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.
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
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
[We've had this discussion before btw.]
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
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
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.
All political activity makes complete sense once the proposition that
all government is basically a self-legalising protection racket, is
An engineers solution to the problem.
But: why bother with taking the air filter off?
"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
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.
Trolls AND TROLL FEEDERS all go in my kill file
On Wednesday, April 27, 2016 at 9:36:38 AM UTC+1, Another John wrote:
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
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
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.
 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
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.
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?
Cheers, T i m
On Saturday, April 30, 2016 at 7:35:03 PM UTC+1, Johnny B Good wrote:
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!!
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.
In my original 'vanity/proof reading' scan, I *had* used the correct
"maximum" word in the above statement. However, even I managed to get
sufficiently confused as to change it to the word "minimum". Apologies
everyone (I do see other posters' justification for criticising my often
overly long (and flowery) postings. :-(
I suspect this must a modern hybrid version of the magneto system where
the functions of a self contained magneto were split into a seperate
generator (permanent magnet rotor keyed directly onto the engine
crankshaft) feeding ac current to what would otherwise be a conventional
ignition coil and CB arrangement normally powered from a battery. The
keying of the rotor of the PM alternator guaranteeing that the current
would always be interrupted at the peak of the generator's waveform.
The generator would also be used to provide electrical power to
headlights and instrumentation (speedo and main beam warning) lamps,
usually from seperate generator coil pairs (it was common practice to
split the generator output into two sources (using either 50:50 or 34:66
ratios) and combine them according to load demands using auxiliary switch
contacts on the side/head light selector switch as a crude charging
control in a battery set up or else to avoid burning out sidlelights when
the extra power wasn't required in a batteryless system (typical of
mopeds and some models of motorbikes, usually models designed for
scrambling events or bush trail riding where the battery would be an
unnecessary luxury and a liability).
In this case, the flywheel generator is most likely powering a
contactless "transistor assisted" ignition module circuit and coil hidden
under that plastic cover. The ignition timing will most likely be
triggered by a magnet and pickup coil sensor (avoids mechanical wear and
tear, hence the 'benefit' of the additional 'electronics' which might
literally be emulating the original magneto idea, replacing the CB points
with a high voltage switching transistor interrupting the appropriate
polarity peak current of the ac output taken directly from the generator
winding or, a little more sophisticated, triggering a capacitor into
discharging some 400v into the low voltage winding of the ignition coil
to generate the necessary 30 odd KV spark pulse almost regardless of any
leakage (damp or fouled plug insulation).
In this case (Capacitor Discharge Ignition) the ignition coil is only
being used as a step up transformer, not as in the original case, for its
inductive properties to generate the necessary 400v pulse *and* its step
up transformer action which relied on there not being any leakage in the
HT side to compromise its ability to rapidly turn off the 3 or 4 amps of
current flowing in the inductance of the coil's primary winding fast
enough to produce the required 400v pulse of back EMF.
Whilst a CDI has a bit more complexity than a naive "Transistor
Assisted" ignition circuit, it does offer the benefit of less erosion on
the spark plug points, much greater immunity to HT leakage current issues
and a more powerful spark to allow improved fuel economy and engine
performance. If you're going to start adding additional electronic parts
(points of failure) to the basic ignition circuit, you may as well "Be
hung for a sheep as for a lamb". :-)
I'd guess what is actually hidden away under that plastic cover rather
depends on the vintage of that B&S engine. If it's using 21st century
technology, there's every chance there's a transistor or 3 involved.
Earlier than that and it's likely to just consist of nothing more
sophisticated than a set of CB points and a 500v 100nF capacitor.
If it's using 21st century technology, the two most common ways, aside
from a mechanical CB points set, were either magnetic or optical
triggering. Since optical triggering was popular on aftermarket
transistorised ignition upgrade kits for 20th century motorcars, the most
likely choice for 'by design' ignition systems built into the later
electronically ignited petrol engines was 'magnetic' triggering which may
be nothing more than a sensing coil embedded into that plastic cover with
a tiny triggering magnet embedded into the flywheel (although the
generator magnets *could* be used for this function, a seperate tiny
magnet offers more precise timing - this may even simply be an extra bit
of iron to divert a fraction of the magnetic flux from one of the
generator magnets rather than an actual seperate tiny magnet).
Careful probing with a small steel rod might reveal the presence of such
a triggering magnetic field 'hot spot' on the inside of the outer rim of
the flywheel. If such a 'hot spot' is detected, you have your answer as
to why there's no sign of a contact breaker.
With regard to testing the sparking efficacy of the ignition system, you
really need to use an air gap of at least quarter of an inch, preferably
a 10mm spark gap at standard atmospheric pressure. The breakdown voltage
per millimetre is roughly proportional to pressure.
The voltage required to jump the the typical 0.8 to 1.00 mm spark plug
gap is only a tenth or less than that needed in actual service. BTW, if
the handbook specifies somewhere close to a 1.5mm gap, you can be
reasonably be sure of the use of a CDI module.
A simple high voltage power transistor switch substitute for the less
elegant electronic version of the Kettering CB points arrangement as used
by the Suzuki GSF600 digital ignitor module - a microprocessor controlled
ignition module let down by the weird choice of electronic CB points over
the technologically correct CDI system - I was *so very not impressed!*,
is just as prone to HT leakage current issues as the traditional CB/
condensor/ignition coil system of yester-millenium.
If you have a spare sacrificial spark plug to hand, you can bend the
earth electrode away from the tip to regap it out to 7 or 8 mm purely for
use as a spark gap tester for ignition systems in general (attach the HT
lead and rest the plug body on any handy metalwork electrically bonded to
the engine if not the engine itself).
Also, a handy insulator such as a piece of thin paxolin or perspex sheet
which can be gently wedged in the spark plug gap to force the spark to
bend around the obstruction for a total spark path length of circa 8 to
10 mm is a non destructive way to check the actual spark plug's
insulation performance after removing it from the engine and letting it
rest in contact with the engine as previously suggested for the test
spark gap plug.
The point is that even a failing ignition system can easily produce
sparks in open air over a 1mm gap leaving you to try and judge the
quality by intensity of the spark alone. Testing with a circa 8 to 10 mm
gap removes such uncertainty from out of the equation.
Or neither, possibly just a coil driven from the flywheel, handling
both the generation and timing of the spark:
Cheers, T i m
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