This mower is under 3 years old and is clean with very low mileage (small garden!).
I was away last w/e and Mrs S said it felt like it was running short of fuel - it starts to rev itself when it is. (The engine runs at same revs whether clutch is engaged or not).
So she filled with unleaded from our usual can - not dirty. The mower now idles and runs roughly, with black puffs of smoke from exhaust, and the plug is wet sooty. I'm assuming this is rich mixture.
I've taken off the air filter which is not clogged and it runs the same without it so I don't think clogged air filter is the cause.
It seems to run OK for the first 30 seconds then go into this lumpy mode. I'm wondering if the fuel supply is restricted (dirty maybe), but can't see how to get at it without going beyond my scope. I hear that putting it in for a service will cost half the original cost of the mower...!
Any quick hints or tips please?
Mods - please feel free to move this to IHAQ is that is the right place...
Stupid question: is the choke still on? Check that it's coming off OK. After checking this and the air filter, I'd be surprised if it's still running rich unless you've fiddled with some adjuster screws.
Revving by itself is in common with the governor trying to keep up with bubbly fuel.
A wild guess: Could this be an ignition problem/misfiring? Check the points and plug gap and readjust if necessary. Clean ignition points if they're dirty/oily.
I would recommend removing the spark plug and checking it is not coked up. IIRC the plug needs a smaller socket than a car plug. Removing the plug usually works in most misfiring problems for me. Briggs and Stratton engines are very simple and require little other servicing other than an oil change. They tend to run for years and years and years. My mower is now about 15 years old and has been continually abused and the engine still runs well, can't say the same for the mower body.
do not tip a briggs engine it allows oil to get into something or other it shouldn't. also does it have the small tank at the side of the engine with small pinholes on the screwtop do not fill these right up as apparently it causes them to run rich and smoke. i am no expert on these but was chatting to an old bloke no more than two weeks ago, when i purchased a second hand briggs engined mower from him. taught me a lot in about twenty minutes. oh and if you have to tip it, you should only tip it forward towards the plug or if to one side always to the side away from the carb. ...cheers...keo ps. the mower i bought is several years old but starts and runs like a new one....£20... bargain. keo.
The chap I work with tells me that his mower will only run properly on genuine leaded 4 star petrol, on unleaded it runs very badly indeed.
I recently bought him some 4 star at one of the special garages near me who still sell it. That should keep him going for a couple of years!
I don't know why there is this difference. Perhaps it's because the 4 star has a higher octane rating than the unleaded. If this is the case then, perhaps, running the mower on Optimax or similar might cure the problem, but my friend hasn't tried this yet.
As it was working fine on unleaded, I think I will try a new batch of fuel. I remember reading the bit about tilting the mower towards the soprak plug, if it HAS to be done... Syphoning is probably easiest. Or tie up the handle and let it run dry... :-)
I had exactly the same problem with my B&S lawnmower a couple of weeks ago, identical symptoms (starts ok, then runs rough, engine hunting,smoking,) - tried fresh fuel, new plug etc, then phoned local repair shop who told me over the phone the likely problem ...the diaphragm in the carburretor had split.
The guy replaced the part (£4.50) + £30 labour. It's probably easy to DIY but didn't want to deprive him of the job as he was so helpful!
He indicated it was a common problem so most likely this will be the problem on yours.
Mobil 1 is far too good for a lawnmower engine, they do not get hot enough for the oil to work properly. If I were you, I would put some ordinary cheap stuff in.
I also agree with the advice about the petrol, get some fresh, I think that unleaded is OK.
Of course I would never put cheap stuff in my Passat, I will stick to Castrol SLX11!
Is your B&S a 3.5 hp engine? It should have an adjustment screw on the carb if it is. I know this wont solve your problem but although your engine oil may look as clean as the day it went in when checked on the dipstick, it can be quite dirty when you drain it. You should change it each season. Also do not tip the mower on its side with the aircleaner at the lowest position. I would go along with the suggestions above re draining old petrol out.In warm weather I give it less priming to sart from cold. My engine is 10 yrs old and still on the original champion plug.
You may have water in your carburettor. It will have collected in the float chamber and semi block the flow of petrol. I think most models have 2 bolts with xcrew heads holding on the top so easy to remove.
(My 1972 B&S 3.5hp - ex my father - is still going strong in a Hayterette rotary mower..not spent a penny on spares but cut a new air filer from foam every 3 years.Used for about 15 hours a year with annual oil changes)
Tipping, eg to clear clogged grass from the blade and body, should be towards the carburettor. If you tip it the other way it fills the silencer with oil - you will recognise the symptoms by the dense clouds of blue smoke, when you eventually get it to start again!
Most B & S I have seen - I have several in various mowers and rotavators, have an automatic choke flap working by suction. It is a pivotted plastic disc just underneath the air intake, shaped so that the air cleaner bolt clears it down a groove. If this gets clogged, eg by grass or the air cleaner bolt not in straight, then the flap doesn't open fully when the engine fires.
Update: I've changed the petrol and cleaned and reset the plug. it runs a lot smoother, except it seems to be lower on power (i.e. moves slower) than I remember it. I also still have exhaust smoke, whcih I don't recall seeing before.
I'm going to get a new plug next...
I browsed the B&S web site and had a long hard look at the engine. I don't believe there is any (useful) adjustment I could make to the carb but I don't mind being proved wrong. There is no manual choke and I can't see any adjustment screws - and the manual for teh engine says "choke - if fitted" which I guess must mean some engines don't have them. There is a primer button for starting.
Our mower which had problems starting recently (see IHAQ) has never needed the choke. The primer button should be sufficient just to flood the jet for starting from cold.
Re it being down on power. It should run at a constant speed, as you say, adjusted by the governor. By down on power, do you mean that the engine speed reduces significantly now when you take up drive? If not it's probably just your perception, or the governor's mean (mal)adjusted, unlikely if you haven't touched it.
If runs ok for few secs then runs rich, clearly carb prob. I had similar prob in a Villiers mower engine - cause was the small brass valve which the float pushes up to regulate amount of petrol entering the float chamber. [years ago I lost it while cleaning carb and had made another out of a brass screw which never fitted very well].
You probably had a bit of grit causing overfilling and rough running. The test is to turn the petrol off. Rough running will eventually return to normal as the mixture weakens, then rev slightly faster briefly, then die as the petrol runs out.
Incidentally, all four of my mowers, average age >30yrs, run better on bog standard unleaded.
Use old engine oil after 10,000m - still perfectly ok for old mower engines.
Suggest filter your old petrol with paper coffee filter.
Recently I found a firm which does spares for ancient mower engines. It's called 'Meetens' - tel 0845 634 0295.
I tinkered this weekend. Fitted a new plug and it's still the same - smoother but feels a bit underpowered and a little smoky.
I bought the plug from a B&S service agent. He said it's probably the diaphragm (mentioned above) which he will swap for c £55. The part costs c £9. Am I going to get in a mess if I try doing it myself?
Plug unlikely to cause prob - mine are as old as the mowers! Sometimes the HT lead can need a bit of a trim at the hot end after 10yrs if the spark not getting thru and the points are ok.
Definitely sounds like a carburettor problem but I don't know anything about 'diaphragms' apart from when found in fuel pumps, which none of my mowers have - all gravity fed.
If it's newish I would take the carburettor to bits, clean carefully, reassemble - and see what happens! If it's black smoky not blue smoky it must be running rich and there aren't too many causes of that.
If it's the diaphram I am thinking of then it's actually a simple fuel pump. The petrol tank is underneath the carburettor, and the fuel is pumped by some kind of process that uses the induction pressure - "sucked" in short. That is why priming is necessary to start the process.
It is a straightforward job, but I have to say when I replaced one once it didn't make the slightest difference. I just replaced the whole carburettor from a scrap mower.
For what he's charging it may be a pig of a job, but it sounds like only a couple of hours work, if that. There's no harm trying and then taking it to him if you hit any problems. As mentioned, plugs don't really cause problems, our mowers still on it's original after 30 years.
Cliff, would the priming button also serve mainly as a cold start "tickler" since there is no choke?
Cliff, would the priming button also serve mainly as a cold
start "tickler" since there is no choke?
That's my understanding of how it works. I think they are really pretty crude engines with rather imprecise fuel measuring systems. Even ones I have had with manual chokes only needed the choke for the first few seconds of running. I think the primer just sloshes in some petrol, it splutters into life, and after that manages to draw the petrol it needs in vaguely the right sort of mixture. The "automatic" flap (choke?) I most commonly observe is just spring-loaded. When the engine is spun on the cord it is like putting your hand over the intake, enriching the mixture (old dodge for cold weather starting). As soon as the engine fires the suction opens the flap again.
I may be wrong, and a B & S spokesman is going to claim that they are masterpieces of precision fueling and that their exhausts are cleaner than the air that went in. But I have read somewhere that mowers and their relatives are actually quite a significant source of pollution, but their engine size makes them unsuitable, or uneconomic, for fuel injection.
You can't really go wrong taking a carburettor to pieces and cleaning it out, unless you are hoping to reuse any of the gaskets and diaphrams. They tend to weld themselves to the castings, and come to pieces when opened up. A thin sharp knife sometimes prises them off intact.
I had this problem with my B&S Classic 5, after fuel, additive etc... I removed the carb (plastic it was too) and the very thin diagphram which acts as the pulsejet pump had shriveled a bit causing the mixure to be all over the place. A new one was £2.50 from Henton and Chattel in Nottingham, put on and it's never run so lovely! The person who said that Mobil 1 is too good as the mower never gets hot enough is totally wrong! It's an aircooled engine and runs much hotter than a car engine!
Ah, reminds me of the time I tried to stop a lawnmower by pulling off the spark plug cable. Took a firm grip on some black rubbery stuff to brace myself and grabbed the plug... Turns out the black rubbery stuff was oil drenched metal. ;-)
lawnmower by pulling off the spark plug cable. Took a
firm grip on some black rubbery stuff to brace myself and
grabbed the plug... Turns out the black rubbery stuff was
oil drenched metal. ;-)
Rest should be self explanatory.
Did a very similar thing myself - mower wouldn't start - thinking perhaps flooded or no spark? Removed plug, reconnected lead, swung engine - fuel gushed out over mower body, ignited by spark plug - still has fried paint + curly plastic badge.
Only in the BR would a thread on this keep going so long!
I had this problem with my B&S Classic 5, after fuel,
additive etc... I removed the carb (plastic it was too) and
the very thin diagphram which acts as the pulsejet pump had
shriveled a bit causing the mixure to be all over the
place. A new one was £2.50 from Henton and Chattel in
Nottingham, put on and it's never run so lovely! The
person who said that Mobil 1 is too good as the
mower never gets hot enough is totally wrong! It's an aircooled
engine and runs much hotter than a car engine!
If you don't believe me that Mobil 1 is too good for a lawnmower, go to a mower shop and look at the spec of the specialist mower oil. I think it is SD or SF.
I was told by a mower repairer to use "cheap" oil.
Oh Joy! Thanks quizman. Smokie described the fault with my B&S exactly, and your suggestion ws spot on. In fact the thin diaphragm had not split, but had become slightly rucked near one of the retaining bolts. All I had to do was straighten it out and make sure it was laid well flat before clamping the carb back together. Thanks - now runs like new.
P.S My three year old mower has been abused with two-stroke oil, overfilled with oil and tipped up the wrong way to empty excess oil out. I have even had to syphon out water from the petrol tank on ocassions and it still runs perfectly well on unleaded!
I had a problem with the carburettor diaphragm on our Briggs and Stratton-engined Mountfield after only a few weeks of use. The service agent said it's a common occurence for the diaphragm to pull out from one or more of its clamping lands, and that even some brand new mowers need a new diaphragm. Having looked at the old diaphragm I'm sure he's right ~ the clamping lands are miniscule and I can well imagine the adjacent metal parts not mating up correctly in places. I think it's probably the result of computer aided design which allows small details to be magnified and for the designer to lose his sense of proportion ~ assuming he had one in the first place. It was much more difficult to draw details too small in the days of drawing boards and pencils.
I have a B and S 6hp intek overhead valve on my mower. When it began to run unevenly and was difficult to start when warm I found the grub screw had fallen out of the little lever adjustment mechanism for fast running on the carb - it's in behind the priming button on this engine.
Because I realised this would be an ongoing problem I substituted a blob of Bluetak to hold the lever in place and it's been fine for three years.
I had my mower in bits at the weekend. It's a knackered old thing that needs some new parts. Briggs and Stratton sidevalve engine. It did start briefly but then spluttered to a stop. Sounds like the diaphragm.
Mobil 1 in a mower. What viscosity Mobil 1. Most mower type engine require a monograde oil like SAE30 or 40, it is not a pumped system and the oil need to coat parts quickly at start up. Depending on your Mobil 1 it mat be far too thin on start up. Regards Peter
My B and S engine handbook that came with the mower 6 years ago only says 'use fully synthetic oil'.
I've always used supermarket 5-30 and changed it every year as per recommendation. It always stays clean and the level doesn't drop.
Briggs and Stratton has a website, IIRC, that is quite useful.
It's funny how this thread can splutter almost seamlessly from 2005 to 2008 and it all seems just as relevant as it was in the original posts. The good old Briggs and Strattons just keep chugging along. Hands up anyone who has actually changed their oil in the meantime?
Still wish I'd bought a mower with a Honda 160 though...
I've been looking at bigger and better mowers recently and it's amazing to see most B and S engines are still sidevalves.
Same sort of American take on technology as Harley Davidson, I guess.
Briggs and Strattons just keep chugging along. Hands up anyone who has actually changed their
oil in the meantime?
Yes - was knocking a lot - [5hp on Ransome Hahn rideon bought second hand 1979, was ancient even then] - took head off - piston had such a thick layer of carbon it was hitting the head! Wouldn't start well, changed [original] plug, refilled with usual oil from 10,000m car change left to stand for several weeks......now works perfectly again. This is the first attention it has had since 1979.
Have a few hayter pedestrians of various ages used commercially and as a rule the older the engine the better it is... Watch for the ceramic inlet manifolds on the later models that develop cracks that also cause running problems...
Have a Hayter Ranger that is 11 months old and here we go, starts will run forever and a day and then when you stop it and go to re start no play or takes an age to get going again....
Heavily sooted plug with a lumpy running... So looks like the diaphram proble as described..
Have a spy at this site I found for briggs bits which may be a help to you guys...
My grateful thanks to smokie, and all who replied suggesting the diaphragm might be at fault.
On Monday, my mower which is about 13 years old and well past its use-by date, started to run badly, stalling, stopping, etc. On Tuesday I went to check up on the current prices of new ones but happily Timnbd resurrected this thread just in time before I parted with the cash.
I purchased a diaphragm kit today for £3.50, fitted it this afternoon and, mercifully, the mower is not just restored to health but running better than it has done for a long time.
My grateful thanks and a virtual pint to all concerned, as befits my username I prefer to make do and mend rather than buy new.
Maybe, but they are still prehistoric.
Would you rather ride a Harley or break up your drive with a road drill?
Answers on a postcard please...
No, on second thoughts, please don't bother. I get trouble with hornets' nests as it is.
I changed the oil in mine when I was fiddling with it the other Sunday. I refilled it with Silkoline fully synth but only because I have 2 small containers of the stuff and nothing else to do with it. The diaphragm should be on it's way as I type. According to the engine code it was made in '78.
I have a problem with my 46s in that it is now sluggish so I took the air filter off and cleaned it, but I noticed that the springs on the carb (I think where you adjust the speed) aren't connected properly as the do nothing. If I move the (choke?) arm in the frony of the carb where the springs are attached nothing happens, and where they attach to an arm that goes up and around (about 3 inches) the fly wheel at the top, if you move that it builds the revs up. Does anyone know how thes springs shpuld attach and how I can alter the speed of the engine which I can do when the air filter is removed but not when it is on.Thanks