwind on an ignition coil isn't designed to step up a mere 12 (or 6 or 24) volt pulse to the 20 to
30 Kilovolt level but rather the three to four hundred volt pulse generated across the contact breaker points when they open to interrupt the 3 or 4 amp current flow (typical 12v system) to generate a back emf voltage. If it weren't for the CB points capacitor, most of this energy would be wasted in arcing across the points. It's also worth noting that the same considerations apply to a magneto CB points setup (but here, at tickover speeds, there's a lot less 'surplus spark energy' to be handled as unwanted CB points arcing energy)
The capacitor value (and voltage rating) is optimised to moderate the voltage rise such that the effective breakdown voltage of the air gap being opened up across the points increases faster than this inductive voltage rise for moderate to maximum engine rpm. Even so, at tick-over rpm, there will still be a good portion of the spark energy being dissipated in arcing at the CB points.
In both the magneto and Kettering ignition systems, it's important not to introduce any 'electrical damping' (eg electrical leakage in the HT circuit - fouled plug or damp getting into the HT circuit) since it robs energy from the high voltage spike being generated at the points sufficiently to stop the HT voltage spike from attaining enough potential to break down the spark plug gap insulation (which depends on the pressure within the cylinder).
This last could be oil or excess fuel vaporising. I'm not familiar with the fuelling system on this mower but, according to others', along with your mention of 'pulsation' (or 'hunting') it does seem as though it's using a slightly more sophisticated diaphragm controlled slide carburetter with some sort of airflow vane control over the engine rpms.
It's surprising that there isn't some sort of fuel enrichment device to aid starting such as a choke or a float depression button (or both) to raise the float chamber fuel level. Also, 'pulsation' can also be a symptom of an overly rich fuel mixture.
Examination of the spark plug insulator nose can be very revealing of this condition. Indeed, you can find any chart of spark plug conditions which provide diagnostic information about the condition of both two and four stroke engines so if you don't have the owner's guide to hand, you should be able to track down one for any petrol engined machine on the internet to aid your diagnosis. Probably your best source would be a spark plug manufacturer's web site where you'll find a comprehensive chart covering both types of engine.
[1] When I "Transistorised" the twin coil ignition on my Triumph Bonneville T120V with a homebuilt custom designed Capacitor Discharge ignition module (way back in the mid 70s), driven from the original CB points, the maintenance period on re-gapping the points went from "Every
3000 miles", in the owner's manual, to "never needed" simply by removing the 3 or 4 amp inductive load and replacing it with a purely resistive
100mA load.
Also, another handy benefit was reduced electrical erosion of the spark plug electrodes (20,000 miles versus 5 to 10 thousand miles regapping/ replacement schedule) along with immunity against leakage effects allowing for a grade harder plug to be used without the higher level of plug fouling that results under stop start urban riding conditions presenting any issues whilst permitting a leaner mid range mixture and the use of a 40 thou gap instead of the original 25 thou gap specification which permitted an even leaner idle mixture setting.
Ton Up motorway cruising speed still remained at the 45mpg mark but
50/60mph "A road" crusing speeds enjoyed a 100mpg fuel economy versus the almost consistent 50mpg I had previously been suffering almost regardless of road conditions and whether solo or two up with a pillion passenger.
If it ran well when it finally started and presumably started eventually then a new carb probably wouldn't make much difference.
Have you tried pulling the starter with the plug out but in the cap and laying on the engine somehow? You should see / hear a nice healthy spark across the gap.
The thing there is you are playing with high voltage and not 'current' as such so even a small cap between the cap contact and the plug top wouldn't make a huge difference, unless ignition was marginal etc.
Ok ...
Not unknown to make an engine more difficult to start.
Basically it's a large coil of copper wire wrapped around an iron core that sits just outside (normally) the flywheel / ignition rotor. This has a magnet inside it and when the magnet passes over the poles of the coil, the coil generates a high voltage > spark.
Hmm, do you mean you try to start it on full throttle? You don't normally I don't think.
Does it have a primer?
They often do because they have a flap sitting in the cooling air that acts as a speed regulator (normally at tickover / lower speeds). When the air pushing against the plate slows and the pressure drops, a fine spring increases the throttle to speed it up again and stop it stalling.
That 'could' just be residual smoke from the heat of the exhaust / silencer that you can't normally see when it's running?
I'm not sure how many of these basic / stationary engines have points and so normally just use the close proximity of the rotor to the stator to generate the spark and determine the ignition timing?
Thanks for the informative reply. You gave some useful info that hadn't occ urred to me concerning cylinder pressure affecting things. As a fellow ex-t inkerer and owner of several British twins (1953 BSA Golden Flash, highly c ustomised, plus a couple of Royal Enfield twins and one 350cc Triumph twin) I rebuilt engines on three occasions, so I expected that a simple little p etrol mower would be a piece of cake to undestand, but I am baffled by bot the way the carb works and also the ignition system. I can see that the mai n flywheel-like thing has a smooth shiny surface that passes within 1mm of something covered in plastic that is about 2" diameter and an inch thiock f rom which the HT lead goes to the spark plug. That must be the coil, is it? I presume that there is no contact-breaker. And this would also mean there is no condenser, yes?
The carb has no air leaks. It has a rubber bulbous primer that I am instruc ted to push three times. I *think* this sucks petrol up out of the tank suf ficiently to fill what appears to be a sort of floatless float chamber. Its ' hard to see how this carb works by looking at it. All my bikes had Amal m onoblock carbs AFAICR, and I could see exactly how they worked. Anyway, thi s is a moot point since by replacing the carb entirely, with no improvement , I think it's safe to assume it's not a carb or fuel problem.
I will try cranking it over with the plug out in the dark and see how brigh t the spark looks within the next 24 hours. If said spark looks feeble, I w ill try buying a new plug. If that makes no diff I will try buying a new co il (or at least, the thing I suspect is the coil)! It doesn't remotely rese mble a motorcycle coil that I've seen (However, my motorcycle familiarity i s limited to 1950-1070 British bikes and 1990-2006 Harley Davidsons. Oh, an d more recently, a Honda 125 scooter, which was IMO, the most practical mac hine of the lot!!
-called 'weed wand' a.k.a. blow lamp with long neck for burning weeds. It w as fairly easy to prop this up against the underside of the air cleaner whi le holding the throttle open and pulling the rope (without needing three ar ms).
new carburettor, complete with diaphragm, gasket and o-rings etc, but it di dn't make a ha'p'orth of difference.
hat didn't help.
but that didn't help.
early there is no contact breaker (unless it's hidden somewhere). Clearly t he spark must be occurring, or it wouldn't run at all.
e max when starting.
ngie after I shut the
Thanks... The instructions tell me to hold the throttle fully open while st artung the engine. Yes, it has a carb primer: a rubber bulb thing that you push three times. I *think* this sucks petrol up up out of the tank enough to fill a little resevoir that feeds the jet, as far as I can make out. I'l l try cranking it over with the plug out, but touching the engine and see i f the spark is bright. If not I will buy a new plug. If that doesn't help, I'll perhaps try a new coil. If that doesn't work, I may just modify the ai r cleaner with a gas blow lamp permanently attached to it so I can implemen t Another John's lifesaving tip each time I start it - without needing to g row a third arm!
Ok, but have you tried other throttle positions when starting OOI?
That's the badger.
Yes, or fill the float bowl if it has one etc. Basically, to get a decent fuel feed up to where it counts. When you pump it, does it stop (pressurise) or can you just keep pumping? If you can keep pumping, I wonder if you would see a spray of fuel from the plug hole if you take the plug out and spin it over after 'over priming'?
Ok. Is this mower electric start?
One of the first things to change as it's easy and cheap. As a quick and dirty check, most plugs in these things are gapped at around 25 thou and that's about the thickness of a thumbnail. ;-)
You can get coils where some of the windings become shorted so that you get enough spark to run the engine but not a good enough spark to start the engine *easily*. The manufacturers sometimes offer ohms check readings and whilst they can help if the coil is badly out, it may not if it's intermittent (under load etc) or a thermal problem (unless you heat the coil with say a hot air gun when testing).
I have seen people mod such things with a small (typically) silicone tube passed though a hole (drilled in) the air cleaner and facing the carb inlet. The outer end of the tube is plugged under normal conditions and therefore giving good / clean access for a squirt of 'Easy Start' or whatever when required.
That said and given all the right circumstances, that shouldn't be required (of course) but sometimes it is the simplest option.
The fact that it does start (easily?) when given something slightly more flammable than fuel (say gas or ether) may indicate a general 'fuelling issue or mask the fact that there is a (minor) issues that is being masked by the 'starting aid'.
It's like having a weak battery on a car. It may not have enough 'guts' to start the car on the starter motor but may have plenty to run the engine (and I experienced on many_an_occasion when starting my Morris Minor with the starting handle). ;-)
Daughter was given an old 2/ leaf blower that wouldn't run, apparently had been 'check out' by a mechanic and that 'needed a new carb'. A couple of pulls on the starter cord suggested it had no compression and quick strip down reviled a seized piston ring and a fairly scored bore. Just to prove that it wasn't the carb, I got her to put a new ring in it, put it back together and whilst it did indeed start and run, it was obviously f'd. ;-(
I have a Honda GX200 powered generator and that became quite intermittent, mainly on starting (although like yours, would start with some Easy Start etc). Once running it generally carried on running and would re-start ok. One day it simply wouldn't start at all and I managed to pin it down to the coil (weak spark). I fitted a new coil and it's been fine ever since. ;-)
At least 4/'s don't suffer with the same issues as 2/ where leaky crankshaft seals can also have a big impact on starting and running (other than leaking oil all over the place). ;-)
I love the challenge of a recalcitrant utility engine as you can generally go though it all quite easily and if it's common / modern enough, replace they key components (like coils or even CDI units) quite cheaply. The very common engines (like Honda specifically) also have a good supply of pattern parts, or in the case of engines also used on things like go-carts, 'racing parts'. ;-)
In message , snipped-for-privacy@aanotes.com writes
Before you throw too much more money at it, try a new plug by all means, but also try a new HT lead. I had a problem with my Minor which was eventually cured by fitting new HT leads. At first, I thought the problem had been cured just by disturbing and remaking the connections, but putting the old leads back on killed it. Threw those in the bin, refitted the new leads and away it went.
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